High Energy Astrophysical Phenomena (astro-ph.HE)
Wed, 28 Jun 2023
1.The mHz quasi-regular modulations of 4U 1630--47 during its 1998 outburst
Authors:Qingchang Zhao, Hongxing Yin, Lian Tao, Zixu Yang, Jinlu Qu, Liang Zhang, Shu Zhang, Erlin Qiao, Qingcui Bu, Shujie Zhao, Panping Li, Yiming Huang, Ruican Ma, Ruijing Tang, Pei Jin, Wei Yu, Hexin Liu, Yue Huang, Xiang Ma, Jingyu Xiao, Xuan Zhang, Kang Zhao
Abstract: We present the results of a detailed timing and spectral analysis of the quasi-regular modulation (QRM) phenomenon in the black hole X-ray binary 4U 1630--47 during its 1998 outburst observed by Rossi X-ray Timing Explore (RXTE). We find that the $\sim$ 50-110 mHz QRM is flux dependent, and the QRM is detected with simultaneous low frequency quasi-periodic oscillations (LFQPOs). According to the behavior of the power density spectrum, we divide the observations into four groups. In the first group, namely behavior A, LFQPOs are detected, but no mHz QRM. The second group, namely behavior B, a QRM with frequency above $\sim$ 88 mHz is detected and the $\sim$ 5 Hz and $\sim$ 7 Hz LFQPOs are almost overlapping. In the third group, namely behavior C, the QRM frequency below $\sim$ 88 mHz is detected and the LFQPOs are significantly separated. In the forth group, namely behavior D, neither QRM nor LFQPOs are detected. We study the energy-dependence of the fractional rms, centroid frequency, and phase-lag of QRM and LFQPOs for behavior B and C. We then study the evolution of QRM and find that the frequency of QRM increases with hardness, while its rms decreases with hardness. We also analyze the spectra of each observation, and find that the QRM rms of behavior B has a positive correlation with $\rm F_{\rm powerlaw}$ / $\rm F_{\rm total}$. Finally, we give our understanding for this mHz QRM phenomena.
2.Collisionless kinetic regimes for quasi-stationary axisymmetric accretion disc plasmas
Authors:Claudio Cremaschini, Massimo Tessarotto
Abstract: This paper is concerned with the kinetic treatment of quasi-stationary axisymmetric collisionless accretion disc plasmas. The conditions of validity of the kinetic description for non-relativistic magnetized and gravitationally-bound plasmas of this type are discussed. A classification of the possible collisionless plasma regimes which can arise in these systems is proposed, which can apply to accretion discs around both stellar-mass compact objects and galactic-center black holes. Two different classifications are determined, which are referred to respectively as energy-based and magnetic field-based classifications. Different regimes are pointed out for each plasma species, depending both on the relative magnitudes of kinetic and potential energies and the magnitude of the magnetic field. It is shown that in all cases, there can be quasi-stationary Maxwellian-like solutions of the Vlasov equation. The perturbative approach outlined here permits unique analytical determination of the functional form for the distribution function consistent, in each kinetic regime, with the explicit inclusion of finite Larmor radius-diamagnetic and/or energy-correction effects.
3.Diffuse neutrino background from past core-collapse supernovae
Authors:Shin'ichiro Ando, Nick Ekanger, Shunsaku Horiuchi, Yusuke Koshio
Abstract: Core-collapse supernovae are among the most powerful explosions in the universe, emitting thermal neutrinos that carry away the majority of the gravitational binding energy released. These neutrinos create a diffuse supernova neutrino background (DSNB), one of the largest energy budgets among all radiation backgrounds. Detecting the DSNB is a crucial goal of modern high-energy astrophysics and particle physics, providing valuable insights in both core-collapse modeling, neutrino physics, and cosmic supernova rate history. In this review, we discuss the key ingredients of DSNB calculation and what we can learn from future detections, including black-hole formation and non-standard neutrino interactions. Additionally, we provide an overview of the latest updates in neutrino experiments, which could lead to the detection of the DSNB in the next decade. With the promise of this breakthrough discovery on the horizon, the study of DSNB holds enormous potential for advancing our understanding of the Universe.
4.Probing Galaxy structure with VHE $γ$ rays
Authors:Constantin Steppa, Kathrin Egberts
Abstract: As an observer from within the Milky Way, it is difficult to determine its global structure. Despite extensive observational data from surveys at different wavelengths, we have no conclusive description of the structure of our own Galaxy. For very-high-energy (VHE) $\gamma$ rays, the most comprehensive catalogue of Galactic sources resulting from the H.E.S.S. Galactic Plane Survey (HGPS) shows a striking asymmetry in the distribution of the sources in the latitudinal direction. This could be the result of a local feature in the spatial distribution of the sources or it could be due to the position of the Sun above the Galactic plane. In this contribution, we estimate the position of the Sun based on the latitudinal flux profile of VHE $\gamma$-ray sources, assuming three mirror-symmetric models for the spatial distribution of the sources in three-dimensional space and taking into account the observational bias of the HGPS. We verify our method using simulations and find values for $z_{\odot}$ between $-6\,\mathrm{pc}$ and $94\,\mathrm{pc}$ depending on the considered model. Our results show that the position of the Sun has a significant impact on the observed source distribution and must therefore be taken into account when modelling the population of Galactic VHE $\gamma$ sources. However, it is not conclusive whether the Sun's offset from the Galactic plane is the only factor leading to the asymmetry in the latitudinal profile.
5.The NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background
Authors:Gabriella Agazie The NANOGrav Collaboration, Akash Anumarlapudi The NANOGrav Collaboration, Anne M. Archibald The NANOGrav Collaboration, Zaven Arzoumanian The NANOGrav Collaboration, Paul T. Baker The NANOGrav Collaboration, Bence Becsy The NANOGrav Collaboration, Laura Blecha The NANOGrav Collaboration, Adam Brazier The NANOGrav Collaboration, Paul R. Brook The NANOGrav Collaboration, Sarah Burke-Spolaor The NANOGrav Collaboration, Rand Burnette The NANOGrav Collaboration, Robin Case The NANOGrav Collaboration, Maria Charisi The NANOGrav Collaboration, Shami Chatterjee The NANOGrav Collaboration, Katerina Chatziioannou The NANOGrav Collaboration, Belinda D. Cheeseboro The NANOGrav Collaboration, Siyuan Chen The NANOGrav Collaboration, Tyler Cohen The NANOGrav Collaboration, James M. Cordes The NANOGrav Collaboration, Neil J. Cornish The NANOGrav Collaboration, Fronefield Crawford The NANOGrav Collaboration, H. Thankful Cromartie The NANOGrav Collaboration, Kathryn Crowter The NANOGrav Collaboration, Curt J. Cutler The NANOGrav Collaboration, Megan E. DeCesar The NANOGrav Collaboration, Dallas DeGan The NANOGrav Collaboration, Paul B. Demorest The NANOGrav Collaboration, Heling Deng The NANOGrav Collaboration, Timothy Dolch The NANOGrav Collaboration, Brendan Drachler The NANOGrav Collaboration, Justin A. Ellis The NANOGrav Collaboration, Elizabeth C. Ferrara The NANOGrav Collaboration, William Fiore The NANOGrav Collaboration, Emmanuel Fonseca The NANOGrav Collaboration, Gabriel E. Freedman The NANOGrav Collaboration, Nate Garver-Daniels The NANOGrav Collaboration, Peter A. Gentile The NANOGrav Collaboration, Kyle A. Gersbach The NANOGrav Collaboration, Joseph Glaser The NANOGrav Collaboration, Deborah C. Good The NANOGrav Collaboration, Kayhan Gultekin The NANOGrav Collaboration, Jeffrey S. Hazboun The NANOGrav Collaboration, Sophie Hourihane The NANOGrav Collaboration, Kristina Islo The NANOGrav Collaboration, Ross J. Jennings The NANOGrav Collaboration, Aaron D. Johnson The NANOGrav Collaboration, Megan L. Jones The NANOGrav Collaboration, Andrew R. Kaiser The NANOGrav Collaboration, David L. Kaplan The NANOGrav Collaboration, Luke Zoltan Kelley The NANOGrav Collaboration, Matthew Kerr The NANOGrav Collaboration, Joey S. Key The NANOGrav Collaboration, Tonia C. Klein The NANOGrav Collaboration, Nima Laal The NANOGrav Collaboration, Michael T. Lam The NANOGrav Collaboration, William G. Lamb The NANOGrav Collaboration, T. Joseph W. Lazio The NANOGrav Collaboration, Natalia Lewandowska The NANOGrav Collaboration, Tyson B. Littenberg The NANOGrav Collaboration, Tingting Liu The NANOGrav Collaboration, Andrea Lommen The NANOGrav Collaboration, Duncan R. Lorimer The NANOGrav Collaboration, Jing Luo The NANOGrav Collaboration, Ryan S. Lynch The NANOGrav Collaboration, Chung-Pei Ma The NANOGrav Collaboration, Dustin R. Madison The NANOGrav Collaboration, Margaret A. Mattson The NANOGrav Collaboration, Alexander McEwen The NANOGrav Collaboration, James W. McKee The NANOGrav Collaboration, Maura A. McLaughlin The NANOGrav Collaboration, Natasha McMann The NANOGrav Collaboration, Bradley W. Meyers The NANOGrav Collaboration, Patrick M. Meyers The NANOGrav Collaboration, Chiara M. F. Mingarelli The NANOGrav Collaboration, Andrea Mitridate The NANOGrav Collaboration, Priyamvada Natarajan The NANOGrav Collaboration, Cherry Ng The NANOGrav Collaboration, David J. Nice The NANOGrav Collaboration, Stella Koch Ocker The NANOGrav Collaboration, Ken D. Olum The NANOGrav Collaboration, Timothy T. Pennucci The NANOGrav Collaboration, Benetge B. P. Perera The NANOGrav Collaboration, Polina Petrov The NANOGrav Collaboration, Nihan S. Pol The NANOGrav Collaboration, Henri A. Radovan The NANOGrav Collaboration, Scott M. Ransom The NANOGrav Collaboration, Paul S. Ray The NANOGrav Collaboration, Joseph D. Romano The NANOGrav Collaboration, Shashwat C. Sardesai The NANOGrav Collaboration, Ann Schmiedekamp The NANOGrav Collaboration, Carl Schmiedekamp The NANOGrav Collaboration, Kai Schmitz The NANOGrav Collaboration, Levi Schult The NANOGrav Collaboration, Brent J. Shapiro-Albert The NANOGrav Collaboration, Xavier Siemens The NANOGrav Collaboration, Joseph Simon The NANOGrav Collaboration, Magdalena S. Siwek The NANOGrav Collaboration, Ingrid H. Stairs The NANOGrav Collaboration, Daniel R. Stinebring The NANOGrav Collaboration, Kevin Stovall The NANOGrav Collaboration, Jerry P. Sun The NANOGrav Collaboration, Abhimanyu Susobhanan The NANOGrav Collaboration, Joseph K. Swiggum The NANOGrav Collaboration, Jacob Taylor The NANOGrav Collaboration, Stephen R. Taylor The NANOGrav Collaboration, Jacob E. Turner The NANOGrav Collaboration, Caner Unal The NANOGrav Collaboration, Michele Vallisneri The NANOGrav Collaboration, Rutger van Haasteren The NANOGrav Collaboration, Sarah J. Vigeland The NANOGrav Collaboration, Haley M. Wahl The NANOGrav Collaboration, Qiaohong Wang The NANOGrav Collaboration, Caitlin A. Witt The NANOGrav Collaboration, Olivia Young The NANOGrav Collaboration
Abstract: We report multiple lines of evidence for a stochastic signal that is correlated among 67 pulsars from the 15-year pulsar-timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. The correlations follow the Hellings-Downs pattern expected for a stochastic gravitational-wave background. The presence of such a gravitational-wave background with a power-law-spectrum is favored over a model with only independent pulsar noises with a Bayes factor in excess of $10^{14}$, and this same model is favored over an uncorrelated common power-law-spectrum model with Bayes factors of 200-1000, depending on spectral modeling choices. We have built a statistical background distribution for these latter Bayes factors using a method that removes inter-pulsar correlations from our data set, finding $p = 10^{-3}$ (approx. $3\sigma$) for the observed Bayes factors in the null no-correlation scenario. A frequentist test statistic built directly as a weighted sum of inter-pulsar correlations yields $p = 5 \times 10^{-5} - 1.9 \times 10^{-4}$ (approx. $3.5 - 4\sigma$). Assuming a fiducial $f^{-2/3}$ characteristic-strain spectrum, as appropriate for an ensemble of binary supermassive black-hole inspirals, the strain amplitude is $2.4^{+0.7}_{-0.6} \times 10^{-15}$ (median + 90% credible interval) at a reference frequency of 1/(1 yr). The inferred gravitational-wave background amplitude and spectrum are consistent with astrophysical expectations for a signal from a population of supermassive black-hole binaries, although more exotic cosmological and astrophysical sources cannot be excluded. The observation of Hellings-Downs correlations points to the gravitational-wave origin of this signal.
6.The second data release from the European Pulsar Timing Array III. Search for gravitational wave signals
Authors:J. Antoniadis, P. Arumugam, S. Arumugam, S. Babak, M. Bagchi, A. -S. Bak Nielsen, C. G. Bassa, A. Bathula, A. Berthereau, M. Bonetti, E. Bortolas, P. R. Brook, M. Burgay, R. N. Caballero, A. Chalumeau, D. J. Champion, S. Chanlaridis, S. Chen, I. Cognard, S. Dandapat, D. Deb, S. Desai, G. Desvignes, N. Dhanda-Batra, C. Dwivedi, M. Falxa, R. D. Ferdman, A. Franchini, J. R. Gair, B. Goncharov, A. Gopakumar, E. Graikou, J. -M. Grießmeier, L. Guillemot, Y. J. Guo, Y. Gupta, S. Hisano, H. Hu, F. Iraci, D. Izquierdo-Villalba, J. Jang, J. Jawor, G. H. Janssen, A. Jessner, B. C. Joshi, F. Kareem, R. Karuppusamy, E. F. Keane, M. J. Keith, D. Kharbanda, T. Kikunaga, N. Kolhe, M. Kramer, M. A. Krishnakumar, K. Lackeos, K. J. Lee, K. Liu, Y. Liu, A. G. Lyne, J. W. McKee, Y. Maan, R. A. Main, M. B. Mickaliger, I. C. Nitu, K. Nobleson, A. K. Paladi, A. Parthasarathy, B. B. P. Perera, D. Perrodin, A. Petiteau, N. K. Porayko, A. Possenti, T. Prabu, H. Quelquejay Leclere, P. Rana, A. Samajdar, S. A. Sanidas, A. Sesana, G. Shaifullah, J. Singha, L. Speri, R. Spiewak, A. Srivastava, B. W. Stappers, M. Surnis, S. C. Susarla, A. Susobhanan, K. Takahashi, P. Tarafdar, G. Theureau, C. Tiburzi, E. van der Wateren, A. Vecchio, V. Venkatraman Krishnan, J. P. W. Verbiest, J. Wang, L. Wang, Z. Wu
Abstract: We present the results of the search for an isotropic stochastic gravitational wave background (GWB) at nanohertz frequencies using the second data release of the European Pulsar Timing Array (EPTA) for 25 millisecond pulsars and a combination with the first data release of the Indian Pulsar Timing Array (InPTA). We analysed (i) the full 24.7-year EPTA data set, (ii) its 10.3-year subset based on modern observing systems, (iii) the combination of the full data set with the first data release of the InPTA for ten commonly timed millisecond pulsars, and (iv) the combination of the 10.3-year subset with the InPTA data. These combinations allowed us to probe the contributions of instrumental noise and interstellar propagation effects. With the full data set, we find marginal evidence for a GWB, with a Bayes factor of four and a false alarm probability of $4\%$. With the 10.3-year subset, we report evidence for a GWB, with a Bayes factor of $60$ and a false alarm probability of about $0.1\%$ ($\gtrsim 3\sigma$ significance). The addition of the InPTA data yields results that are broadly consistent with the EPTA-only data sets, with the benefit of better noise modelling. Analyses were performed with different data processing pipelines to test the consistency of the results from independent software packages. The inferred spectrum from the latest EPTA data from new generation observing systems is rather uncertain and in mild tension with the common signal measured in the full data set. However, if the spectral index is fixed at 13/3, the two data sets give a similar amplitude of ($2.5\pm0.7)\times10^{-15}$ at a reference frequency of $1\,{\rm yr}^{-1}$. By continuing our detection efforts as part of the International Pulsar Timing Array (IPTA), we expect to be able to improve the measurement of spatial correlations and better characterise this signal in the coming years.
7.Search for an isotropic gravitational-wave background with the Parkes Pulsar Timing Array
Authors:Daniel J. Reardon, Andrew Zic, Ryan M. Shannon, George B. Hobbs, Matthew Bailes, Valentina Di Marco, Agastya Kapur, Axl F. Rogers, Eric Thrane, Jacob Askew, N. D. Ramesh Bhat, Andrew Cameron, Małgorzata Curyło, William A. Coles, Shi Dai, Boris Goncharov, Matthew Kerr, Atharva Kulkarni, Yuri Levin, Marcus E. Lower, Richard N. Manchester, Rami Mandow, Matthew T. Miles, Rowina S. Nathan, Stefan Osłowski, Christopher J. Russell, Renée Spiewak, Songbo Zhang, Xing-Jiang Zhu
Abstract: Pulsar timing arrays aim to detect nanohertz-frequency gravitational waves (GWs). A background of GWs modulates pulsar arrival times and manifests as a stochastic process, common to all pulsars, with a signature spatial correlation. Here we describe a search for an isotropic stochastic gravitational-wave background (GWB) using observations of 30 millisecond pulsars from the third data release of the Parkes Pulsar Timing Array (PPTA), which spans 18 years. Using current Bayesian inference techniques we recover and characterize a common-spectrum noise process. Represented as a strain spectrum $h_c = A(f/1 {\rm yr}^{-1})^{\alpha}$, we measure $A=3.1^{+1.3}_{-0.9} \times 10^{-15}$ and $\alpha=-0.45 \pm 0.20$ respectively (median and 68% credible interval). For a spectral index of $\alpha=-2/3$, corresponding to an isotropic background of GWs radiated by inspiraling supermassive black hole binaries, we recover an amplitude of $A=2.04^{+0.25}_{-0.22} \times 10^{-15}$. However, we demonstrate that the apparent signal strength is time-dependent, as the first half of our data set can be used to place an upper limit on $A$ that is in tension with the inferred common-spectrum amplitude using the complete data set. We search for spatial correlations in the observations by hierarchically analyzing individual pulsar pairs, which also allows for significance validation through randomizing pulsar positions on the sky. For a process with $\alpha=-2/3$, we measure spatial correlations consistent with a GWB, with an estimated false-alarm probability of $p \lesssim 0.02$ (approx. $2\sigma$). The long timing baselines of the PPTA and the access to southern pulsars will continue to play an important role in the International Pulsar Timing Array.
8.Searching for the nano-Hertz stochastic gravitational wave background with the Chinese Pulsar Timing Array Data Release I
Authors:Heng Xu, Siyuan Chen, Yanjun Guo, Jinchen Jiang, Bojun Wang, Jiangwei Xu, Zihan Xue, R. Nicolas Caballero, Jianping Yuan, Yonghua Xu, Jingbo Wang, Longfei Hao, Jingtao Luo, Kejia Lee, Jinlin Han, Peng Jiang, Zhiqiang Shen, Min Wang, Na Wang, Renxin Xu, Xiangping Wu, Richard Manchester, Lei Qian, Xin Guan, Menglin Huang, Chun Sun, Yan Zhu
Abstract: Observing and timing a group of millisecond pulsars (MSPs) with high rotational stability enables the direct detection of gravitational waves (GWs). The GW signals can be identified from the spatial correlations encoded in the times-of-arrival of widely spaced pulsar-pairs. The Chinese Pulsar Timing Array (CPTA) is a collaboration aiming at the direct GW detection with observations carried out using Chinese radio telescopes. This short article serves as a `table of contents' for a forthcoming series of papers related to the CPTA Data Release 1 (CPTA DR1) which uses observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Here, after summarizing the time span and accuracy of CPTA DR1, we report the key results of our statistical inference finding a correlated signal with amplitude $\log A_{\rm c}= -14.4 \,^{+1.0}_{-2.8}$ for spectral index in the range of $\alpha\in [-1.8, 1.5]$ assuming a GW background (GWB) induced quadrupolar correlation. The search for the Hellings-Downs (HD) correlation curve is also presented, where some evidence for the HD correlation has been found that a 4.6-$\sigma$ statistical significance is achieved using the discrete frequency method around the frequency of 14 nHz. We expect that the future International Pulsar Timing Array data analysis and the next CPTA data release will be more sensitive to the nHz GWB, which could verify the current results.
9.The NANOGrav 15-year Data Set: Observations and Timing of 68 Millisecond Pulsars
Authors:Gabriella Agazie for the NANOGrav Collaboration, Md Faisal Alam for the NANOGrav Collaboration, Akash Anumarlapudi for the NANOGrav Collaboration, Anne M. Archibald for the NANOGrav Collaboration, Zaven Arzoumanian for the NANOGrav Collaboration, Paul T. Baker for the NANOGrav Collaboration, Laura Blecha for the NANOGrav Collaboration, Victoria Bonidie for the NANOGrav Collaboration, Adam Brazier for the NANOGrav Collaboration, Paul R. Brook for the NANOGrav Collaboration, Sarah Burke-Spolaor for the NANOGrav Collaboration, Bence Bécsy for the NANOGrav Collaboration, Christopher Chapman for the NANOGrav Collaboration, Maria Charisi for the NANOGrav Collaboration, Shami Chatterjee for the NANOGrav Collaboration, Tyler Cohen for the NANOGrav Collaboration, James M. Cordes for the NANOGrav Collaboration, Neil J. Cornish for the NANOGrav Collaboration, Fronefield Crawford for the NANOGrav Collaboration, H. Thankful Cromartie for the NANOGrav Collaboration, Kathryn Crowter for the NANOGrav Collaboration, Megan E. DeCesar for the NANOGrav Collaboration, Paul B. Demorest for the NANOGrav Collaboration, Timothy Dolch for the NANOGrav Collaboration, Brendan Drachler for the NANOGrav Collaboration, Elizabeth C. Ferrara for the NANOGrav Collaboration, William Fiore for the NANOGrav Collaboration, Emmanuel Fonseca for the NANOGrav Collaboration, Gabriel E. Freedman for the NANOGrav Collaboration, Nate Garver-Daniels for the NANOGrav Collaboration, Peter A. Gentile for the NANOGrav Collaboration, Joseph Glaser for the NANOGrav Collaboration, Deborah C. Good for the NANOGrav Collaboration, Kayhan Gültekin for the NANOGrav Collaboration, Jeffrey S. Hazboun for the NANOGrav Collaboration, Ross J. Jennings for the NANOGrav Collaboration, Cody Jessup for the NANOGrav Collaboration, Aaron D. Johnson for the NANOGrav Collaboration, Megan L. Jones for the NANOGrav Collaboration, Andrew R. Kaiser for the NANOGrav Collaboration, David L. Kaplan for the NANOGrav Collaboration, Luke Zoltan Kelley for the NANOGrav Collaboration, Matthew Kerr for the NANOGrav Collaboration, Joey S. Key for the NANOGrav Collaboration, Anastasia Kuske for the NANOGrav Collaboration, Nima Laal for the NANOGrav Collaboration, Michael T. Lam for the NANOGrav Collaboration, William G. Lamb for the NANOGrav Collaboration, T. Joseph W. Lazio for the NANOGrav Collaboration, Natalia Lewandowska for the NANOGrav Collaboration, Ye Lin for the NANOGrav Collaboration, Tingting Liu for the NANOGrav Collaboration, Duncan R. Lorimer for the NANOGrav Collaboration, Jing Luo for the NANOGrav Collaboration, Ryan S. Lynch for the NANOGrav Collaboration, Chung-Pei Ma for the NANOGrav Collaboration, Dustin R. Madison for the NANOGrav Collaboration, Kaleb Maraccini for the NANOGrav Collaboration, Alexander McEwen for the NANOGrav Collaboration, James W. McKee for the NANOGrav Collaboration, Maura A. McLaughlin for the NANOGrav Collaboration, Natasha McMann for the NANOGrav Collaboration, Bradley W. Meyers for the NANOGrav Collaboration, Chiara M. F. Mingarelli for the NANOGrav Collaboration, Andrea Mitridate for the NANOGrav Collaboration, Cherry Ng for the NANOGrav Collaboration, David J. Nice for the NANOGrav Collaboration, Stella Koch Ocker for the NANOGrav Collaboration, Ken D. Olum for the NANOGrav Collaboration, Elisa Panciu for the NANOGrav Collaboration, Timothy T. Pennucci for the NANOGrav Collaboration, Benetge B. P. Perera for the NANOGrav Collaboration, Nihan S. Pol for the NANOGrav Collaboration, Henri A. Radovan for the NANOGrav Collaboration, Scott M. Ransom for the NANOGrav Collaboration, Paul S. Ray for the NANOGrav Collaboration, Joseph D. Romano for the NANOGrav Collaboration, Laura Salo for the NANOGrav Collaboration, Shashwat C. Sardesai for the NANOGrav Collaboration, Carl Schmiedekamp for the NANOGrav Collaboration, Ann Schmiedekamp for the NANOGrav Collaboration, Kai Schmitz for the NANOGrav Collaboration, Brent J. Shapiro-Albert for the NANOGrav Collaboration, Xavier Siemens for the NANOGrav Collaboration, Joseph Simon for the NANOGrav Collaboration, Magdalena S. Siwek for the NANOGrav Collaboration, Ingrid H. Stairs for the NANOGrav Collaboration, Daniel R. Stinebring for the NANOGrav Collaboration, Kevin Stovall for the NANOGrav Collaboration, Abhimanyu Susobhanan for the NANOGrav Collaboration, Joseph K. Swiggum for the NANOGrav Collaboration, Stephen R. Taylor for the NANOGrav Collaboration, Jacob E. Turner for the NANOGrav Collaboration, Caner Unal for the NANOGrav Collaboration, Michele Vallisneri for the NANOGrav Collaboration, Sarah J. Vigeland for the NANOGrav Collaboration, Haley M. Wahl for the NANOGrav Collaboration, Qiaohong Wang for the NANOGrav Collaboration, Caitlin A. Witt for the NANOGrav Collaboration, Olivia Young for the NANOGrav Collaboration
Abstract: We present observations and timing analyses of 68 millisecond pulsars (MSPs) comprising the 15-year data set of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). NANOGrav is a pulsar timing array (PTA) experiment that is sensitive to low-frequency gravitational waves. This is NANOGrav's fifth public data release, including both "narrowband" and "wideband" time-of-arrival (TOA) measurements and corresponding pulsar timing models. We have added 21 MSPs and extended our timing baselines by three years, now spanning nearly 16 years for some of our sources. The data were collected using the Arecibo Observatory, the Green Bank Telescope, and the Very Large Array between frequencies of 327 MHz and 3 GHz, with most sources observed approximately monthly. A number of notable methodological and procedural changes were made compared to our previous data sets. These improve the overall quality of the TOA data set and are part of the transition to new pulsar timing and PTA analysis software packages. For the first time, our data products are accompanied by a full suite of software to reproduce data reduction, analysis, and results. Our timing models include a variety of newly detected astrometric and binary pulsar parameters, including several significant improvements to pulsar mass constraints. We find that the time series of 23 pulsars contain detectable levels of red noise, 10 of which are new measurements. In this data set, we find evidence for a stochastic gravitational-wave background.
10.The NANOGrav 15-Year Data Set: Detector Characterization and Noise Budget
Authors:Gabriella Agazie for the Nanograv Collaboration, Akash Anumarlapudi for the Nanograv Collaboration, Anne M. Archibald for the Nanograv Collaboration, Zaven Arzoumanian for the Nanograv Collaboration, Paul T. Baker for the Nanograv Collaboration, Bence Bécsy for the Nanograv Collaboration, Laura Blecha for the Nanograv Collaboration, Adam Brazier for the Nanograv Collaboration, Paul R. Brook for the Nanograv Collaboration, Sarah Burke-Spolaor for the Nanograv Collaboration, Maria Charisi for the Nanograv Collaboration, Shami Chatterjee for the Nanograv Collaboration, Tyler Cohen for the Nanograv Collaboration, James M. Cordes for the Nanograv Collaboration, Neil J. Cornish for the Nanograv Collaboration, Fronefield Crawford for the Nanograv Collaboration, H. Thankful Cromartie for the Nanograv Collaboration, Kathryn Crowter for the Nanograv Collaboration, Megan E. Decesar for the Nanograv Collaboration, Paul B. Demorest for the Nanograv Collaboration, Timothy Dolch for the Nanograv Collaboration, Brendan Drachler for the Nanograv Collaboration, Elizabeth C. Ferrara for the Nanograv Collaboration, William Fiore for the Nanograv Collaboration, Emmanuel Fonseca for the Nanograv Collaboration, Gabriel E. Freedman for the Nanograv Collaboration, Nate Garver-Daniels for the Nanograv Collaboration, Peter A. Gentile for the Nanograv Collaboration, Joseph Glaser for the Nanograv Collaboration, Deborah C. Good for the Nanograv Collaboration, Lydia Guertin for the Nanograv Collaboration, Kayhan Gültekin for the Nanograv Collaboration, Jeffrey S. Hazboun for the Nanograv Collaboration, Ross J. Jennings for the Nanograv Collaboration, Aaron D. Johnson for the Nanograv Collaboration, Megan L. Jones for the Nanograv Collaboration, Andrew R. Kaiser for the Nanograv Collaboration, David L. Kaplan for the Nanograv Collaboration, Luke Zoltan Kelley for the Nanograv Collaboration, Matthew Kerr for the Nanograv Collaboration, Joey S. Key for the Nanograv Collaboration, Nima Laal for the Nanograv Collaboration, Michael T. Lam for the Nanograv Collaboration, William G. Lamb for the Nanograv Collaboration, T. Joseph W. Lazio for the Nanograv Collaboration, Natalia Lewandowska for the Nanograv Collaboration, Tingting Liu for the Nanograv Collaboration, Duncan R. Lorimer for the Nanograv Collaboration, Jing Luo for the Nanograv Collaboration, Ryan S. Lynch for the Nanograv Collaboration, Chung-Pei Ma for the Nanograv Collaboration, Dustin R. Madison for the Nanograv Collaboration, Alexander Mcewen for the Nanograv Collaboration, James W. Mckee for the Nanograv Collaboration, Maura A. Mclaughlin for the Nanograv Collaboration, Natasha Mcmann for the Nanograv Collaboration, Bradley W. Meyers for the Nanograv Collaboration, Chiara M. F. Mingarelli for the Nanograv Collaboration, Andrea Mitridate for the Nanograv Collaboration, Cherry Ng for the Nanograv Collaboration, David J. Nice for the Nanograv Collaboration, Stella Koch Ocker for the Nanograv Collaboration, Ken D. Olum for the Nanograv Collaboration, Timothy T. Pennucci for the Nanograv Collaboration, Benetge B. P. Perera for the Nanograv Collaboration, Nihan S. Pol for the Nanograv Collaboration, Henri A. Radovan for the Nanograv Collaboration, Scott M. Ransom for the Nanograv Collaboration, Paul S. Ray for the Nanograv Collaboration, Joseph D. Romano for the Nanograv Collaboration, Shashwat C. Sardesai for the Nanograv Collaboration, Ann Schmiedekamp for the Nanograv Collaboration, Carl Schmiedekamp for the Nanograv Collaboration, Kai Schmitz for the Nanograv Collaboration, Brent J. Shapiro-Albert for the Nanograv Collaboration, Xavier Siemens for the Nanograv Collaboration, Joseph Simon for the Nanograv Collaboration, Magdalena S. Siwek for the Nanograv Collaboration, Ingrid H. Stairs for the Nanograv Collaboration, Daniel R. Stinebring for the Nanograv Collaboration, Kevin Stovall for the Nanograv Collaboration, Abhimanyu Susobhanan for the Nanograv Collaboration, Joseph K. Swiggum for the Nanograv Collaboration, Stephen R. Taylor for the Nanograv Collaboration, Jacob E. Turner for the Nanograv Collaboration, Caner Unal for the Nanograv Collaboration, Michele Vallisneri for the Nanograv Collaboration, Sarah J. Vigeland for the Nanograv Collaboration, Haley M. Wahl for the Nanograv Collaboration, Caitlin A. Witt for the Nanograv Collaboration, Olivia Young for the Nanograv Collaboration
Abstract: Pulsar timing arrays (PTAs) are galactic-scale gravitational wave detectors. Each individual arm, composed of a millisecond pulsar, a radio telescope, and a kiloparsecs-long path, differs in its properties but, in aggregate, can be used to extract low-frequency gravitational wave (GW) signals. We present a noise and sensitivity analysis to accompany the NANOGrav 15-year data release and associated papers, along with an in-depth introduction to PTA noise models. As a first step in our analysis, we characterize each individual pulsar data set with three types of white noise parameters and two red noise parameters. These parameters, along with the timing model and, particularly, a piecewise-constant model for the time-variable dispersion measure, determine the sensitivity curve over the low-frequency GW band we are searching. We tabulate information for all of the pulsars in this data release and present some representative sensitivity curves. We then combine the individual pulsar sensitivities using a signal-to-noise-ratio statistic to calculate the global sensitivity of the PTA to a stochastic background of GWs, obtaining a minimum noise characteristic strain of $7\times 10^{-15}$ at 5 nHz. A power law-integrated analysis shows rough agreement with the amplitudes recovered in NANOGrav's 15-year GW background analysis. While our phenomenological noise model does not model all known physical effects explicitly, it provides an accurate characterization of the noise in the data while preserving sensitivity to multiple classes of GW signals.
11.The NANOGrav 15-year Data Set: Search for Signals from New Physics
Authors:Adeela Afzal for the NANOGrav Collaboration, Gabriella Agazie for the NANOGrav Collaboration, Akash Anumarlapudi for the NANOGrav Collaboration, Anne M. Archibald for the NANOGrav Collaboration, Zaven Arzoumanian for the NANOGrav Collaboration, Paul T. Baker for the NANOGrav Collaboration, Bence Bécsy for the NANOGrav Collaboration, Jose Juan Blanco-Pillado for the NANOGrav Collaboration, Laura Blecha for the NANOGrav Collaboration, Kimberly K. Boddy for the NANOGrav Collaboration, Adam Brazier for the NANOGrav Collaboration, Paul R. Brook for the NANOGrav Collaboration, Sarah Burke-Spolaor for the NANOGrav Collaboration, Rand Burnette for the NANOGrav Collaboration, Robin Case for the NANOGrav Collaboration, Maria Charisi for the NANOGrav Collaboration, Shami Chatterjee for the NANOGrav Collaboration, Katerina Chatziioannou for the NANOGrav Collaboration, Belinda D. Cheeseboro for the NANOGrav Collaboration, Siyuan Chen for the NANOGrav Collaboration, Tyler Cohen for the NANOGrav Collaboration, James M. Cordes for the NANOGrav Collaboration, Neil J. Cornish for the NANOGrav Collaboration, Fronefield Crawford for the NANOGrav Collaboration, H. Thankful Cromartie for the NANOGrav Collaboration, Kathryn Crowter for the NANOGrav Collaboration, Curt J. Cutler for the NANOGrav Collaboration, Megan E. DeCesar for the NANOGrav Collaboration, Dallas DeGan for the NANOGrav Collaboration, Paul B. Demorest for the NANOGrav Collaboration, Heling Deng for the NANOGrav Collaboration, Timothy Dolch for the NANOGrav Collaboration, Brendan Drachler for the NANOGrav Collaboration, Richard von Eckardstein for the NANOGrav Collaboration, Elizabeth C. Ferrara for the NANOGrav Collaboration, William Fiore for the NANOGrav Collaboration, Emmanuel Fonseca for the NANOGrav Collaboration, Gabriel E. Freedman for the NANOGrav Collaboration, Nate Garver-Daniels for the NANOGrav Collaboration, Peter A. Gentile for the NANOGrav Collaboration, Kyle A. Gersbach for the NANOGrav Collaboration, Joseph Glaser for the NANOGrav Collaboration, Deborah C. Good for the NANOGrav Collaboration, Lydia Guertin for the NANOGrav Collaboration, Kayhan Gültekin for the NANOGrav Collaboration, Jeffrey S. Hazboun for the NANOGrav Collaboration, Sophie Hourihane for the NANOGrav Collaboration, Kristina Islo for the NANOGrav Collaboration, Ross J. Jennings for the NANOGrav Collaboration, Aaron D. Johnson for the NANOGrav Collaboration, Megan L. Jones for the NANOGrav Collaboration, Andrew R. Kaiser for the NANOGrav Collaboration, David L. Kaplan for the NANOGrav Collaboration, Luke Zoltan Kelley for the NANOGrav Collaboration, Matthew Kerr for the NANOGrav Collaboration, Joey S. Key for the NANOGrav Collaboration, Nima Laal for the NANOGrav Collaboration, Michael T. Lam for the NANOGrav Collaboration, William G. Lamb for the NANOGrav Collaboration, T. Joseph W. Lazio for the NANOGrav Collaboration, Vincent S. H. Lee for the NANOGrav Collaboration, Natalia Lewandowska for the NANOGrav Collaboration, Rafael R. Lino dos Santos for the NANOGrav Collaboration, Tyson B. Littenberg for the NANOGrav Collaboration, Tingting Liu for the NANOGrav Collaboration, Duncan R. Lorimer for the NANOGrav Collaboration, Jing Luo for the NANOGrav Collaboration, Ryan S. Lynch for the NANOGrav Collaboration, Chung-Pei Ma for the NANOGrav Collaboration, Dustin R. Madison for the NANOGrav Collaboration, Alexander McEwen for the NANOGrav Collaboration, James W. McKee for the NANOGrav Collaboration, Maura A. McLaughlin for the NANOGrav Collaboration, Natasha McMann for the NANOGrav Collaboration, Bradley W. Meyers for the NANOGrav Collaboration, Patrick M. Meyers for the NANOGrav Collaboration, Chiara M. F. Mingarelli for the NANOGrav Collaboration, Andrea Mitridate for the NANOGrav Collaboration, Jonathan Nay for the NANOGrav Collaboration, Priyamvada Natarajan for the NANOGrav Collaboration, Cherry Ng for the NANOGrav Collaboration, David J. Nice for the NANOGrav Collaboration, Stella Koch Ocker for the NANOGrav Collaboration, Ken D. Olum for the NANOGrav Collaboration, Timothy T. Pennucci for the NANOGrav Collaboration, Benetge B. P. Perera for the NANOGrav Collaboration, Polina Petrov for the NANOGrav Collaboration, Nihan S. Pol for the NANOGrav Collaboration, Henri A. Radovan for the NANOGrav Collaboration, Scott M. Ransom for the NANOGrav Collaboration, Paul S. Ray for the NANOGrav Collaboration, Joseph D. Romano for the NANOGrav Collaboration, Shashwat C. Sardesai for the NANOGrav Collaboration, Ann Schmiedekamp for the NANOGrav Collaboration, Carl Schmiedekamp for the NANOGrav Collaboration, Kai Schmitz for the NANOGrav Collaboration, Tobias Schröder for the NANOGrav Collaboration, Levi Schult for the NANOGrav Collaboration, Brent J. Shapiro-Albert for the NANOGrav Collaboration, Xavier Siemens for the NANOGrav Collaboration, Joseph Simon for the NANOGrav Collaboration, Magdalena S. Siwek for the NANOGrav Collaboration, Ingrid H. Stairs for the NANOGrav Collaboration, Daniel R. Stinebring for the NANOGrav Collaboration, Kevin Stovall for the NANOGrav Collaboration, Peter Stratmann for the NANOGrav Collaboration, Jerry P. Sun for the NANOGrav Collaboration, Abhimanyu Susobhanan for the NANOGrav Collaboration, Joseph K. Swiggum for the NANOGrav Collaboration, Jacob Taylor for the NANOGrav Collaboration, Stephen R. Taylor for the NANOGrav Collaboration, Tanner Trickle for the NANOGrav Collaboration, Jacob E. Turner for the NANOGrav Collaboration, Caner Unal for the NANOGrav Collaboration, Michele Vallisneri for the NANOGrav Collaboration, Sonali Verma for the NANOGrav Collaboration, Sarah J. Vigeland for the NANOGrav Collaboration, Haley M. Wahl for the NANOGrav Collaboration, Qiaohong Wang for the NANOGrav Collaboration, Caitlin A. Witt for the NANOGrav Collaboration, David Wright for the NANOGrav Collaboration, Olivia Young for the NANOGrav Collaboration, Kathryn M. Zurek for the NANOGrav Collaboration
Abstract: The 15-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) shows positive evidence for the presence of a low-frequency gravitational-wave (GW) background. In this paper, we investigate potential cosmological interpretations of this signal, specifically cosmic inflation, scalar-induced GWs, first-order phase transitions, cosmic strings, and domain walls. We find that, with the exception of stable cosmic strings of field theory origin, all these models can reproduce the observed signal. When compared to the standard interpretation in terms of inspiraling supermassive black hole binaries (SMBHBs), many cosmological models seem to provide a better fit resulting in Bayes factors in the range from 10 to 100. However, these results strongly depend on modeling assumptions about the cosmic SMBHB population and, at this stage, should not be regarded as evidence for new physics. Furthermore, we identify excluded parameter regions where the predicted GW signal from cosmological sources significantly exceeds the NANOGrav signal. These parameter constraints are independent of the origin of the NANOGrav signal and illustrate how pulsar timing data provide a new way to constrain the parameter space of these models. Finally, we search for deterministic signals produced by models of ultralight dark matter (ULDM) and dark matter substructures in the Milky Way. We find no evidence for either of these signals and thus report updated constraints on these models. In the case of ULDM, these constraints outperform torsion balance and atomic clock constraints for ULDM coupled to electrons, muons, or gluons.
12.The NANOGrav 15-year Data Set: Constraints on Supermassive Black Hole Binaries from the Gravitational Wave Background
Authors:Gabriella Agazie The NANOGrav Collaboration, Akash Anumarlapudi The NANOGrav Collaboration, Anne M. Archibald The NANOGrav Collaboration, Paul T. Baker The NANOGrav Collaboration, Bence Bécsy The NANOGrav Collaboration, Laura Blecha The NANOGrav Collaboration, Alexander Bonilla The NANOGrav Collaboration, Adam Brazier The NANOGrav Collaboration, Paul R. Brook The NANOGrav Collaboration, Sarah Burke-Spolaor The NANOGrav Collaboration, Rand Burnette The NANOGrav Collaboration, Robin Case The NANOGrav Collaboration, J. Andrew Casey-Clyde The NANOGrav Collaboration, Maria Charisi The NANOGrav Collaboration, Shami Chatterjee The NANOGrav Collaboration, Katerina Chatziioannou The NANOGrav Collaboration, Belinda D. Cheeseboro The NANOGrav Collaboration, Siyuan Chen The NANOGrav Collaboration, Tyler Cohen The NANOGrav Collaboration, James M. Cordes The NANOGrav Collaboration, Neil J. Cornish The NANOGrav Collaboration, Fronefield Crawford The NANOGrav Collaboration, H. Thankful Cromartie The NANOGrav Collaboration, Kathryn Crowter The NANOGrav Collaboration, Curt J. Cutler The NANOGrav Collaboration, Daniel J. D'Orazio The NANOGrav Collaboration, Megan E. DeCesar The NANOGrav Collaboration, Dallas DeGan The NANOGrav Collaboration, Paul B. Demorest The NANOGrav Collaboration, Heling Deng The NANOGrav Collaboration, Timothy Dolch The NANOGrav Collaboration, Brendan Drachler The NANOGrav Collaboration, Elizabeth C. Ferrara The NANOGrav Collaboration, William Fiore The NANOGrav Collaboration, Emmanuel Fonseca The NANOGrav Collaboration, Gabriel E. Freedman The NANOGrav Collaboration, Emiko Gardiner The NANOGrav Collaboration, Nate Garver-Daniels The NANOGrav Collaboration, Peter A. Gentile The NANOGrav Collaboration, Kyle A. Gersbach The NANOGrav Collaboration, Joseph Glaser The NANOGrav Collaboration, Deborah C. Good The NANOGrav Collaboration, Kayhan Gültekin The NANOGrav Collaboration, Jeffrey S. Hazboun The NANOGrav Collaboration, Sophie Hourihane The NANOGrav Collaboration, Kristina Islo The NANOGrav Collaboration, Ross J. Jennings The NANOGrav Collaboration, Aaron Johnson The NANOGrav Collaboration, Megan L. Jones The NANOGrav Collaboration, Andrew R. Kaiser The NANOGrav Collaboration, David L. Kaplan The NANOGrav Collaboration, Luke Zoltan Kelley The NANOGrav Collaboration, Matthew Kerr The NANOGrav Collaboration, Joey S. Key The NANOGrav Collaboration, Nima Laal The NANOGrav Collaboration, Michael T. Lam The NANOGrav Collaboration, William G. Lamb The NANOGrav Collaboration, T. Joseph W. Lazio The NANOGrav Collaboration, Natalia Lewandowska The NANOGrav Collaboration, Tyson B. Littenberg The NANOGrav Collaboration, Tingting Liu The NANOGrav Collaboration, Jing Luo The NANOGrav Collaboration, Ryan S. Lynch The NANOGrav Collaboration, Chung-Pei Ma The NANOGrav Collaboration, Dustin R. Madison The NANOGrav Collaboration, Alexander McEwen The NANOGrav Collaboration, James W. McKee The NANOGrav Collaboration, Maura A. McLaughlin The NANOGrav Collaboration, Natasha McMann The NANOGrav Collaboration, Bradley W. Meyers The NANOGrav Collaboration, Patrick M. Meyers The NANOGrav Collaboration, Chiara M. F. Mingarelli The NANOGrav Collaboration, Andrea Mitridate The NANOGrav Collaboration, Priyamvada Natarajan The NANOGrav Collaboration, Cherry Ng The NANOGrav Collaboration, David J. Nice The NANOGrav Collaboration, Stella Koch Ocker The NANOGrav Collaboration, Ken D. Olum The NANOGrav Collaboration, Timothy T. Pennucci The NANOGrav Collaboration, Benetge B. P. Perera The NANOGrav Collaboration, Polina Petrov The NANOGrav Collaboration, Nihan S. Pol The NANOGrav Collaboration, Henri A. Radovan The NANOGrav Collaboration, Scott M. Ransom The NANOGrav Collaboration, Paul S. Ray The NANOGrav Collaboration, Joseph D. Romano The NANOGrav Collaboration, Jessie C. Runnoe The NANOGrav Collaboration, Shashwat C. Sardesai The NANOGrav Collaboration, Ann Schmiedekamp The NANOGrav Collaboration, Carl Schmiedekamp The NANOGrav Collaboration, Kai Schmitz The NANOGrav Collaboration, Levi Schult The NANOGrav Collaboration, Brent J. Shapiro-Albert The NANOGrav Collaboration, Xavier Siemens The NANOGrav Collaboration, Joseph Simon The NANOGrav Collaboration, Magdalena S. Siwek The NANOGrav Collaboration, Ingrid H. Stairs The NANOGrav Collaboration, Daniel R. Stinebring The NANOGrav Collaboration, Kevin Stovall The NANOGrav Collaboration, Jerry P. Sun The NANOGrav Collaboration, Abhimanyu Susobhanan The NANOGrav Collaboration, Joseph K. Swiggum The NANOGrav Collaboration, Jacob Taylor The NANOGrav Collaboration, Stephen R. Taylor The NANOGrav Collaboration, Jacob E. Turner The NANOGrav Collaboration, Caner Unal The NANOGrav Collaboration, Michele Vallisneri The NANOGrav Collaboration, Sarah J. Vigeland The NANOGrav Collaboration, Jeremy M. Wachter The NANOGrav Collaboration, Haley M. Wahl The NANOGrav Collaboration, Qiaohong Wang The NANOGrav Collaboration, Caitlin A. Witt The NANOGrav Collaboration, David Wright The NANOGrav Collaboration, Olivia Young The NANOGrav Collaboration
Abstract: The NANOGrav 15-year data set shows evidence for the presence of a low-frequency gravitational-wave background (GWB). While many physical processes can source such low-frequency gravitational waves, here we analyze the signal as coming from a population of supermassive black hole (SMBH) binaries distributed throughout the Universe. We show that astrophysically motivated models of SMBH binary populations are able to reproduce both the amplitude and shape of the observed low-frequency gravitational-wave spectrum. While multiple model variations are able to reproduce the GWB spectrum at our current measurement precision, our results highlight the importance of accurately modeling binary evolution for producing realistic GWB spectra. Additionally, while reasonable parameters are able to reproduce the 15-year observations, the implied GWB amplitude necessitates either a large number of parameters to be at the edges of expected values, or a small number of parameters to be notably different from standard expectations. While we are not yet able to definitively establish the origin of the inferred GWB signal, the consistency of the signal with astrophysical expectations offers a tantalizing prospect for confirming that SMBH binaries are able to form, reach sub-parsec separations, and eventually coalesce. As the significance grows over time, higher-order features of the GWB spectrum will definitively determine the nature of the GWB and allow for novel constraints on SMBH populations.
13.The NANOGrav 15-year Data Set: Search for Anisotropy in the Gravitational-Wave Background
Authors:Gabriella Agazie for the NANOGrav Collaboration, Akash Anumarlapudi for the NANOGrav Collaboration, Anne M. Archibald for the NANOGrav Collaboration, Zaven Arzoumanian for the NANOGrav Collaboration, Paul T. Baker for the NANOGrav Collaboration, Bence Bécsy for the NANOGrav Collaboration, Laura Blecha for the NANOGrav Collaboration, Adam Brazier for the NANOGrav Collaboration, Paul R. Brook for the NANOGrav Collaboration, Sarah Burke-Spolaor for the NANOGrav Collaboration, J. Andrew Casey-Clyde for the NANOGrav Collaboration, Maria Charisi for the NANOGrav Collaboration, Shami Chatterjee for the NANOGrav Collaboration, Tyler Cohen for the NANOGrav Collaboration, James M. Cordes for the NANOGrav Collaboration, Neil J. Cornish for the NANOGrav Collaboration, Fronefield Crawford for the NANOGrav Collaboration, H. Thankful Cromartie for the NANOGrav Collaboration, Kathryn Crowter for the NANOGrav Collaboration, Megan E. DeCesar for the NANOGrav Collaboration, Paul B. Demorest for the NANOGrav Collaboration, Timothy Dolch for the NANOGrav Collaboration, Brendan Drachler for the NANOGrav Collaboration, Elizabeth C. Ferrara for the NANOGrav Collaboration, William Fiore for the NANOGrav Collaboration, Emmanuel Fonseca for the NANOGrav Collaboration, Gabriel E. Freedman for the NANOGrav Collaboration, Emiko Gardiner for the NANOGrav Collaboration, Nate Garver-Daniels for the NANOGrav Collaboration, Peter A. Gentile for the NANOGrav Collaboration, Joseph Glaser for the NANOGrav Collaboration, Deborah C. Good for the NANOGrav Collaboration, Kayhan Gültekin for the NANOGrav Collaboration, Jeffrey S. Hazboun for the NANOGrav Collaboration, Ross J. Jennings for the NANOGrav Collaboration, Aaron D. Johnson for the NANOGrav Collaboration, Megan L. Jones for the NANOGrav Collaboration, Andrew R. Kaiser for the NANOGrav Collaboration, David L. Kaplan for the NANOGrav Collaboration, Luke Zoltan Kelley for the NANOGrav Collaboration, Matthew Kerr for the NANOGrav Collaboration, Joey S. Key for the NANOGrav Collaboration, Nima Laal for the NANOGrav Collaboration, Michael T. Lam for the NANOGrav Collaboration, William G. Lamb for the NANOGrav Collaboration, T. Joseph W. Lazio for the NANOGrav Collaboration, Natalia Lewandowska for the NANOGrav Collaboration, Tingting Liu for the NANOGrav Collaboration, Duncan R. Lorimer for the NANOGrav Collaboration, Jing Luo for the NANOGrav Collaboration, Ryan S. Lynch for the NANOGrav Collaboration, Chung-Pei Ma for the NANOGrav Collaboration, Dustin R. Madison for the NANOGrav Collaboration, Alexander McEwen for the NANOGrav Collaboration, James W. McKee for the NANOGrav Collaboration, Maura A. McLaughlin for the NANOGrav Collaboration, Natasha McMann for the NANOGrav Collaboration, Bradley W. Meyers for the NANOGrav Collaboration, Chiara M. F. Mingarelli for the NANOGrav Collaboration, Andrea Mitridate for the NANOGrav Collaboration, Cherry Ng for the NANOGrav Collaboration, David J. Nice for the NANOGrav Collaboration, Stella Koch Ocker for the NANOGrav Collaboration, Ken D. Olum for the NANOGrav Collaboration, Timothy T. Pennucci for the NANOGrav Collaboration, Benetge B. P. Perera for the NANOGrav Collaboration, Nihan S. Pol for the NANOGrav Collaboration, Henri A. Radovan for the NANOGrav Collaboration, Scott M. Ransom for the NANOGrav Collaboration, Paul S. Ray for the NANOGrav Collaboration, Joseph D. Romano for the NANOGrav Collaboration, Shashwat C. Sardesai for the NANOGrav Collaboration, Ann Schmiedekamp for the NANOGrav Collaboration, Carl Schmiedekamp for the NANOGrav Collaboration, Kai Schmitz for the NANOGrav Collaboration, Levi Schult for the NANOGrav Collaboration, Brent J. Shapiro-Albert for the NANOGrav Collaboration, Xavier Siemens for the NANOGrav Collaboration, Joseph Simon for the NANOGrav Collaboration, Magdalena S. Siwek for the NANOGrav Collaboration, Ingrid H. Stairs for the NANOGrav Collaboration, Daniel R. Stinebring for the NANOGrav Collaboration, Kevin Stovall for the NANOGrav Collaboration, Abhimanyu Susobhanan for the NANOGrav Collaboration, Joseph K. Swiggum for the NANOGrav Collaboration, Stephen R. Taylor for the NANOGrav Collaboration, Jacob E. Turner for the NANOGrav Collaboration, Caner Unal for the NANOGrav Collaboration, Michele Vallisneri for the NANOGrav Collaboration, Sarah J. Vigeland for the NANOGrav Collaboration, Haley M. Wahl for the NANOGrav Collaboration, Caitlin A. Witt for the NANOGrav Collaboration, Olivia Young for the NANOGrav Collaboration
Abstract: The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has reported evidence for the presence of an isotropic nanohertz gravitational wave background (GWB) in its 15 yr dataset. However, if the GWB is produced by a population of inspiraling supermassive black hole binary (SMBHB) systems, then the background is predicted to be anisotropic, depending on the distribution of these systems in the local Universe and the statistical properties of the SMBHB population. In this work, we search for anisotropy in the GWB using multiple methods and bases to describe the distribution of the GWB power on the sky. We do not find significant evidence of anisotropy, and place a Bayesian $95\%$ upper limit on the level of broadband anisotropy such that $(C_{l>0} / C_{l=0}) < 20\%$. We also derive conservative estimates on the anisotropy expected from a random distribution of SMBHB systems using astrophysical simulations conditioned on the isotropic GWB inferred in the 15-yr dataset, and show that this dataset has sufficient sensitivity to probe a large fraction of the predicted level of anisotropy. We end by highlighting the opportunities and challenges in searching for anisotropy in pulsar timing array data.
14.The NANOGrav 15-year Data Set: Bayesian Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries
Authors:Gabriella Agazie for the NANOGrav Collaboration, Akash Anumarlapudi for the NANOGrav Collaboration, Anne M. Archibald for the NANOGrav Collaboration, Zaven Arzoumanian for the NANOGrav Collaboration, Paul T. Baker for the NANOGrav Collaboration, Bence Bécsy for the NANOGrav Collaboration, Laura Blecha for the NANOGrav Collaboration, Adam Brazier for the NANOGrav Collaboration, Paul R. Brook for the NANOGrav Collaboration, Sarah Burke-Spolaor for the NANOGrav Collaboration, Robin Case for the NANOGrav Collaboration, J. Andrew Casey-Clyde for the NANOGrav Collaboration, Maria Charisi for the NANOGrav Collaboration, Shami Chatterjee for the NANOGrav Collaboration, Tyler Cohen for the NANOGrav Collaboration, James M. Cordes for the NANOGrav Collaboration, Neil Cornish for the NANOGrav Collaboration, Fronefield Crawford for the NANOGrav Collaboration, H. Thankful Cromartie for the NANOGrav Collaboration, Kathryn Crowter for the NANOGrav Collaboration, Megan DeCesar for the NANOGrav Collaboration, Paul B. Demorest for the NANOGrav Collaboration, Matthew C. Digman for the NANOGrav Collaboration, Timothy Dolch for the NANOGrav Collaboration, Brendan Drachler for the NANOGrav Collaboration, Elizabeth C. Ferrara for the NANOGrav Collaboration, William Fiore for the NANOGrav Collaboration, Emmanuel Fonseca for the NANOGrav Collaboration, Gabriel Freedman for the NANOGrav Collaboration, Nathaniel Garver-Daniels for the NANOGrav Collaboration, Peter Gentile for the NANOGrav Collaboration, Joseph Glaser for the NANOGrav Collaboration, Deborah Good for the NANOGrav Collaboration, Kayhan Gültekin for the NANOGrav Collaboration, Jeffrey Hazboun for the NANOGrav Collaboration, Sophie Hourihane for the NANOGrav Collaboration, Ross Jennings for the NANOGrav Collaboration, Aaron D. Johnson for the NANOGrav Collaboration, Megan Jones for the NANOGrav Collaboration, Andrew R. Kaiser for the NANOGrav Collaboration, David Kaplan for the NANOGrav Collaboration, Luke Zoltan Kelley for the NANOGrav Collaboration, Matthew Kerr for the NANOGrav Collaboration, Joey Key for the NANOGrav Collaboration, Nima Laal for the NANOGrav Collaboration, Michael Lam for the NANOGrav Collaboration, William G. Lamb for the NANOGrav Collaboration, T. Joseph W. Lazio for the NANOGrav Collaboration, Natalia Lewandowska for the NANOGrav Collaboration, Tingting Liu for the NANOGrav Collaboration, Duncan R. Lorimer for the NANOGrav Collaboration, Jing Santiago Luo for the NANOGrav Collaboration, Ryan S. Lynch for the NANOGrav Collaboration, Chung-Pei Ma for the NANOGrav Collaboration, Dustin Madison for the NANOGrav Collaboration, Alexander McEwen for the NANOGrav Collaboration, James W. McKee for the NANOGrav Collaboration, Maura McLaughlin for the NANOGrav Collaboration, Natasha McMann for the NANOGrav Collaboration, Bradley W. Meyers for the NANOGrav Collaboration, Patrick M. Meyers for the NANOGrav Collaboration, Chiara M. F. Mingarelli for the NANOGrav Collaboration, andrea mitridate for the NANOGrav Collaboration, priya natarajan for the NANOGrav Collaboration, Cherry Ng for the NANOGrav Collaboration, David Nice for the NANOGrav Collaboration, Stella Koch Ocker for the NANOGrav Collaboration, Ken Olum for the NANOGrav Collaboration, Timothy T. Pennucci for the NANOGrav Collaboration, Benetge Perera for the NANOGrav Collaboration, Polina Petrov for the NANOGrav Collaboration, Nihan Pol for the NANOGrav Collaboration, Henri A. Radovan for the NANOGrav Collaboration, Scott Ransom for the NANOGrav Collaboration, Paul S. Ray for the NANOGrav Collaboration, Joseph Romano for the NANOGrav Collaboration, Shashwat C. Sardesai for the NANOGrav Collaboration, Ann Schmiedekamp for the NANOGrav Collaboration, Carl Schmiedekamp for the NANOGrav Collaboration, Kai Schmitz for the NANOGrav Collaboration, Brent J. Shapiro-Albert for the NANOGrav Collaboration, Xavier Siemens for the NANOGrav Collaboration, Joseph Simon for the NANOGrav Collaboration, Magdalena Siwek for the NANOGrav Collaboration, Ingrid Stairs for the NANOGrav Collaboration, Dan Stinebring for the NANOGrav Collaboration, Kevin Stovall for the NANOGrav Collaboration, Abhimanyu Susobhanan for the NANOGrav Collaboration, Joseph Swiggum for the NANOGrav Collaboration, Jacob Taylor for the NANOGrav Collaboration, Stephen Taylor for the NANOGrav Collaboration, Jacob E. Turner for the NANOGrav Collaboration, Caner Unal for the NANOGrav Collaboration, Michele Vallisneri for the NANOGrav Collaboration, Rutger van Haasteren for the NANOGrav Collaboration, Sarah J. Vigeland for the NANOGrav Collaboration, Haley M. Wahl for the NANOGrav Collaboration, Caitlin Witt for the NANOGrav Collaboration, Olivia Young for the NANOGrav Collaboration
Abstract: Evidence for a low-frequency stochastic gravitational wave background has recently been reported based on analyses of pulsar timing array data. The most likely source of such a background is a population of supermassive black hole binaries, the loudest of which may be individually detected in these datasets. Here we present the search for individual supermassive black hole binaries in the NANOGrav 15-year dataset. We introduce several new techniques, which enhance the efficiency and modeling accuracy of the analysis. The search uncovered weak evidence for two candidate signals, one with a gravitational-wave frequency of $\sim$4 nHz, and another at $\sim$170 nHz. The significance of the low-frequency candidate was greatly diminished when Hellings-Downs correlations were included in the background model. The high-frequency candidate was discounted due to the lack of a plausible host galaxy, the unlikely astrophysical prior odds of finding such a source, and since most of its support comes from a single pulsar with a commensurate binary period. Finding no compelling evidence for signals from individual binary systems, we place upper limits on the strain amplitude of gravitational waves emitted by such systems.
15.The NANOGrav 15-year Gravitational-Wave Background Analysis Pipeline
Authors:Aaron D. Johnson for the NANOGrav Collaboration, Patrick M. Meyers for the NANOGrav Collaboration, Paul T. Baker for the NANOGrav Collaboration, Neil J. Cornish for the NANOGrav Collaboration, Jeffrey S. Hazboun for the NANOGrav Collaboration, Tyson B. Littenberg for the NANOGrav Collaboration, Joseph D. Romano for the NANOGrav Collaboration, Stephen R. Taylor for the NANOGrav Collaboration, Michele Vallisneri for the NANOGrav Collaboration, Sarah J. Vigeland for the NANOGrav Collaboration, Ken D. Olum for the NANOGrav Collaboration, Xavier Siemens for the NANOGrav Collaboration, Justin A. Ellis for the NANOGrav Collaboration, Rutger van Haasteren for the NANOGrav Collaboration, Sophie Hourihane for the NANOGrav Collaboration, Gabriella Agazie for the NANOGrav Collaboration, Akash Anumarlapudi for the NANOGrav Collaboration, Anne M. Archibald for the NANOGrav Collaboration, Zaven Arzoumanian for the NANOGrav Collaboration, Laura Blecha for the NANOGrav Collaboration, Adam Brazier for the NANOGrav Collaboration, Paul R. Brook for the NANOGrav Collaboration, Sarah Burke-Spolaor for the NANOGrav Collaboration, Bence Bécsy for the NANOGrav Collaboration, J. Andrew Casey-Clyde for the NANOGrav Collaboration, Maria Charisi for the NANOGrav Collaboration, Shami Chatterjee for the NANOGrav Collaboration, Katerina Chatziioannou for the NANOGrav Collaboration, Tyler Cohen for the NANOGrav Collaboration, James M. Cordes for the NANOGrav Collaboration, Fronefield Crawford for the NANOGrav Collaboration, H. Thankful Cromartie for the NANOGrav Collaboration, Kathryn Crowter for the NANOGrav Collaboration, Megan E. DeCesar for the NANOGrav Collaboration, Paul B. Demorest for the NANOGrav Collaboration, Timothy Dolch for the NANOGrav Collaboration, Brendan Drachler for the NANOGrav Collaboration, Elizabeth C. Ferrara for the NANOGrav Collaboration, William Fiore for the NANOGrav Collaboration, Emmanuel Fonseca for the NANOGrav Collaboration, Gabriel E. Freedman for the NANOGrav Collaboration, Nate Garver-Daniels for the NANOGrav Collaboration, Peter A. Gentile for the NANOGrav Collaboration, Joseph Glaser for the NANOGrav Collaboration, Deborah C. Good for the NANOGrav Collaboration, Kayhan Gültekin for the NANOGrav Collaboration, Ross J. Jennings for the NANOGrav Collaboration, Megan L. Jones for the NANOGrav Collaboration, Andrew R. Kaiser for the NANOGrav Collaboration, David L. Kaplan for the NANOGrav Collaboration, Luke Zoltan Kelley for the NANOGrav Collaboration, Matthew Kerr for the NANOGrav Collaboration, Joey S. Key for the NANOGrav Collaboration, Nima Laal for the NANOGrav Collaboration, Michael T. Lam for the NANOGrav Collaboration, William G. Lamb for the NANOGrav Collaboration, T. Joseph W. Lazio for the NANOGrav Collaboration, Natalia Lewandowska for the NANOGrav Collaboration, Tingting Liu for the NANOGrav Collaboration, Duncan R. Lorimer for the NANOGrav Collaboration, Ryan S. Lynch for the NANOGrav Collaboration, Chung-Pei Ma for the NANOGrav Collaboration, Dustin R. Madison for the NANOGrav Collaboration, Alexander McEwen for the NANOGrav Collaboration, James W. McKee for the NANOGrav Collaboration, Maura A. McLaughlin for the NANOGrav Collaboration, Natasha McMann for the NANOGrav Collaboration, Bradley W. Meyers for the NANOGrav Collaboration, Chiara M. F. Mingarelli for the NANOGrav Collaboration, Andrea Mitridate for the NANOGrav Collaboration, Cherry Ng for the NANOGrav Collaboration, David J. Nice for the NANOGrav Collaboration, Stella Koch Ocker for the NANOGrav Collaboration, Timothy T. Pennucci for the NANOGrav Collaboration, Benetge B. P. Perera for the NANOGrav Collaboration, Nihan S. Pol for the NANOGrav Collaboration, Henri A. Radovan for the NANOGrav Collaboration, Scott M. Ransom for the NANOGrav Collaboration, Paul S. Ray for the NANOGrav Collaboration, Shashwat C. Sardesai for the NANOGrav Collaboration, Carl Schmiedekamp for the NANOGrav Collaboration, Ann Schmiedekamp for the NANOGrav Collaboration, Kai Schmitz for the NANOGrav Collaboration, Brent J. Shapiro-Albert for the NANOGrav Collaboration, Joseph Simon for the NANOGrav Collaboration, Magdalena S. Siwek for the NANOGrav Collaboration, Ingrid H. Stairs for the NANOGrav Collaboration, Daniel R. Stinebring for the NANOGrav Collaboration, Kevin Stovall for the NANOGrav Collaboration, Abhimanyu Susobhanan for the NANOGrav Collaboration, Joseph K. Swiggum for the NANOGrav Collaboration, Jacob E. Turner for the NANOGrav Collaboration, Caner Unal for the NANOGrav Collaboration, Haley M. Wahl for the NANOGrav Collaboration, Caitlin A. Witt for the NANOGrav Collaboration, Olivia Young for the NANOGrav Collaboration
Abstract: This paper presents rigorous tests of pulsar timing array methods and software, examining their consistency across a wide range of injected parameters and signal strength. We discuss updates to the 15-year isotropic gravitational-wave background analyses and their corresponding code representations. Descriptions of the internal structure of the flagship algorithms \texttt{Enterprise} and \texttt{PTMCMCSampler} are given to facilitate understanding of the PTA likelihood structure, how models are built, and what methods are currently used in sampling the high-dimensional PTA parameter space. We introduce a novel version of the PTA likelihood that uses a two-step marginalization procedure that performs much faster when the white noise parameters remain fixed. We perform stringent tests of consistency and correctness of the Bayesian and frequentist analysis software. For the Bayesian analysis, we test prior recovery, injection recovery, and Bayes factors. For the frequentist analysis, we test that the cross-correlation-based optimal statistic, when modified to account for a non-negligible gravitational-wave background, accurately recovers the amplitude of the background. We also summarize recent advances and tests performed on the optimal statistic in the literature from both GWB detection and parameter estimation perspectives. The tests presented here validate current and future analyses of PTA data.
16.The second data release from the European Pulsar Timing Array I. The dataset and timing analysis
Authors:J. Antoniadis, S. Babak, A. -S. Bak Nielsen, C. G. Bassa, A. Berthereau, M. Bonetti, E. Bortolas, P. R. Brook, M. Burgay, R. N. Caballero, A. Chalumeau, D. J. Champion, S. Chanlaridis, S. Chen, I. Cognard, G. Desvignes, M. Falxa, R. D. Ferdman, A. Franchini, J. R. Gair, B. Goncharov, E. Graikou, J. -M. Grießmeier, L. Guillemot, Y. J. Guo, H. Hu, F. Iraci, D. Izquierdo-Villalba, J. Jang, J. Jawor, G. H. Janssen, A. Jessner, R. Karuppusamy, E. F. Keane, M. J. Keith, M. Kramer, M. A. Krishnakumar, K. Lackeos, K. J. Lee, K. Liu, Y. Liu, A. G. Lyne, J. W. McKee, R. A. Main, M. B. Mickaliger, I. C. Nitu, A. Parthasarathy, B. B. P. Perera, D. Perrodin, A. Petiteau, N. K. Porayko, A. Possenti, H. Quelquejay Leclere A. Samajdar, S. A. Sanidas, A. Sesana, G. Shaifullah, L. Speri, R. Spiewak, B. W. Stappers, S. C. Susarla, G. Theureau, C. Tiburzi, E. van der Wateren, A. Vecchio, V. Venkatraman Krishnan, J. P. W. Verbiest, J. Wang, L. Wang, Z. Wu
Abstract: Pulsar timing arrays offer a probe of the low-frequency gravitational wave spectrum (1 - 100 nanohertz), which is intimately connected to a number of markers that can uniquely trace the formation and evolution of the Universe. We present the dataset and the results of the timing analysis from the second data release of the European Pulsar Timing Array (EPTA). The dataset contains high-precision pulsar timing data from 25 millisecond pulsars collected with the five largest radio telescopes in Europe, as well as the Large European Array for Pulsars. The dataset forms the foundation for the search for gravitational waves by the EPTA, presented in associated papers. We describe the dataset and present the results of the frequentist and Bayesian pulsar timing analysis for individual millisecond pulsars that have been observed over the last ~25 years. We discuss the improvements to the individual pulsar parameter estimates, as well as new measurements of the physical properties of these pulsars and their companions. This data release extends the dataset from EPTA Data Release 1 up to the beginning of 2021, with individual pulsar datasets with timespans ranging from 14 to 25 years. These lead to improved constraints on annual parallaxes, secular variation of the orbital period, and Shapiro delay for a number of sources. Based on these results, we derived astrophysical parameters that include distances, transverse velocities, binary pulsar masses, and annual orbital parallaxes.
17.The second data release from the European Pulsar Timing Array II. Customised pulsar noise models for spatially correlated gravitational waves
Authors:J. Antoniadis, P. Arumugam, S. Arumugam, S. Babak, M. Bagchi, A. S. Bak Nielsen, C. G. Bassa, A. Bathula, A. Berthereau, M. Bonetti, E. Bortolas, P. R. Brook, M. Burgay, R. N. Caballero, A. Chalumeau, D. J. Champion, S. Chanlaridis, S. Chen, I. Cognard, S. Dandapat, D. Deb, S. Desai, G. Desvignes, N. Dhanda-Batra, C. Dwivedi, M. Falxa, R. D. Ferdman, A. Franchini, J. R. Gair, B. Goncharov, A. Gopakumar, E. Graikou, J. -M. Grießmeier, L. Guillemot, Y. J. Guo, Y. Gupta, S. Hisano, H. Hu, F. Iraci, D. Izquierdo-Villalba, J. Jang, J. Jawor, G. H. Janssen, A. Jessner, B. C. Joshi, F. Kareem, R. Karuppusamy, E. F. Keane, M. J. Keith, D. Kharbanda, T. Kikunaga, N. Kolhe, M. Kramer, M. A. Krishnakumar, K. Lackeos, K. J. Lee, K. Liu, Y. Liu, A. G. Lyne, J. W. McKee, Y. Maan, R. A. Main, M. B. Mickaliger, I. C. Niţu, K. Nobleson, A. K. Paladi, A. Parthasarathy, B. B. P. Perera, D. Perrodin, A. Petiteau, N. K. Porayko, A. Possenti, T. Prabu, H. Quelquejay Leclere, P. Rana, A. Samajdar, S. A. Sanidas, A. Sesana, G. Shaifullah, J. Singha, L. Speri, R. Spiewak, A. Srivastava, B. W. Stappers, M. Surnis, S. C. Susarla, A. Susobhanan, K. Takahashi, P. Tarafdar, G. Theureau, C. Tiburzi, E. van der Wateren, A. Vecchio, V. Venkatraman Krishnan, J. P. W. Verbiest, J. Wang, L. Wang, Z. Wu
Abstract: The nanohertz gravitational wave background (GWB) is expected to be an aggregate signal of an ensemble of gravitational waves emitted predominantly by a large population of coalescing supermassive black hole binaries in the centres of merging galaxies. Pulsar timing arrays, ensembles of extremely stable pulsars, are the most precise experiments capable of detecting this background. However, the subtle imprints that the GWB induces on pulsar timing data are obscured by many sources of noise. These must be carefully characterized to increase the sensitivity to the GWB. In this paper, we present a novel technique to estimate the optimal number of frequency coefficients for modelling achromatic and chromatic noise and perform model selection. We also incorporate a new model to fit for scattering variations in the pulsar timing package temponest and created realistic simulations of the European Pulsar Timing Array (EPTA) datasets that allowed us to test the efficacy of our noise modelling algorithms. We present an in-depth analysis of the noise properties of 25 millisecond pulsars (MSPs) that form the second data release (DR2) of the EPTA and investigate the effect of incorporating low-frequency data from the Indian PTA collaboration. We use enterprise and temponest packages to compare noise models with those reported with the EPTA DR1. We find that, while in some pulsars we can successfully disentangle chromatic from achromatic noise owing to the wider frequency coverage in DR2, in others the noise models evolve in a more complicated way. We also find evidence of long-term scattering variations in PSR J1600$-$3053. Through our simulations, we identify intrinsic biases in our current noise analysis techniques and discuss their effect on GWB searches. The results presented here directly help improve sensitivity to the GWB and are already being used as part of global PTA efforts.
18.The second data release from the European Pulsar Timing Array IV. Search for continuous gravitational wave signals
Authors:J. Antoniadis, P. Arumugam, S. Arumugam, S. Babak, M. Bagchi, A. S. Bak Nielsen, C. G. Bassa, A. Bathula, A. Berthereau, M. Bonetti, E. Bortolas, P. R. Brook, M. Burgay, R. N. Caballero, A. Chalumeau, D. J. Champion, S. Chanlaridis, S. Chen, I. Cognard, S. Dandapat, D. Deb, S. Desai, G. Desvignes, N. Dhanda-Batra, C. Dwivedi, M. Falxa, I. Ferranti, R. D. Ferdman, A. Franchini, J. R. Gair, B. Goncharov, A. Gopakumar, E. Graikou, J. M. Grießmeier, L. Guillemot, Y. J. Guo, Y. Gupta, S. Hisano, H. Hu, F. Iraci, D. Izquierdo-Villalba, J. Jang, J. Jawor, G. H. Janssen, A. Jessner, B. C. Joshi, F. Kareem, R. Karuppusamy, E. F. Keane, M. J. Keith, D. Kharbanda, T. Kikunaga, N. Kolhe, M. Kramer, M. A. Krishnakumar, K. Lackeos, K. J. Lee, K. Liu, Y. Liu, A. G. Lyne, J. W. McKee, Y. Maan, R. A. Main, S. Manzini, M. B. Mickaliger, I. C. Nitu, K. Nobleson, A. K. Paladi, A. Parthasarathy, B. B. P. Perera, D. Perrodin, A. Petiteau, N. K. Porayko, A. Possenti, T. Prabu, H. Quelquejay Leclere, P. Rana, A. Samajdar, S. A. Sanidas, A. Sesana, G. Shaifullah, J. Singha, L. Speri, R. Spiewak, A. Srivastava, B. W. Stappers, M. Surnis, S. C. Susarla, A. Susobhanan, K. Takahashi, P. Tarafdar, G. Theureau, C. Tiburzi, E. van der Wateren, A. Vecchio, V. Venkatraman Krishnan, J. P. W. Verbiest, J. Wang, L. Wang, Z. Wu
Abstract: We present the results of a search for continuous gravitational wave signals (CGWs) in the second data release (DR2) of the European Pulsar Timing Array (EPTA) collaboration. The most significant candidate event from this search has a gravitational wave frequency of 4-5 nHz. Such a signal could be generated by a supermassive black hole binary (SMBHB) in the local Universe. We present the results of a follow-up analysis of this candidate using both Bayesian and frequentist methods. The Bayesian analysis gives a Bayes factor of 4 in favor of the presence of the CGW over a common uncorrelated noise process, while the frequentist analysis estimates the p-value of the candidate to be 1%, also assuming the presence of common uncorrelated red noise. However, comparing a model that includes both a CGW and a gravitational wave background (GWB) to a GWB only, the Bayes factor in favour of the CGW model is only 0.7. Therefore, we cannot conclusively determine the origin of the observed feature, but we cannot rule it out as a CGW source. We present results of simulations that demonstrate that data containing a weak gravitational wave background can be misinterpreted as data including a CGW and vice versa, providing two plausible explanations of the EPTA DR2 data. Further investigations combining data from all PTA collaborations will be needed to reveal the true origin of this feature.
19.The second data release from the European Pulsar Timing Array: VI. Challenging the ultralight dark matter paradigm
Authors:Clemente Smarra, Boris Goncharov, Enrico Barausse, J. Antoniadis, S. Babak, A. -S. Bak Nielsen, C. G. Bassa, A. Berthereau, M. Bonetti, E. Bortolas, P. R. Brook, M. Burgay, R. N. Caballero, A. Chalumeau, D. J. Champion, S. Chanlaridis, S. Chen, I. Cognard, G. Desvignes, M. Falxa, R. D. Ferdman, A. Franchini, J. R. Gair, E. Graikou, J. -M. Grie, L. Guillemot, Y. J. Guo, H. Hu, F. Iraci, D. Izquierdo-Villalba, J. Jang, J. Jawor, G. H. Janssen, A. Jessner, R. Karuppusamy, E. F. Keane, M. J. Keith, M. Kramer, M. A. Krishnakumar, K. Lackeos, K. J. Lee, K. Liu, Y. Liu, A. G. Lyne, J. W. McKee, R. A. Main, M. B. Mickaliger, I. C. Niţu, A. Parthasarathy, B. B. P. Perera, D. Perrodin, A. Petiteau, N. K. Porayko, A. Possenti, H. Quelquejay Leclere, A. Samajdar, S. A. Sanidas, A. Sesana, G. Shaifullah, L. Speri, R. Spiewak, B. W. Stappers, S. C. Susarla, G. Theureau, C. Tiburzi, E. van der Wateren, A. Vecchio, V. Venkatraman Krishnan, J. Wang, L. Wang, Z. Wu
Abstract: Pulsar Timing Array experiments probe the presence of possible scalar/pseudoscalar ultralight dark matter particles through decade-long timing of an ensemble of galactic millisecond radio pulsars. With the second data release of the European Pulsar Timing Array, we focus on the most robust scenario, in which dark matter interacts only gravitationally with ordinary baryonic matter. Our results show that ultralight particles with masses $10^{-24.0}~\text{eV} \lesssim m \lesssim 10^{-23.2}~\text{eV}$ cannot constitute $100\%$ of the measured local dark matter density, but can have at most local density $\rho\lesssim 0.15$ GeV/cm$^3$.
20.The gravitational-wave background null hypothesis: Characterizing noise in millisecond pulsar arrival times with the Parkes Pulsar Timing Array
Authors:Daniel J. Reardon, Andrew Zic, Ryan M. Shannon, Valentina Di Marco, George B. Hobbs, Agastya Kapur, Marcus E. Lower, Rami Mandow, Hannah Middleton, Matthew T. Miles, Axl F. Rogers, Jacob Askew, Matthew Bailes, N. D. Ramesh Bhat, Andrew Cameron, Matthew Kerr, Atharva Kulkarni, Richard N. Manchester, Rowina S. Nathan, Christopher J. Russell, Stefan Osłowski, Xing-Jiang Zhu
Abstract: The noise in millisecond pulsar (MSP) timing data can include contributions from observing instruments, the interstellar medium, the solar wind, solar system ephemeris errors, and the pulsars themselves. The noise environment must be accurately characterized in order to form the null hypothesis from which signal models can be compared, including the signature induced by nanohertz-frequency gravitational waves (GWs). Here we describe the noise models developed for each of the MSPs in the Parkes Pulsar Timing Array (PPTA) third data release, which have been used as the basis of a search for the isotropic stochastic GW background. We model pulsar spin noise, dispersion measure variations, scattering variations, events in the pulsar magnetospheres, solar wind variability, and instrumental effects. We also search for new timing model parameters and detected Shapiro delays in PSR~J0614$-$3329 and PSR~J1902$-$5105. The noise and timing models are validated by testing the normalized and whitened timing residuals for Gaussianity and residual correlations with time. We demonstrate that the choice of noise models significantly affects the inferred properties of a common-spectrum process. Using our detailed models, the recovered common-spectrum noise in the PPTA is consistent with a power law with a spectral index of $\gamma=13/3$, the value predicted for a stochastic GW background from a population of supermassive black hole binaries driven solely by GW emission.
21.The Parkes Pulsar Timing Array Third Data Release
Authors:Andrew Zic, Daniel J. Reardon, Agastya Kapur, George Hobbs, Rami Mandow, Małgorzata Curyło, Ryan M. Shannon, Jacob Askew, Matthew Bailes, N. D. Ramesh Bhat, Andrew Cameron, Zu-Cheng Chen, Shi Dai, Valentina Di Marco, Yi Feng, Matthew Kerr, Atharva Kulkarni, Marcus E. Lower, Rui Luo, Richard N. Manchester, Matthew T. Miles, Rowina S. Nathan, Stefan Osłowski, Axl F. Rogers, Christopher J. Russell, Renée Spiewak, Nithyanandan Thyagarajan, Lawrence Toomey, Shuangqiang Wang, Lei Zhang, Songbo Zhang, Xing-Jiang Zhu
Abstract: We present the third data release from the Parkes Pulsar Timing Array (PPTA) project. The release contains observations of 32 pulsars obtained using the 64-m Parkes ``Murriyang'' radio telescope. The data span is up to 18 years with a typical cadence of 3 weeks. This data release is formed by combining an updated version of our second data release with $\sim 3$ years of more recent data primarily obtained using an ultra-wide-bandwidth receiver system that operates between 704 and 4032 MHz. We provide calibrated pulse profiles, flux-density dynamic spectra, pulse times of arrival, and initial pulsar timing models. We describe methods for processing such wide-bandwidth observations, and compare this data release with our previous release.
22.Setting an upper limit for the total TeV neutrino flux from the disk of our Galaxy
Authors:Vittoria Vecchiotti, Francesco L. Villante, Giulia Pagliaroli
Abstract: We set an upper limit for the total TeV neutrino flux expected from the disk of our Galaxy in the region $|l|<30^{\circ}$ and $|b|<2^{\circ}$ probed by the ANTARES experiment. We include both the diffuse emission, due to the interaction of cosmic rays with the interstellar medium, and the possible contribution produced by gamma-ray Galactic sources. The neutrino diffuse emission is calculated under different assumptions for the cosmic ray spatial and energy distribution in our Galaxy. In particular, we assume that the total gamma-ray flux produced by all the sources, resolved and unresolved by H.E.S.S., is produced via hadronic interaction and, hence, is coupled with neutrino emission. We compare our total neutrino flux with the recent ANTARES measurement of the neutrino from the Galactic Ridge. We show that the ANTARES best-fit flux requires the existence of a large source component, close to or even larger than the most optimistic predictions obtained with our approach.
23.Sensitivity of He Flames in X-ray Bursts to Nuclear Physics
Authors:Zhi Chen, Michael Zingale, Kiran Eiden
Abstract: Through the use of axisymmetric 2D hydrodynamic simulations, we further investigate laterally propagating flames in X-ray bursts (XRBs). Our aim is to understand the sensitivity of a propagating helium flame to different nuclear physics. Using the Castro simulation code, we confirm the phenomenon of enhanced energy generation shortly after a flame is established after by adding ${}^{12}$C(p, ${\gamma}$)${}^{13}$N(${\alpha}$, p)${}^{16}$O to the network, in agreement with the past literature. This sudden outburst of energy leads to a short accelerating phase, causing a drastic alteration in the overall dynamics of the flame in XRBs. Furthermore, we investigate the influence of different plasma screening routines on the propagation of the XRB flame. We finally examine the performance of simplified-SDC, a novel approach to hydrodynamics and reaction coupling incorporated in Castro, as an alternative to operator-splitting.
24.A new method for short duration transient detection in radio images: Searching for transient sources in MeerKAT data of NGC 5068
Authors:S. Fijma, A. Rowlinson, R. A. M. J. Wijers, I. de Ruiter, W. J. G. de Blok, S. Chastain, A. J. van der Horst, Z. S. Meyers, K. van der Meulen, R. Fender, P. A. Woudt, A. Andersson, A. Zijlstra, J. Healy, F. M. Maccagni
Abstract: Transient surveys are a vital tool in exploring the dynamic universe, with radio transients acting as beacons for explosive and highly energetic astrophysical phenomena. However, performing commensal transient surveys using radio imaging can require a significant amount of computing power, data storage and time. With the instrumentation available to us, and with new and exciting radio interferometers in development, it is essential that we develop efficient methods to probe the radio transient sky. In this paper, we present results from an commensal short duration transient survey, on time scales of 8 seconds, 128 seconds and 1 hour, using data from the MeerKAT radio telescope. The dataset used was obtained as part of a galaxy observing campaign, and we focus on the field of NGC 5068. We present a quick, wide field imaging strategy to enable fast imaging of large datasets, and develop methods to efficiently filter detected transient candidates. No transient candidates were identified on the time scales of 8 seconds, 128 seconds and 1 hour, leading to competitive limits on the transient surface densities of $6.7{\times}10^{-5}$ deg$^{-1}$, $1.1{\times}10^{-3}$ deg$^{-1}$, and $3.2{\times}10^{-2}$ deg$^{-1}$ at sensitivities of 56.4 mJy, 19.2 mJy, and 3.9 mJy for the respective time scales. We find one possible candidate that could be associated with a stellar flare, that was rejected due to strict image quality control. Further short time-scale radio observations of this candidate could give definite results to its origin.
25.Constraining MeV-scale axion-like particles with Fermi-LAT observations of SN 2023ixf
Authors:Eike Müller, Pierluca Carenza, Christopher Eckner, Ariel Goobar
Abstract: The Fermi-LAT observations of SN 2023ixf, a Type II supernova in the nearby Pinwheel Galaxy, Messier 101 (M101), presents us with an excellent opportunity to constrain MeV-scale Axion-Like Particles (ALPs). By examining the photon decay signature from heavy ALPs that could be produced in the explosion, we improve the existing constraints on the ALP-photon coupling by up to a factor of $ \sim 2 $ for masses $ m_a \lesssim 3 $ MeV, with the exact value depending mostly on plasma properties of the collapsing core. This study demonstrates the relevance of core-collapse supernovae, also beyond the Magellanic Clouds, as probes of fundamental physics.