By: Omer Blaes, Yan-Fei Jiang, Jean-Pierre Lasota, Galina Lipunova
We review our current knowledge of thermal and viscous instabilities in accretion discs around compact objects. We begin with classical disc models based on analytic viscosity prescriptions, discussing physical uncertainties and exploring time-dependent solutions of disc evolution. We also review the ionization instability responsible for outbursting dwarf nova and X-ray binary systems, including some detailed comparisons between alpha-base... more
We review our current knowledge of thermal and viscous instabilities in accretion discs around compact objects. We begin with classical disc models based on analytic viscosity prescriptions, discussing physical uncertainties and exploring time-dependent solutions of disc evolution. We also review the ionization instability responsible for outbursting dwarf nova and X-ray binary systems, including some detailed comparisons between alpha-based models and the observed characteristics of these systems. We then review modern theoretical work based on ideas around angular momentum transport mediated by magnetic fields, focusing in particular on knowledge gained through local and global computer simulations of MHD processes in discs. We discuss how MHD may alter our understanding of outbursts in white dwarf and X-ray binary systems. Finally, we turn to the putative thermal/viscous instabilities that were predicted to exist in the inner, radiation pressure-dominated regions of black hole and neutron star discs, in apparent contradiction to the observed stability of the high/soft state in black hole X-ray binaries. less
By: M. C. Bezuidenhout, N. D. R. Bhat, M. Caleb, L. N. Driessen, F. Jankowski, M. Kramer, V. Morello, I. Pastor-Marazuela, K. Rajwade, J. Roy, B. W. Stappers, M. Surnis, J. Tian
PSR J0901$-$4046, a likely radio-loud neutron star with a period of 75.88 seconds, challenges conventional models of neutron star radio emission. Here, we showcase results from 46 hours of follow-up observations of PSR J0901$-$4046 using the MeerKAT, Murriyang, GMRT, and MWA radio telescopes. We demonstrate the intriguing stability of the source's timing solution over more than three years, leading to an RMS arrival-time uncertainty of just... more
PSR J0901$-$4046, a likely radio-loud neutron star with a period of 75.88 seconds, challenges conventional models of neutron star radio emission. Here, we showcase results from 46 hours of follow-up observations of PSR J0901$-$4046 using the MeerKAT, Murriyang, GMRT, and MWA radio telescopes. We demonstrate the intriguing stability of the source's timing solution over more than three years, leading to an RMS arrival-time uncertainty of just $\sim$10$^{-4}$ of the rotation period. Furthermore, non-detection below 500 MHz may indicate a low-frequency turnover in the source's spectrum, while no secular decline in the flux density of the source over time, as was apparent from previous observations, has been observed. Using high time-resolution MeerKAT data, we demonstrate two distinct quasi-periodic oscillation modes present in single pulses, with characteristic time scales of 73 ms and 21 ms. We also observe a statistically significant change in the relative prevalence of distinct pulse morphologies compared to previous observations, possibly indicating a shift in the magnetospheric composition over time. Finally, we show that the W$_{50}$ pulse width is nearly constant from 544-4032 MHz, consistent with zero radius-to-frequency mapping. The very short duty cycle ($\sim$1.4$^{\circ}$) is more similar to radio pulsars with periods $>$5 seconds than to radio-loud magnetars. This, along with the lack of magnetar-like outbursts or timing glitches, complicates the identification of the source with ultra-long period magnetar models. less
By: P. Romano INAF/OAB, H. I. Cohen CUA CSST Uni. Maryland NASA GSFC, E. Bozzo Univ. Geneve INAF/OAB, N. Islam CSST Uni. Maryland NASA GSFC, A. Lange GWU CSST Uni. Maryland NASA GSFC, R. H. D. Corbet CRESST and CSST NASA GSFC Maryland ICA, B. Coley Howard Univ CRESST, K. Pottschmidt Uni. Maryland CRESST
We report on an observational campaign performed with Swift/XRT on the wind-fed supergiant X-ray binary 4U 1909+07 to investigate the nature of the orbital and superorbital modulation of its X-ray emission. A total of 137 XRT observations have been carried out, summing up to a total effective exposure time of 114 ks and covering a total of 66 orbital and 19 superorbital cycles of the source. The XRT data folded on the orbital period of the ... more
We report on an observational campaign performed with Swift/XRT on the wind-fed supergiant X-ray binary 4U 1909+07 to investigate the nature of the orbital and superorbital modulation of its X-ray emission. A total of 137 XRT observations have been carried out, summing up to a total effective exposure time of 114 ks and covering a total of 66 orbital and 19 superorbital cycles of the source. The XRT data folded on the orbital period of the source confirmed and improved the previously reported variability in intensity and absorption column density, which can be ascribed to the neutron star accreting from the wind of its B supergiant companion across a fairly circular orbit. The XRT data folded on the superorbital period did not provide evidence of significant variations in either the absorption column density and/or the power-law photon index. This may be due to a significant weakening of the superorbital modulation during the times when the XRT observations were carried out, as confirmed by the BAT dynamic power spectrum. We discuss the implications of these findings within the corotating interaction region model proposed to interpret the superorbital variability in wind-fed supergiant X-ray binaries. less
By: Miljenko Čemeljić, Włodek Kluźniak, Sukalpa Kundu
Context. Several instances of low frequency Quasi-Periodic Oscillations (QPOs) have been reported in ultraluminous X-ray sources (ULXs), including three in pulsating ones (PULXs) to date. The nature of many ULXs is still unclear, as are the detailed properties of accretion in PULXs. Aims. We seek an answer to questions such as: Is there a QPO model that fits the data? Can mHz QPOs be used to constrain the magnetic field and accretion rate o... more
Context. Several instances of low frequency Quasi-Periodic Oscillations (QPOs) have been reported in ultraluminous X-ray sources (ULXs), including three in pulsating ones (PULXs) to date. The nature of many ULXs is still unclear, as are the detailed properties of accretion in PULXs. Aims. We seek an answer to questions such as: Is there a QPO model that fits the data? Can mHz QPOs be used to constrain the magnetic field and accretion rate of the neutron stars in PULXs? Are all the low frequency QPOs in ULXs a manifestation of the same phenomenon? Methods. We apply Dong Lai's precession model to the PULX data, with the magnetic threading of the accretion disk constrained by recent simulations. Results. Based on the magnetic precession model, and on recent progress in understanding the inner structure of accretion disks, we predict an inverse scaling of QPO frequency with the neutron star period in PULXs. The theoretical curve is largely independent of the stellar magnetic field or mass accretion rate and agrees with the data for the known QPOs in PULXs. The flat-top QPOs detected in ULXs have observational properties that seem to be very different from the QPOs detected in PULXs, indicating they might have a different origin. less
By: Christopher Eckner, Noemi Anau Montel, Florian List, Francesca Calore, Christoph Weniger
Over the past 16 years, the Fermi Large Area Telescope (LAT) has significantly advanced our view of the GeV gamma-ray sky, yet several key questions remain - such as the composition of the isotropic gamma-ray background, the origin of the Fermi Bubbles or the potential presence of signatures from exotic physics like dark matter. Addressing these challenges requires sophisticated astrophysical modeling and robust statistical methods capable ... more
Over the past 16 years, the Fermi Large Area Telescope (LAT) has significantly advanced our view of the GeV gamma-ray sky, yet several key questions remain - such as the composition of the isotropic gamma-ray background, the origin of the Fermi Bubbles or the potential presence of signatures from exotic physics like dark matter. Addressing these challenges requires sophisticated astrophysical modeling and robust statistical methods capable of handling high-dimensional parameter spaces. In this work, we analyze 14 years of high-latitude ($|b|\geq30^{\circ}$) Fermi-LAT data in the range from 1 to 10 GeV using simulation-based inference (SBI) via neural ratio estimation. This approach allows us to detect individual gamma-ray sources and derive a list of significant gamma-ray emitters containing more than 98\% of all sources listed in the Fermi-LAT Fourth Source Catalog (4FGL) with a flux $S>3\times10^{-10}\;\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$ (about a factor of three larger than the flux above which 4FGL is nearly complete), without any non-4FGL source detected in that flux range. Additionally, we reconstruct the source-count distribution in both parametric and non-parametric forms, achieving large agreement with previous literature results as well as those sources detected by our SBI pipeline. We also quantitatively validate our gamma-ray emission simulator via an anomaly detection technique, demonstrating that the synthetic data closely reproduces the complexity of the real observations. less
By: Bhagya M. Subrayan, David J. Sand, K. Azalee Bostroem, Saurabh W. Jha, Aravind P. Ravi, Michaela Schwab, Jennifer E. Andrews, Griffin Hosseinzadeh, Stefano Valenti, Yize Dong, Jeniveve Pearson, Manisha Shrestha, Lindsey A. Kwok, Emily Hoang, Jeonghee Rho, Seong Hyun Park, Sung-Chul Yoon, T. R. Geball, Joshua Haislip, Daryl Janzen, Vladimir Kouprianov, Darshana Mehta, Nicolás Meza Retamal, Daniel E. Reichart, Moira Andrews, Joseph Farah, Megan Newsome, D. Andrew Howell, Curtis McCully
We present early multi-wavelength photometric and spectroscopic observations of the Type IIb supernova SN 2024uwq, capturing its shock-cooling emission phase and double-peaked light curve evolution. Early spectra reveal broad H-alpha (v ~ 15,500 km s$^{-1}$) and He I P-Cygni profiles of similar strengths. Over time the He I lines increase in strength while the H-alpha decreases, consistent with a hydrogen envelope ($M_{env}$ = 0.7 - 1.35 $M... more
We present early multi-wavelength photometric and spectroscopic observations of the Type IIb supernova SN 2024uwq, capturing its shock-cooling emission phase and double-peaked light curve evolution. Early spectra reveal broad H-alpha (v ~ 15,500 km s$^{-1}$) and He I P-Cygni profiles of similar strengths. Over time the He I lines increase in strength while the H-alpha decreases, consistent with a hydrogen envelope ($M_{env}$ = 0.7 - 1.35 $M_\odot$ ) overlying helium-rich ejecta. Analytic modeling of early shock cooling emission and bolometric light analysis constrains the progenitor to a partially stripped star with radius R = 10 - 60 $R_\odot$, consistent with a blue/yellow supergiant with an initial ZAMS mass of 12 - 20 $M_\odot$ , likely stripped via binary interaction. SN 2024uwq occupies a transitional position between compact and extended Type IIb supernovae, highlighting the role of binary mass-transfer efficiency in shaping a continuum of stripped-envelope progenitors. Our results underscore the importance of both early UV/optical observations to characterize shock breakout signatures critical to map the diversity in evolutionary pathways of massive stars. Upcoming time domain surveys including Rubin Observatory's LSST and UV missions like ULTRASAT and UVEX will revolutionise our ability to systematically capture these early signatures, probing the full diversity of stripped progenitors and their explosive endpoints. less
Radiative magnetohydrodynamics simulation of minidisks in equal-mass massive black hole binaries
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By: Chi-Ho Chan, Vishal Tiwari, Tamara Bogdanović, Yan-Fei Jiang, Shane W. Davis
We are on the cusp of detecting gravitational waves (GWs) from individual massive black hole binaries (MBHBs) with the Laser Interferometer Space Antenna and pulsar-timing arrays. These MBHBs may be surrounded by circumbinary disks and minidisks, the electromagnetic emission from which are essential for localizing the MBHBs on the sky. Here we present the first radiative magnetohydrodynamics (RMHD) minidisk simulation that directly solves t... more
We are on the cusp of detecting gravitational waves (GWs) from individual massive black hole binaries (MBHBs) with the Laser Interferometer Space Antenna and pulsar-timing arrays. These MBHBs may be surrounded by circumbinary disks and minidisks, the electromagnetic emission from which are essential for localizing the MBHBs on the sky. Here we present the first radiative magnetohydrodynamics (RMHD) minidisk simulation that directly solves the radiative transfer equation on discretized grid rays. The simulation examines one of the minidisks in an equal-mass $2\times10^7\,M_\odot$ MBHB separated by 100 gravitational radii. Minidisks simulated with and without radiative effects resemble each other qualitatively but differ in several key aspects. The RMHD minidisk is denser and geometrically thinner than the magnetohydrodynamics minidisk. Furthermore, the RMHD minidisk, with a nonaxisymmetric photosphere and temperature distribution, produces an anisotropic illumination pattern. As a result, the observed radiative flux of two RMHD minidisks orbiting each other varies at half the binary orbital period, a feature independent of relativistic boosting and lensing effects. Such periodic light curves, if identified in upcoming optical transient surveys, could reveal the existence of MBHBs on the way to merger, particularly if they are in a constant phase relation with detected GWs. less
By: Liang-Gui Zhu, Xian Chen
The formation channels of the gravitational-wave (GW) sources detected by LIGO/Virgo/KAGRA (LVK) remain poorly constrained. Active galactic nucleus (AGN) has been proposed as one of the potential hosts but the fraction of GW events originating from AGNs has not been quantified. Here, we constrain the AGN-origin fraction $f_{\rm agn}$ by analyzing the spatial correlation between GW source localizations ($O1\!-\!O4$a) and AGN distributions (S... more
The formation channels of the gravitational-wave (GW) sources detected by LIGO/Virgo/KAGRA (LVK) remain poorly constrained. Active galactic nucleus (AGN) has been proposed as one of the potential hosts but the fraction of GW events originating from AGNs has not been quantified. Here, we constrain the AGN-origin fraction $f_{\rm agn}$ by analyzing the spatial correlation between GW source localizations ($O1\!-\!O4$a) and AGN distributions (SDSS DR16). We find evidence of an excess of low-luminosity ($L_{\rm bol} \le 10^{45}~\!\mathrm{erg~s}^{-1}$) as well as low-Eddington ratio ($\lambda_{\rm Edd} \le 0.05$) AGNs around the LVK events, the explanation of which requires $f_{\rm agn} = 0.39^{+0.41}_{-0.32}$ and $0.29^{+0.40}_{-0.25}$ (90\% confidence level) of the LVK events originating from these respective AGN populations. Monte Carlo simulations confirm that this correlation is unlikely to arise from random coincidence, further supported by anomalous variation of the error of $f_{\rm agn}$ with GW event counts. These results provide the first observational evidence for GW sources coming from either low-luminosity or low-accretion-rate AGNs, offering critical insights into the environmental dependencies of the formation of GW sources. less
By: Mayank Pathak, Banibrata Mukhopadhyay
Ultraluminous X-ray sources (ULXs) have captivated researchers for decades due to their exceptionally high luminosities and unique spectral characteristics. Some of these sources defy expectations by exhibiting super-Eddington luminosities with respect to stellar mass sources even in their low-hard state. Numerical steady-state calculations suggest that ULXs in this state can be explained as highly magnetized advective accretion sources aro... more
Ultraluminous X-ray sources (ULXs) have captivated researchers for decades due to their exceptionally high luminosities and unique spectral characteristics. Some of these sources defy expectations by exhibiting super-Eddington luminosities with respect to stellar mass sources even in their low-hard state. Numerical steady-state calculations suggest that ULXs in this state can be explained as highly magnetized advective accretion sources around stellar-mass black holes. To explore this further, we employ GRMHD simulations using the publicly available code, BHAC (Black Hole Accretion Code), to model the behavior of highly magnetized advective accretion flows around a black hole. Our simulations demonstrate that such systems can indeed produce the intense luminosities observed in ULXs. Additionally, we validate that the magnetic fields required for these high emissions are of the order of $10^7$ Gauss, consistent with previous numerical steady-state findings. less
By: Mridweeka Singh, Lindsey A. Kwok, Saurabh W. Jha, R. Dastidar, Conor Larison, Alexei V. Filippenko, Jennifer E. Andrews, Moira Andrews, G. C. Anupama, Prasiddha Arunachalam, Katie Auchettl, Dominik BÁnhidi, Barnabas Barna, K. Azalee Bostroem, Thomas G. Brink, RÉgis Cartier, Ping Chen, Collin T. Christy, David A. Coulter, Sofia Covarrubias, Kyle W. Davis, Connor B. Dickinson, Yize Dong, Joseph Farah, Andreas FlÖrs, Ryan J. Foley, Noah Franz, Christoffer Fremling, LluÍs Galbany, Anjasha Gangopadhyay, Aarna Garg, Elinor L. Gates, Or Graur, Alexa C. Gordon, Daichi Hiramatsu, Emily Hoang, D. Andrew Howell, Brian Hsu, Joel Johansson, Arti Joshi, Lordrick A. Kahinga, Ravjit Kaur, Sahana Kumar, Piramon Kumnurdmanee, Hanindyo Kuncarayakti, Natalie Lebaron, C. Lidman, Chang Liu, Keiichi Maeda, Kate Maguire, Bailey Martin, Curtis Mccully, Darshana Mehta, Luca M. Menotti, Anne J. Metevier, A. A. Miller, Kuntal Misra, C. Tanner Murphey, Megan Newsome, Estefania Padilla Gonzalez, Kishore C. Patra, Jeniveve Pearson, Anthony L. Piro, Abigail Polin, Aravind P. Ravi, Armin Rest, Nabeel Rehemtulla, Nicolas Meza Retamal, O. M. Robinson, CÉsar Rojas-Bravo, Devendra K. Sahu, David J. Sand, Brian P. Schmidt, Steve Schulze, Michaela Schwab, Manisha Shrestha, Matthew R. Siebert, Sunil Simha, Nathan Smith, Jesper Sollerman, Shubham Srivastav, Bhagya M. Subrayan, TamÁs Szalai, Kirsty Taggart, Rishabh Singh Teja, Jacco H. Terwel, Samaporn Tinyanont, Stefano Valenti, JÓzsef VinkÓ, Aya L. Westerling, Yi Yang, Weikang Zheng
We present extensive ultraviolet to optical photometric and optical to near-infrared (NIR) spectroscopic follow-up observations of the nearby intermediate-luminosity ($M_V = -$16.81$\pm$0.19~mag) Type Iax supernova (SN) 2024pxl in NGC 6384. SN~2024pxl exhibits a faster light curve evolution than the high-luminosity members of this class, and slower than low-luminosity events. The observationally well-constrained rise time of $\sim$10 days a... more
We present extensive ultraviolet to optical photometric and optical to near-infrared (NIR) spectroscopic follow-up observations of the nearby intermediate-luminosity ($M_V = -$16.81$\pm$0.19~mag) Type Iax supernova (SN) 2024pxl in NGC 6384. SN~2024pxl exhibits a faster light curve evolution than the high-luminosity members of this class, and slower than low-luminosity events. The observationally well-constrained rise time of $\sim$10 days and an estimated synthesized $^{56}$Ni mass of 0.03 M$_\odot$, based on analytical modeling of the pseudobolometric light curve, are consistent with models of the weak deflagration of a carbon-oxygen white dwarf. Our optical spectral sequence of SN~2024pxl shows weak \ion{Si}{2} lines and spectral evolution similar to other high-luminosity Type Iax SNe, but also prominent early-time \ion{C}{2} line, like lower-luminosity Type Iax SNe. The late-time optical spectrum of SN~2024pxl closely matches that of SN 2014dt, and its NIR spectral evolution aligns with those of other well-studied, high-luminosity Type Iax SNe. The spectral-line expansion velocities of SN~2024pxl are at the lower end of the Type Iax SN velocity distribution, and the velocity distribution of iron-group elements compared to intermediate-mass elements suggests that the ejecta are mixed on large scales, as expected in pure deflagration models. SN~2024pxl exhibits characteristics intermediate between those of high-luminosity and low-luminosity Type~Iax SNe, further establishing a link across this diverse class. less