High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: Some of the accreting X-ray pulsars are reported to exhibit a peculiar spectral feature at $\sim$10 keV, known as the "10 keV feature". The feature has been characterized as either an emission line or an absorption line, and its origin is unknown. It has been found in multiple observations of the same source by different observatories, but not all the observations of any particular source consistently showed the presence of it. In this work, we have carried out a systematic investigation for the presence of the "10 keV feature" using data from NuSTAR, a low background spectroscopic observatory having uninterrupted wide band coverage on either side of 10 keV. We performed a systematic spectral analysis on 58 archival NuSTAR observations of 30 bright X-ray pulsars. The 3$-$79 keV spectral continua of these selected sources were fitted with a model chosen on the basis of its fitting quality in 3$-$15 keV and model simplicity, and then inspected for the presence of the "10 keV feature". Our analysis indicates the presence of such a feature in 16 out of 58 the NuSTAR observations of 11 different sources and is fitted with a Gaussian absorption model centered around 10 keV. Our analysis also suggests that such a feature could be wrongly detected if flare data is not analyzed separately from persistent emission.

Abstract: Spectro-polarimetric signatures of accretion disks in X-ray binaries and active galactic nuclei contain information about the masses and spins of their central black holes, as well as the geometry of matter close to the compact objects. This information can be extracted using the means of X-ray polarimetry. In this work, we present a fast analytical ray-tracing technique for polarized light \textsc{artpol} that helps obtain the spinning black hole parameters from the observed properties. This technique can replace the otherwise time-consuming numerical ray-tracing calculations. We show that \textsc{artpol} proves accurate for Kerr black holes with dimensionless spin parameter $a\leq0.94$ while being over four orders of magnitude faster than direct ray-tracing calculations. This approach opens broad prospects for directly fitting the spectro-polarimetric data from the \textit{Imaging X-ray Polarimetry Explorer}.

Abstract: We present a summary of the flavor composition measurements for the diffuse astrophysical neutrino flux using data from the IceCube Neutrino Observatory at the South Pole. IceCube has identified candidate astrophysical tau neutrinos through two different approaches. One approach used a dedicated particle identification algorithm for the classification and reconstruction of the 'Double Cascade' event topology, a signature of tau neutrino charged current interactions. This first approach is applied to the High Energy Starting Events (HESE) sample, an all-sky, all-flavor set of neutrino events with energy above 60~TeV encompassing 12 years of IceCube livetime. We show that the addition of more years of data and updated ice properties on the HESE sample delivers tighter constraints on the flavor composition of the astrophysical neutrino flux than previous IceCube analyses, in particular when it is fit in combination with high statistics samples of through-going tracks and cascades. A second approach uses a sensitive machine-learning-based selection technique that finds seven candidate events in 9.7 years of IceCube data. This approach excludes the zero astrophysical tau neutrino hypothesis at the highest statistical significance to date.

Abstract: The observation of an astrophysical neutrino flux in IceCube and its detection capability to separate between the different neutrino flavors has led IceCube to constraint the flavor content of this flux. IceCube-Gen2 is the planned extension of the current IceCube detector, which will be about 8 times larger than the current instrumented volume. In this work, we study the sensitivity of IceCube-Gen2 to the astrophysical neutrino flavor composition and investigate its tau neutrino identification capabilities. We apply the IceCube analysis on a simulated IceCube-Gen2 dataset that mimics the High Energy Starting Event (HESE) classification. Reconstructions are performed using sensors that have 3 times higher quantum efficiency and isotropic angular acceptance compared to the current IceCube optical modules. We present the projected sensitivity for 10 years of data on constraining the flavor ratio of the astrophysical neutrino flux at Earth by IceCube-Gen2.

Abstract: The equation of state (EOS) is a key ingredient for understanding the structure and composition of neutron stars (NSs). Observation of several pulsars with masses $\approx$ 2 Msun, inference of tidal deformabilities from gravitational waves signals in binary neutron stars mergers and joint mass and radius estimates of two millisecond pulsars contributed to better constraining the behavior of NS EOS beyond the nuclear saturation density. We aim to build families of EOSs subjected to various minimal sets of constraints and identify the role some of these constraints play. We also aim to establish correlations between properties of nuclear matter (NM) and properties of NSs. The non-relativistic mean field theory of NM and the standard Skyrme parametrization of the nucleonic effective interactions are used to generate, within a Bayesian framework, models of EOSs. The constraints we pose come from empirical parameters of NM, ab initio calculations of pure neutron matter (PNM) and the 2 Msun lower limit on the maximum NS mass. EOSs also have to be causal. A purely nucleonic composition is hypothesized. EOSs are generated and investigated for five sets of constraints. Marginalized posteriors of the effective interaction's parameters; empirical parameters of NM; selected global properties of NSs are plotted and analyzed. Correlations among parameters in the isoscalar and isovector channels as well as with NS properties are studied. EOSs of NM and NSs are very sensitive to the set of constraints, including whether correlations among the values that the energy per nucleon in PNM takes at different densities are accounted for. In each of the five sets that we have generated there is a significant number of models that comply at 50\% credible region with joint mass and radius constraints from PSR J0030+045 and J0740+6620, while a tension exists with similar data from the NS in HESS J1731--34.

Abstract: The Event Horizon Telescope (EHT) observed in 2017 the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), at a frequency of 228.1 GHz ($\lambda$=1.3 mm). The fundamental physics tests that even a single pulsar orbiting Sgr A* would enable motivate searching for pulsars in EHT datasets. The high observing frequency means that pulsars - which typically exhibit steep emission spectra - are expected to be very faint. However, it also negates pulse scattering, an effect that could hinder pulsar detections in the Galactic Center. Additionally, magnetars or a secondary inverse Compton emission could be stronger at millimeter wavelengths than at lower frequencies. We present a search for pulsars close to Sgr A* using the data from the three most-sensitive stations in the EHT 2017 campaign: the Atacama Large Millimeter/submillimeter Array, the Large Millimeter Telescope and the IRAM 30 m Telescope. We apply three detection methods based on Fourier-domain analysis, the Fast-Folding-Algorithm and single pulse search targeting both pulsars and burst-like transient emission; using the simultaneity of the observations to confirm potential candidates. No new pulsars or significant bursts were found. Being the first pulsar search ever carried out at such high radio frequencies, we detail our analysis methods and give a detailed estimation of the sensitivity of the search. We conclude that the EHT 2017 observations are only sensitive to a small fraction ($\lesssim$2.2%) of the pulsars that may exist close to Sgr A*, motivating further searches for fainter pulsars in the region.