High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: Recently, some fast radio bursts (FRBs) have been reported to exhibit complex and diverse variations in Faraday rotation measurements (RM) and polarization, suggesting that dynamically evolving magnetization environments may surround them. In this paper, we investigate the Faraday conversion (FC) effect in a binary system involving an FRB source and analyze the polarization evolution of FRBs. For an strongly magnetized high-mass companion binary (HMCB), when an FRB with $\sim100\%$ linear polarization passes through the radial magnetic field of the companion star, the circular polarization (CP) component will be induced and oscillate symmetrically around the point with the CP degree equal to zero, the rate and amplitude of the oscillation decrease as the frequency increases. The very strong plasma column density in the HMCBs can cause CP to oscillate with frequency at a very drastic rate, which may lead to depolarization. Near the superior conjunction of the binary orbit, the DM varies significantly due to the dense plasma near the companion, and the significant FC also occurs in this region. As the pulsar moves away from the superior conjunction, the CP gradually tends towards zero and then returns to its value before incidence. We also investigate the effect of the rotation of the companion star. We find that a sufficiently significant RM reversal can be produced at large magnetic inclinations and the RM variation is very diverse. Finally, we apply this model to explain some polarization observations of PSR B1744-24A and FRB 20201124A.

Abstract: Fast Radio Bursts (FRBs) are extragalactic transients of (sub-)millisecond duration that show wide-ranging spectral, temporal, and polarimetric properties. The polarimetric analysis of FRBs can be used to probe intervening media, study the emission mechanism, and test possible progenitor models. In particular, low frequency depolarisation of FRBs can identify dense, turbulent, magnetised, ionised plasma thought to be near the FRB progenitor. An ensemble of repeating FRBs has shown low-frequency depolarisation. The depolarisation is quantified by the parameter $\sigma_{\rm RM}$, which correlates with proxies for both the turbulence and mean magnetic field strength of the putative plasma. However, while many non-repeating FRBs show comparable scattering (and hence inferred turbulence) to repeating FRBs, it is unclear whether their surrounding environments are comparable to those of repeating FRBs. To test this, we analyse the spectro-polarimetric properties of five one-off FRBs and one repeating FRB, detected and localised by the Australian Square Kilometer Array Pathfinder. We search for evidence of depolarisation due to $\sigma_{\rm RM}$ and consider models where the depolarisation is intrinsic to the source. We find no evidence (for or against) the sample showing spectral depolarisation. Under the assumption that FRBs have multipath propagation-induced depolarisation, the correlation between our constraint on $\sigma_{\rm RM}$ and RM is consistent with repeating FRBs only if the values of $\sigma_{\rm RM}$ are much smaller than our upper limits. The observations provide further evidence for differences in the environments and sources of one-off and repeating FRBs.

Abstract: In the inner region of the disc of an active galactic nucleus (AGN), the collision of two white dwarfs (WDs) through Jacobi capture might be inevitable, leading to a Type Ia supernova (SN Ia) explosion. This transient event, influenced by the disc gas and the gravity of the supermassive black hole (SMBH), exhibits distinct characteristics compared to normal SNe Ia. The energy of the explosion is mainly stored in the ejecta in the form of kinetic energy. Typically, the ejecta is not effectively decelerated by the AGN disc and rushes rapidly out of the AGN disc. However, under the influence of the SMBH, most of the ejecta falls back toward the AGN disc. As the fallback ejecta becomes more dispersed, it interacts with the disc gas, converting its kinetic energy into thermal energy. This results in a high-energy transient characterized by a rapid initial rise followed by a decay with $L\propto t^{-2.8}$. The time-scale of the transient ranges from hours to weeks, depending on the mass of the SMBH. This process generates high-energy radiation spanning from hard X-rays to the soft $\gamma$ range. Additionally, the subsequent damage to the disc may result in changing-look AGNs. Moreover, the falling back of SNe Ia ejecta onto the AGN disc significantly increases the metallicity of the AGN and can even generate heavy elements within the AGN discs.

Abstract: Recent Event Horizon Telescope observations of M87* and Sgr A* strongly suggests the presence of supermassive black hole at their respective cores. In this work, we use the semi-analytic Radiatively Inefficient Accretion Flows (RIAF) model to investigate the resulting images of Joshi-Malafarina-Narayan (JMN-1) naked singularity and the Schwarzschild BH. We aim at choosing the JMN-1 naked singularity model and compare the synchrotron images with the Schwarzschild solution to search any distinct features which can distinguish the two objects and find alternative to the black hole solution. We perform general relativistic ray-tracing and radiative transfer simulations using Brahma code to generate synchrotron emission images utilising thermal distribution function for emissivity and absorptivity. We investigate effects in the images by varying inclination angle, disk width and frequency. The shadow images simulated by the JMN-1 model closely resemble those generated by the Schwarzschild black hole. When we compare these images, we find that the disparities between them are minimal. We conduct simulations using various plasma parameters, but the resulting images remain largely consistent for both scenarios. This similarity is evident in the horizontal cross-sectional brightness profiles of the two instances. Notably, the JMN-1 model exhibits slightly higher intensity in comparison to the Schwarzschild black hole. We conclude that JMN-1 presents itself as a viable substitute for the black hole scenario. This conclusion is not solely grounded in the fact that they are indistinguishable from their respective shadow observations, but also in the consideration that JMN-1 emerges as an end state of a continual gravitational collapse. This paradigm not only allows for constraints on spacetime but also provides a good probe for the nature of the central compact object.

Abstract: Besides the neutrino source detected by IceCube, NGC 1068, the association of the IceCube-170922A neutrino with the blazar in a flaring state among several wavelengths (from radio up to high-energy (HE) gamma-rays), the site and mechanisms of production of HE neutrino remains in discussion. Extragalactic sources such as Quasars, Blazars, Radio galaxies, and Gamma-ray bursts have been proposed as progenitors of HE neutrinos. In this work, we study the Blazars reported by Fermi-LAT in the 4LAC catalog, which are embedded inside the 90\% error of the best-fit position from the neutrinos reported by IceCube. We propose a one-zone lepto-hadronic scenario to describe the broadband Spectral Energy Distribution and then estimate the number of neutrinos to compare with those in the direction of each source. A brief discussion is provided of the results.