High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: We present the properties of relativistic, inviscid, low angular momentum, advective accretion flow in the framework of modified theory of gravity. We adopt a $f(R)$ gravity model that satisfactorily mimics the asymptotically flat vacuum solutions of the Einstein's equations, where $R$ is the scalar curvature. With this, we solve the governing equations that describe the accretion flow and obtain the transonic global accretion solutions in terms of the input parameters, namely energy (${\cal E}$), angular momentum ($\lambda$) and gravity parameter ($A$) that determines the gravity effect. We carry out the critical point analysis and find that depending on the input parameters, flow may contain either single or multiple critical points. In addition, we examine the role of input parameters in obtaining the accretion solutions in modified gravity and observe that gravity parameter ($A$) regulates the overall character of the accretion solutions. We separate the effective domain of the parameter space in $\lambda-{\cal E}$ plane that admits accretion solutions possessing multiple critical points and observe that solution of this kind continues to form for wide range of the flow parameters. We examine the modification of the parameter space and reveal that it gradually shrinks with the decrease of $A$, and ultimately disappears for $A=-2.34$. Finally, we calculate the disk luminosity ($L$) considering bremsstrahlung emission process and find that global accretion solutions passing through the inner critical point are more luminous compared to the outer critical point solutions.

Abstract: Supernova (SN) 2023ixf in M101 is the closest SN explosion observed in the last decade. Therefore it is a suitable test bed to study the role of jets in powering the SN ejecta. With this aim, we explored the idea that high-energy neutrinos could be produced during the interaction between the jets and the intense radiation field produced in the SN explosion and eventually be observed by the IceCube neutrino telescope. The lack of detection of such neutrinos has significantly constrained both the fraction of stellar collapses that produce jets and/or the theoretical models for neutrino production. Finally, we investigated the possibility of detecting low-energy neutrinos from SN 2023ixf with the Super- and Hyper-Kamiokande experiments, obtaining in both cases sub-threshold estimates.

Abstract: Very-high-energy (VHE) photons around TeV energies from a gamma-ray burst (GRB) jet will play an essential role in the multi-messenger era, with a fair fraction of the events being observed off-axis to the jet. We show that different energy photons (MeV and TeV photons in particular) arrive from different emission zones for off-axis observers even if the emission radius is the same. The location of the emission region depends on the jet structure of the surface brightness, and the structures are generally different at different energies, mainly due to the attenuation of VHE photons by electron-positron pair creation. This off-axis zone-shift effect does not justify the usual one-zone approximation and also produces a time-delay of VHE photons comparable to the GRB duration, which is crucial for future VHE observations, such as by the Cherenkov Telescope Array.