Gamma-ray astronomy

Abstract:

The search for sources of high-energy astrophysical neutrinos can be significantly advanced through a multimessenger approach, which seeks to detect the γ-rays that accompany neutrinos as they are produced at their sources. Multimessenger observations have so far provided the first evidence for a neutrino source, illustrated by the joint detection of the flaring blazar TXS 0506+056 in high-energy (E > 1 GeV) and very-high-energy (VHE; E > 100 GeV) γ-rays in coincidence with the high-energy neutrino IceCube-170922A, identified by IceCube. Imaging atmospheric Cherenkov telescopes (IACTs), namely FACT, H.E.S.S., MAGIC, and VERITAS, continue to conduct extensive neutrino target-of-opportunity follow-up programs. These programs have two components: follow-up observations of single astrophysical neutrino candidate events (such as IceCube-170922A), and observation of known γ-ray sources after the identification of a cluster of neutrino events by IceCube. Here we present a comprehensive analysis of follow-up observations of high-energy neutrino events observed by the four IACTs between 2017 September (after the IceCube-170922A event) and 2021 January. Our study found no associations between γ-ray sources and the observed neutrino events. We provide a detailed overview of each neutrino event and its potential counterparts. Furthermore, a joint analysis of all IACT data is included, yielding combined upper limits on the VHE γ-ray flux.

Abstract:

The origin of cosmic rays (CRs) from outside the Solar system is unknown, as they are deflected by the interstellar magnetic field. Supernova remnants are the main candidate for CRs up to PeV energies but due to lack of evidence, they cannot be concluded as the sources of the most energetic Galactic CRs. We investigate discrete ejecta produced in state transitions of black hole X-ray binary systems as a potential source of CRs, motivated by recent TeV -ray detections by LHAASO. Starting from MAXI J1820+070, we examine the multi-wavelength observations and find that efficient particle acceleration may take place (i.e. into a robust power law), up to eV, where is the ratio of particle energy to magnetic energy. From these calculations, we estimate the global contribution of ejecta to the entire Galactic spectrum to be , with the CR contribution rising to at PeV energies, assuming roughly equal energy in non-thermal protons, non-thermal electrons, and magnetic fields. In addition, we calculate associated -ray and neutrino spectra of the MAXI J1820+070 ejecta to investigate new detection methods with CTAO, which provide strong constraints on initial ejecta size of order Schwarzschild radii ( pc) assuming a period of adiabatic expansion.