Revisiting the Earth's atmospheric scattering of X-ray/$\gamma$-rays and its effect on space observation: Implication for GRB spectral analysis

Abstract

A considerable fraction of incident high-energy photons from astrophysical transients such as Gamma-Ray Bursts (GRBs) is Compton scattered by the Earth's atmosphere. These photons, sometimes referred to as the "reflection component", contribute to the signal detected by space-borne X-ray/$\gamma$-ray instruments. The effectiveness and reliability of source parameters such as position, flux, spectra, and polarization, inferred by these instruments are therefore highly dependent on the accurate estimation of this scattered component. Current missions use dedicated response matrices to account for these effects. However, these databases are not readily adaptable for other missions, including many upcoming transient search and gravitational wave high-energy Electromagnetic counterpart detectors. Furthermore, possible systematic effects in these complex simulations have not been thoroughly examined and verified in the literature. We are in the process of investigation of the effect with detailed Monte Carlo simulations in GEANT4 for a Low Earth Orbit (LEO) X-ray detector. Here, we discuss the outcome of our simulation in form of the Atmospheric Response Matrix (ARM) and its implications of any systematic errors in the determination of source spectral characteristics. We intend to apply our results in data processing and analysis for AstroSat-CZTI observation of such sources in near future. Our simulation output and source codes will be made publicly available for use by a large number of upcoming high energy transient missions, as well as for scrutiny and systematic comparisons with other missions