A Study of Background Conditions for Sphinx--The Satellite-Borne Gamma-Ray Burst Polarimeter

TL;DR

This paper analyzes background radiation conditions for the SPHiNX satellite gamma-ray burst polarimeter, using simulations to optimize signal detection and estimate background noise levels for future polarisation measurements.

Contribution

It provides detailed background estimates for SPHiNX using Geant4 simulations, aiding in mission design and sensitivity analysis.

Findings

Total background outside SAA is about 323 counts/s.

Cosmic X-ray background and albedo gamma rays dominate the background.

Total background level is approximately 513 counts/s, enabling polarization detection of ~50 GRBs.

Abstract

SPHiNX is a proposed satellite-borne gamma-ray burst polarimeter operating in the energy range 50-500 keV. The mission aims to probe the fundamental mechanism responsible for gamma-ray burst prompt emission through polarisation measurements. Optimising the signal-to-background ratio for SPHiNX is an important task during the design phase. The Geant4 Monte Carlo toolkit is used in this work. From the simulation, the total background outside the South Atlantic Anomaly (SAA) is about 323 counts/s, which is dominated by the cosmic X-ray background and albedo gamma rays, which contribute ~60% and ~35% of the total background, respectively. The background from albedo neutrons and primary and secondary cosmic rays is negligible. The delayed background induced by the SAA-trapped protons is about 190 counts/s when SPHiNX operates in orbit for one year. The resulting total background level of ~513 counts/s allows the polarisation of ~50 GRBs with minimum detectable polarisation less than 30% to be determined during the two-year mission lifetime.