Background estimation in a wide-field background-limited instrument such as Fermi GBM

TL;DR

This paper introduces a novel background estimation technique for wide-field gamma-ray instruments like Fermi GBM, enabling detection of low-level, long-lived emissions that are challenging to identify with traditional polynomial interpolation methods.

Contribution

The paper presents a new method using adjacent days' data to improve background estimation in background-limited gamma-ray detectors, especially for long-duration emissions.

Findings

Effective separation of low-level emission from instrumental background.

Application of the technique to Fermi GBM data demonstrates improved detection capabilities.

Method applicable to general background-limited detectors in non-equatorial orbits.

Abstract

The supporting instrument on board the Fermi Gamma-ray Space Telescope, the Gamma-ray Burst Monitor (GBM) is a wide-field gamma-ray monitor composed of 14 individual scintillation detectors, with a field of view which encompasses the entire unocculted sky. Primarily designed as transient monitors, the conventional method for background determination with GBM-like instruments is to time interpolate intervals before and after the source as a polynomial. This is generally sufficient for sharp impulsive phenomena such as Gamma-Ray Bursts (GRBs) which are characterised by impulsive peaks with sharp rises, often highly structured, and easily distinguishable against instrumental backgrounds. However, smoother long lived emission, such as observed in solar flares and some GRBs, would be difficult to detect in a background-limited instrument using this method. We present here a description of a technique which uses the rates from adjacent days when the satellite has approximately the same geographical footprint to distinguish low-level emission from the instrumental background. We present results from the application of this technique to GBM data and discuss the implementation of it in a generalised background limited detector in a non-equatorial orbit.