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.Comment: Proceedings of SPIE, Vo. 8443, Paper No. 8443-3