The Role of the BATSE Instrument Response in Creating the GRB E-Peak Distribution

All gamma-ray bursts are observed to have approximately the same characteristic gamma-ray energy. We show in this article that for bursts in the BATSE data set, this property as measured by the E-peak value is not an instrumental effect, but a physical property of gamma-ray bursts. 1. The Characteristic Photon Energy of Bursts The prompt emission of gamma-ray bursts is observed predominately at several hundred keV. This is one of the most interesting features of gamma-ray bursts, since it is not easily explained by most theories of prompt gamma-ray emission. In particular, the internal and external shock theories predict a wide variation in the characteristic gamma-ray energy both during a burst and between bursts. It is therefore imperative to know whether this is a physical characteristic of bursts, or a consequence of an instrumental effect. In this article, we discuss the instrumental effects that arise when observing gamma-ray bursts with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory (CGRO).[2] The primary points that affect the observed distribution function are the ability to correctly define the characteristic energy of the gamma-ray burst and the ability to trigger on a gamma-ray burst with a characteristic energy outside of the trigger energy range. These effects are discussed in §2 and §3 below. Models of simple distributions for the characteristic energy are given in §4, and their consistency with the observations is discussed in §5. We find that the observed distribution of characteristic energies cannot be explained through instrumental effects alone. A model distribution with a characteristic energy is fit to the observations to quantify the type of physical theory required by the observations. Our conclusion is that the existence of approximately the same characteristic energy for all gamma-ray burst spectra is a physical property of gamma-ray bursts that any viable theory must explain.