Evidence for Postquiescent , High - Energy Emission from Gamma - Ray Burst 990104

It is well known that high-energy emission (MeV-GeV) has been observed in a number of gamma-ray bursts, and temporally-extended emission from lower energy gamma rays through radio wavelengths is well established. An important observed characteristic of some bursts at low energy is quiescence: an initial emission followed by a quiet period before a second (postquiescent) emission. Evidence for significant high-energy, postquiescent emission has been lacking. Here we present evidence for high-energy emission, coincident with lower energy emission, from the postquiescent emission episode of the very bright and long burst, GRB 990104. We show light curves and spectra that confirm emission above 50 MeV, approximately 152 seconds after the BATSE trigger and initial emission episode. Between the initial emission episode and the main peak, seen at both low and high energy, there was a quiescent period of ∼100 s during which the burst was relatively quiet. This burst was found as part of an ongoing search for high-energy emission in gamma-ray bursts using the EGRET fixed interval (32 s) accumulation spectra, which provide sensitivity to later, high-energy emission that is otherwise missed by the standard EGRET BATSE-triggered burst spectra. 1. introduction Over a period of nine years, the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) detected ∼2700 gamma-ray bursts 1 at a rate of approximately one per day (Meegan et al. 1992). Only a few are known for MeV-GeV emission From two of these bursts, EGRET detected high-energy photons after low-energy emission ended: from GRB 940217 EGRET observed a ∼20 GeV photon, which arrived approximately 75 minutes after the cessation of low-energy emission (Hurley et al. 1994); and from GRB 930131, EGRET observed MeV emission that continued for some time after the end of low-energy emission (Sommer et al. 1994). Indeed, MeV emission from some bursts is coincident with low-energy emission, while from some bursts it is not (Dingus, Catelli, & Schneid 1998). It is also known that bursts with a qui-escent period between an initial emission episode (prequi-escent) and a latter emission episode (postquiescent) are not uncommon (Ramirez-Ruiz & Merloni 2001). However, significant high-energy spectral information has previously only been observed during prequiescent episodes (GRB 910503; Schneid et al. 1992), and high-energy spectral information from postquiescent emission has been lacking. 2 This was due, in part, to data analysis choices that limited the emission search time intervals to those that were shorter than the …