The X-ray afterglow of dark GRB 970815: A common origin for gamma-ray bursts and X-ray flashes?

GRB 970815 is a well- localized gamma- ray burst ( GRB) detected by the All- Sky Monitor ( ASM) on the Rossi X- Ray Timing Explorer ( RXTE) for which no afterglow was identified despite follow- up ASCA and ROSAT pointings and optical imaging to limiting magnitude R > 23. Although an X- ray source, AX/ RX J1606.8+ 8130, was detected just outside the ASM error box, it was never associated with the GRB because it was not clearly fading and because no optical afterglow was ever found. We recently obtained an upper limit for this source with Chandra that is at least a factor of 100 fainter than the ASCA detection. We also made deep optical observations of the AX/ RX J1606.8+ 8130 position, which is blank to limits V > 25: 2 and I > 24: 0. In view of these extreme limits, we conclude that AX/ RX J1606.8+ 8130 is indeed the afterglow of GRB 970815, which corresponds to an optically " dark'' GRB. AX/ RX J1606.8+ 8130 can therefore be ruled out as the counterpart of the persistent EGRET source 3EG J1621+ 8203. The early light curves from BATSE and the RXTE ASM show spectral softening between multiple peaks of prompt emission. We propose that GRB 970815 might be a case in which the properties of an X- ray flash and a " normal'' GRB coincide in a single event.