GRB 050717: A Long, Short-Lag, High Peak Energy Burst Observed by Swift and Konus

Abstract

The long burst GRB 050717 was observed simultaneously by the Burst Alert Telescope (BAT) on Swift and the Konus instrument on Wind. Significant hard to soft spectral evolution was seen. Early gamma-ray and X-ray emission was detected by both BAT and the X-Ray Telescope (XRT) on Swift. The XRT continued to observe the burst for 7.1 days and detect it for 1.4 days. The X-ray light curve showed a classic decay pattern; the afterglow was too faint for a jet break to be detected. No optical, infrared or ultraviolet counterpart was discovered despite deep searches within 14 hours of the burst. Two particular features of the prompt emission make GRB 050717 a very unusual burst. First, the peak of the nu Fnu spectrum was observed to be 2401(-568/+781) keV for the main peak, which is the highest value of Epeak ever observed. Secondly, the spectral lag for GRB 050717 was determined to be 2.5 +- 2.6 ms, consistent with zero and unusually short for a long burst. This lag measurement suggests that this burst has a high intrinsic luminosity and hence is at high redshift (z > 2.7). Despite these unusual features GRB 050717 exhibits the classic prompt and afterglow behaviour of a gamma-ray burst.