Gamma-Ray Bursts

There were 54 gamma-ray bursts detected by BATSE that were in the field of view of Milagrito while data taking was in progress. One of these bursts appears to have generated very-high-energy gamma rays that were detected by Milagrito. The first plot below shows a distribution of probabilities for the 54 bursts. The probabilities given are the probability that the observation was simply a fluctuation of the background (called the chance probability). One usually interprets a small chance probability as a signal.

Since Milagrito looked at 54 different bursts the probability given in the above graph must be multiplied by 54. So the probability that Milagrito only observed fluctuations of the background over the entire ensemble of gamma-ray bursts is 2x10^-3. The burst with the small probability is known as GRB970417a (1997, April 17), after the date is was observed. The picture below shows the event as seen by Milagrito. This burst lasted for 8 seconds.

The signal was 18 events on a background of 3.4 events. The above figure shows the 1-s (black ellipse) and 2-s (the entire region) error boxes from BATSE detector. Since the angular resolution of Milagrito is significantly better that that of BATSE we had to superimpose many search bins onto the BATSE error region. The smaller colored circles are the integration regions for Milagrito. The scale on the right gives the color coding for the number of observed events in each of the smaller integration regions.

This results has been published in the Astrophysical Journal Letters (Atkins, R., et al., 2000, Evidence for TeV Emission from GRB 970417a, Ap J Lett, Vol 533, L119.)