WFC Science highlights - Gamma-ray burst 970228

The gamma ray burst, GRB 970228, was discovered simultaneously in the data from the Gamma Ray Burst Monitor and Wide Field Cameras onboard by the team of italian/dutch scientists at 05:00 am on Friday 28th february who continually monitor the satellite data at the Scientific Operations Center in Nuova Telespazio, Rome.

Three time intervals around GRB 970228 were imaged with WFC data: 23,400 sec before, 135 sec during, and 24,500 sec after the burst. The Crab nebula was in the same field of view and is visible in all three images. GRB 970228 flashes only during the 135 sec interval. The overlayed graph in the middle image shows the photon count rate of the WFC during the burst, the background is responsible for the general rise of the rate.

On the basis of experience gained in January 1997 (when a similar gamma-ray burst was observed in the constellation of Serpens by BeppoSAX, and studied in detail 16 hours later, in comparison with the previous record for a X-ray observation of about 18 days) the italian scientists responsible for the gamma-ray burst detector onboard the satellite (E. Costa, CNR, and F. Frontera, University of Ferrara), in conjunction with the Mission Scientist (L. Piro, CNR) and the Mission Director (R. C. Butler, ASI), were able to reschedule the satellite observations and point the BeppoSAX narrow field X-ray telescopes in only 8 hours at the gamma-ray burst source. In consequence a X-ray source never before seen was discovered and localized with an accuracy of one hundreth of a degree. The source is actually in the constellation of Orion.

A second follow-up has been performed with narrow field instruments after about 2 days. This second observation has shown a strong drop, about 20 times lower, in the source flux. The pictures taken during the two follow-up observations are shown in the figure below.

The bright source in the field is the new X-ray source

1SAX J0501.7+1146

associated with the Gamma Ray Burst GRB 970228.
SAX J0501.7+1146 was detected by the MECS and LECS at the same position (R.A. = 5h01m44s, Decl. = +11o46'.7, equinox 2000.0; estimated error radius 50"). This position lies at the edge of the reported BeppoSAX WFC error box, see following figure

The optical counterpart was detected at the intersection of the three error boxes of the gamma-ray burst and X-ray afterglow.

The BeppoSAX have been published in a Nature paper by Enrico Costa and collaborators (Costa et al. 1997, Nature 387, 783).

Jean in 't Zand, SRON, March 12, 1999