Evidence for a late-time outburst of the X−ray afterglow of GB970508 from BeppoSAX

The γ−ray burst GB970508 was observed simultaneously by the Gamma Ray Burst Monitor (GRBM) and one of the X−ray Wide Field Cameras (WFC) aboard BeppoSAX. The latter provided a position within 1.9 radius. A series of follow-up observations with the Narrow Field Intruments (NFI) was then performed in a period from ∼ 6 hours to 6 days after the main event. A previously unknown source, which we associate with the afterglow of the GRB, was discovered in the error box. We find that, after the initial burst, X−ray emission is still present and decays as ∼ t up to ∼ 6× 10 s. This is followed by a burst of activity with a duration ∼ 10 s. The energy produced in this event is a substantial fraction of the total energy of the GRB, which means that the afterglow is not a remnant of the initial burst (the GRB) that fades away smoothly. Our results support the idea that the processes generating the GRB and its afterglow are the same.