Evidence for Diverse Optical Emission from Gamma‐Ray Burst Sources

Optical Transients from gamma-ray burst sources, in addition to offering adistance determination, convey important information on the physics of theemission mechanism, and perhaps also about the underlying energy source. As thegamma-ray phenomenon is extremely diverse, with time scales spanning severalorders of magnitude, some diversity in optical counterpart signatures appearsplausible. We have studied the Optical Transient, which accompanied thegamma-ray burst of May 8, 1997 (GRB 970508). Observations conducted at the2.5-m Nordic Optical Telescope (NOT) and the 2.2-m telescope at theGerman-Spanish Calar Alto observatory (CAHA) cover the time interval starting 3hours 5 minutes to 96 days after the high energy event. This brackets all otherpublished observations, including radio. When analyzed in conjunction withoptical data from other observatories, evidence emerges for a composite lightcurve. The first interval, from 3 to 8 hours after the event was characterizedby a constant, or slowly declining brightness. At a later moment the brightnessstarted increasing rapidly, and reached a maximum approximately 40 hours afterthe GRB. From that moment the GRB brightness decayed approximately as apower-law of index -1.21. The last observation, after 96 days, m_R =24.28+-0.10, is brighter than the extrapolated power-law, and hints that aconstant component, m_R = 25.50+-0.40 is present. The OT is unresolved (FWHM0.83") at the faintest magnitude level. The brightness of the opticaltransient, its duration and the general shape of the light curve sets thissource apart from the single other optical transient known, that of theFebruary 28, 1997 event.Comment: 13 pages, LaTeX (aaspp4.sty) 2 figures. To appear in ApJ 496 (March 20, 1998