Discovery of the Optical Transient of GRB 990308

Abstract

The optical transient of the faint gamma-ray burst GRB 990308 was detected by the QUEST camera on the Venezuelan 1 m Schmidt telescope starting 3.28 hr after the burst. Our photometry gives V=18.32+/-0.07, R=18.14+/-0.06, B=18.65+/-0.23, and R=18.22+/-0.05 for times ranging from 3.28 to 3.47 hr after the burst. The colors correspond to a spectral slope of close to f$_ ν$åisebox-0.5ex ensuremat hν$^1/3$. Within the standard synchrotron fireball model, this requires that the external medium be less dense than 10$^4$ cm$^-3$, the electrons contain more than 20% of the shock energy, and the magnetic field energy be less than 24% of the energy in the electrons for normal interstellar or circumstellar densities. We also report upper limits of V>12.0 at 132 s (with LOTIS), V>13.4 from 132 to 1029 s (with LOTIS), V>15.3 at 28.2 minutes (with Super-LOTIS), and a 8.5 GHz flux of less than 114 μJy at 110 days (with the Very Large Array). Wisconsin-Indiana-Yale-NOAO 3.5 m and Keck 10 m telescopes reveal this location to be empty of any host galaxy to R>25.7 and K>23.3. The lack of a host galaxy likely implies that it is either substantially subluminous or more distant than a redshift of i̊sebox-0.5ex 1.2.

Publication
Astrophysical Journal Letters

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