Discovery of the Optical Transient of the Gamma Ray Burst 990308

The optical transient of the faint Gamma Ray Burst 990308 was detected by the QUEST camera on the Venezuelan 1-m Schmidt telescope starting 3.28 hours after the burst. Our photometry gives $V = 18.32 \pm 0.07$, $R = 18.14 \pm 0.06$, $B = 18.65 \pm 0.23$, and $R = 18.22 \pm 0.05$ for times ranging from 3.28 to 3.47 hours after the burst. The colors correspond to a spectral slope of close to $f_{\nu} \propto \nu^{1/3}$. Within the standard synchrotron fireball model, this requires that the external medium be less dense than $10^{4} cm^{-3}$, the electrons contain $> 20$ of the shock energy, and the magnetic field energy must 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 132-1029s (with LOTIS), $V > 15.3$ at 28.2 min (with Super-LOTIS), and a 8.5 GHz flux of $< 114 \mu Jy$ at 110 days (with the Very Large Array). WIYN 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 red shift of $\sim 1.2$.Comment: ApJ Lett submitted, 5 pages, 2 figures, no space for 12 coauthor

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 ‫ע‬ 0.07 , , and for times ranging from 3.28 to 3.47 hr after the R = 18.14 ‫ע‬ 0.06 B = 18.65 ‫ע‬ 0.23 R = 18.22 ‫ע‬ 0.05 burst. The colors correspond to a spectral slope of close to . Within the standard synchrotron fireball f ∝ n n model, this requires that the external medium be less dense than 10 4 cm Ϫ3 , 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 at 132 s (with LOTIS), V 1 12.0 V 1 13.4 from 132 to 1029 s (with LOTIS), at 28.2 minutes (with Super-LOTIS), and a 8.5 GHz flux of less V 1 15.3 than 114 mJy 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 and . The lack of a host R 1 25.7 K 1 23.3 galaxy likely implies that it is either substantially subluminous or more distant than a redshift of ∼1.2