Optical Observations of GRB 970508:                               #030

J. S. Bloom, S. R. Kulkarni, S. G. Djorgovski, Caltech and
D. A. Frail, NARO report on behalf of the Caltech GRB effort:

"Optical follow-up of GRB 970508 on the Keck-II 10m was conducted on
the nights of Nov 28-29, 1997 and Feb 22-23, 1998. In both cases,
imaging observations were obtained with the Low Resolution Imaging
Spectrograph (LRIS).  Coincident with the position of the optical
transient, a faint source is easily detected in both B- and R-bands.
Differencing photometry with a number of field objects provided by V.
Sokolov (see their paper at http://xxx.lanl.gov/astro-ph/9802341),
suggest that the light curve has levelled off from the initial
power-law decline seen from 2-100 days from the time of the burst
(Sokolov et al. ibid).

We find:
B (UT Nov 29.6, 1997) = 26.32 +/- 0.26 mag (integ., 2400s),
B (UT Feb 23.4, 1998) = 26.27 +/- 0.14 mag (integ., 2400s),
R (UT Nov 28.6-29.5, 1997) =  25.09 +/- 0.14 mag (integ., 5400s), and
R (UT Feb 22.4, 1998) =  25.29 +/ 0.16 mag. (integ., 3600 s).
The systematic uncertainty of the Sokolov photometric zero-point (0.05
mag) is not included in the error analysis.  Had the power-law decline
continued the predicted brightness at the time of observations would be
[B,R] 26.65,25.63 (Nov) and 27.11,26.07 (Feb).  The 1-sigma errors for
the power-law light curve are estimated at 0.25 and 0.2 for B and R,
respectively, at each epoch. At the second epoch, then, the source is
significantly brighter then predicted by ~0.8 mags in each band.

Thus, although the object does not appear extended (consistent
with the HST findings of Fruchter et al. IAUC 6674) beyond that which
is expected from the seeing (~0.8"), we interpret the level-off as due
to the faint host galaxy--itself responsible for the persistent [OII]
emission. 

In fact, the lack of extent of the galaxy may be connected with its
previously inferred underluminous nature (Fruchter et al., and
Natarajan et al. 1997).  Furthermore, the B-R color (+1) is consistent
with an irregular galaxy at z=0.8 (Fukugita et al. 1995).

We thank V. Sokolov for collegial help in providing photometry of
secondary stars fainter than those reported in their Huntsville
poster.

This circular is a citable reference."