GCN Circular 92

GRB 980519 Optical Observations

R. R. Gal, J. S. Bloom, S. G. Djorgovski, and S. R. Kulkarni on behalf
of the Caltech GRB Collaboration, report:

"In addition to the I and R photometry reported by Bloom et al. (GCN
#87), we have photometered the B and V Palomar 200inch images of the
field of GRB 980519.  Derived magnitudes of the following 7 secondary
stars are in the Landolt BV system (see GCN #87 for details):

Star      B    err(B)    V    err(V)
A       19.51  0.13    18.31  0.12
B       21.74  0.34    20.05  0.09
C       19.27  0.11    18.35  0.08
E       22.88  0.11    21.07  0.24
F       22.61  0.17    20.83  0.13
G       21.51  0.19    19.74  0.10
H       21.94  0.29    20.26  0.13


The derived magnitudes of the OT are:

         May 20.4485 UT, B = 22.53 +/- .14
         May 20.466  UT, V = 21.74 +/- .16
             
         May 21.448 UT, B > 22.9 (3-sigma upper limit)
         May 21.476 UT, V > 22.0 (3-sigma upper limit) 

The non-detection on May 21 is consistent with the power-law decline
slope of beta = -1.98 reported in Djorgovski et al. (GCN #79).
Moreover, broadband colors circa May 20.4 (BV reported herein; RI
reported in Bloom #87) suggest a spectral index of alpha = -1.26 +/-
0.3. This is also consistent with the simple blastwave model
(eg. Wijers, Rees and Meszaros MNRAS 288, L51.) prediction that alpha
= (2/3) beta.  The V-band point is somewhat higher (~3 sigma) than the
extrapolated spectrum, suggesting the spectrum of the transient may
not have been a pure power-law.  Lastly, given the general consistency
of spectral slope with flux time decay, we note the absence of strong
extinction in the OT (in contrast to GRB 980329).

This report may be cited."