GCN Circular 313

T.J. Galama, P.M. Vreeswijk, E. Rol (U. of Amsterdam), N. Tanvir
(Institute of Astronomy, Cambridge, UK), E. Palazzi, E. Pian,
N. Masetti, F. Frontera (ITESRE, CNR, Bologna), K. Pollard (U. of
Canterbury), J. Menzies (SAAO), P. Sackett (U. of Groningen), K. Sahu
(STScI), J. van Paradijs (U. of Amsterdam and U. of Alabama in
Huntsville), and C. Kouveliotou (USRA/MSFC) report on behalf of the
PLANET microlensing team, and the Bologna and Amsterdam/Huntsville GRB
optical follow-up teams:

Comparison of European Southern Observatory 2.2m and SAAO Sutherland
1.0m telescope (South Africa) R-band observations, of the error box of
GRB 990510 (Dadina et al. 1999; IAU Circular 7160), shows that the
proposed optical counterpart to GRB 990510 (Vreeswijk et al., GCN
#310) is decaying in brightness. We obtain the following magnitudes
(measured with respect to the USNO star at RA = 13:38:00.82, 
DEC=-80:29:11.7, for which we have assumed R = 17.2):

	R mag	  err	May (UT)   Telescope	

	19.11    0.02    10.73	   SAAO 1m
	19.17    0.02    10.76	   SAAO 1m
	19.25    0.02    10.79     SAAO 1m
 	19.61    0.01    10.99     ESO 2.2m

The absolute uncertainty in the calibration is not included. The decay
is consistent with a power law, with temporal decay index -0.85 +- 0.04.

The light curve can be found at http://www.astro.uva.nl/~titus/grb990510.html

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