TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  329
SUBJECT: GRB 990510: optical observations
DATE:    99/05/16 23:26:57 GMT
FROM:    Gabriele Ghisellini at Obs.Astro. di Brera  <gabriele@merate.mi.astro.it>

G. Marconi and G.L. Israel (Observ. of Rome, Italy), D. Lazzati, S. Covino 
and G. Ghisellini (Observ. of Brera, Milan, Italy) report:

A 30 min exposure taken at ESO-NTT-SUSI2 1999, May 16.10984, with a 
seeing of 1.8" shows the OT of GRB990510 at the magnitude R=23.0+-0.1
(assuming R=16.5 for the USNO star at RA = 13:38:00.82, DEC=-80:29:11.7).
The faintest detected objects in the field have R=24.5.
No candidate host galaxy can be seen. 

We have collected all available photometric data in V, R and I.
The light curve in the three bands can be fitted by the following 4 
parameters formula (similar to that given by Bloom et al. in GCN 323):
      F_nu(t) = k t^(-al1) / [1+ (t/t_*)^(al2-al1)]
where k is a normalization constant, such that the magnitude is given by
m(t)=-2.5*log(F_nu). This formula fits very well the light curve 
in the 3 bands varying only k.
 We find:  al1 = 0.85, al2 = 2.6, t_* = 1.8e5 sec = 2.08 days.

The collection of data together with the above fit and the NTT image are
posted at: 
      http://www.merate.mi.astro.it/~gabriele/990510/ 

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