[vsnet-grb-info 4633] GRB 070508: Multicolor optical observations and possible redshift

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6389
SUBJECT: GRB 070508: Multicolor optical observations and possible redshift
DATE:    07/05/08 13:33:42 GMT
FROM:    Christina Thoene at Niels Bohr Institute,DARK Cosmo Ctr  <cthoene at astro.ku.dk>

Christina C. Thoene, Johan P. U. Fynbo (DARK) and Andrew Williams (Perth
Observatory) report:

We observed the OT of GRB 070508 (GCN 6383, Grupe et al.) with DFOSC at
the Danish 1.54m on La Silla in V, R and I band, starting at May 8, 08:15
UT (3.8h after the burst) until sunrise.
The burst is clearly detected in 300s exposures in R and I band, but
undetected in 600s V band images. At a mean time of 4.1h after the burst,
we measure the following magnitudes (based on preliminary photometric
zeropoints):

I = 20.5 +- 0.2
R = 21.5 +- 0.2
V > 22.9

which indicates a red color. The nondetection in V together with the red
color might suggest a redshift of z > 3.5.
The afterglow is clearly fading between 3.8h and 5.8h after the burst,
showing a rather steep temporal slope in the I band of alpha = 3.1 +-0.6.