TITLE:   GCN CIRCULAR
NUMBER:  7327
SUBJECT: ESO optical and NIR observations of GRB 080218B
DATE:    08/02/22 16:51:29 GMT
FROM:    Paul Vreeswijk at Dark Cosmology Centre,U.of Copenhagen  <pmv@dark-cosmology.dk>

Paul Vreeswijk, Johan Fynbo, Bo Milvang-Jensen, Daniele Malesani,
Jens Hjorth (DARK/NBI), Pall Jakobsson (U. Hertfordshire),
Andreas Jaunsen (U. Oslo), Nial Tanvir (U. Leicester) report
on behalf of a larger collaboration:

The Swift localization of GRB 080218B (Schady et al., GCN 7314) was
observed with FORS2 mounted at the VLT Antu telescope on Cerro Paranal
in Rapid-Response Mode, and with SOFI at the NTT on La Silla. Starting
at Feb 19, 1:03 UT (roughly 1 hour after the burst) VLT/FORS2
performed the imaging sequence R-I-z-V-B-R (30-150s per filter)
through thin cirrus, at an airmass of 2.5, and with an effective
seeing of 1.5". NTT/SOFI imaged the afterglow location at an airmass
of 1.3-1.5 and in 1.4" seeing conditions in Ks-J-H-Ks (10 min. per
filter), starting at Feb 19, 2:52 UT.

No object is detected within the enhanced XRT error circle (Osborne et
al., GCN 7320) down to the following limiting magnitudes (2 sigma):
B>22.1, V>22.7, R>22.9, I>22.6, J>20.6, H>20.1 and Ks>19.4.  The
corresponding upper limit on the optical to X-ray spectral slope,
beta_OX, is 0.24, rendering the burst dark according to the definition
proposed by Jakobsson et al. (2004, ApJ, 617, L21). A spectrum of an
object just outside the XRT error circle at R.A. 11:51:49.25,
decl. -53:05:50.1 (J2000), shows it to be a Galactic star of type M6V.

We acknowledge the excellent support of ESO's night astronomers at La
Silla Paranal Observatory, and in particular that of Linda
Schmidtobreick, Colin Snodgrass, Stephane Brillant and Lorenzo Monaco.