TITLE:   GCN CIRCULAR
NUMBER:  6178
SUBJECT: GRB 070306: NOT observations
DATE:    07/03/07 15:24:07 GMT
FROM:    Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center  <malesani@astro.ku.dk>

D. Malesani (DARK/NBI), A.O. Jaunsen (Univ. Oslo), C.C. Thoene, J. Hjorth
(DARK/NBI), D. Paraficz (NOT), E. Leitet, and B. Caldwell (Univ. Uppsala),
report on behalf of a larger collaboration:

We observed the field of GRB 070306 (Pandey et al., GCN 6169) with the NOT
equipped with StanCam. Imaging was carried out in the I filter, starting
on Mar 6.946 UT (6 hr after the GRB), with a seeing of 1". The object
visible in the SDSS (SDSS J095223.31+102855.4) and reported by Rol et al.
(GCN 6174) and Levan et al. (GCN 6176) is seen in our images.

Preliminary photometry calibrated against the SDSS i-band magnitudes (Cool
et al., GCN 6170) provides i(AB) = 22.65+-0.30 (mean time Mar 6.954 UT).
This is consistent, within the large errors, with the value measured in
the SDSS for this object:  i(AB) = 22.8+-0.4, and indicates little
contribution from the afterglow in the I band at the epoch of our
observation.

Comparison with the magnitudes reported by Rol et al. (GCN 6174) further
suggests that either the host galaxy, or the afterglow, or both, are quite
red.

Using the X-ray flux at 6 hr from Nat Butler's web page
(http://astro.berkeley.edu/~nat/swift/00263361/bat_xrt.jpg; see also Page
et al., GCN 6172), we can measure a limit on the broad-band spectral index
beta_OX < 0.2 at 6 hr after the GRB. According to the criterion by
Jakobsson et al. (2004, ApJ, 617, L21), this burst is classified as dark.
The detection of the relatively bright host in the SDSS g filter and the
presence of a large absorbing column in the X-ray spectrum (Page et al.,
GCN 6172; Grupe et al. 2007, AJ, in press; astro-ph/0612104) imply the
redshift is not very large. Therefore this afterglow was likely suffering
significant dust extinction.