TITLE:   GCN CIRCULAR
NUMBER:  10910
SUBJECT: GRB 100628A: GROND optical/NIR observations of the host candidate
DATE:    10/06/29 22:04:47 GMT
FROM:    Adria C. Updike at Clemson U  <aupdike@clemson.edu>

A. Updike (Clemson University), M. Nardini, P. Afonso, A. Rau, T.
Kruehler, and J. Greiner (all MPE Garching) report on behalf of the
GROND team:

We observed the field of GRB 100628A (Immler et al., GCN 10895)
simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120,
405) mounted at the 2.2 m MPI/ESO telescope at La Silla
Observatory (Chile).

Observations started at 01:24 UT on June 29, 17.1 hours after the GRB
trigger, and continued for 1.5 hours.

In stacked images of 75 min total integration time in g'r'i'z' and 60
minutes in JHK we detect the possible host galaxy candidate G1 first
noted in Berger et al., GCN #10902 and Berger, GCN #10908 inside the
refined XRT position (Starling et al. GCN 10907) in all filters.

Preliminary photometry yields the following AB magnitudes:

g' = 22.8 +- 0.2
r' = 21.5 +- 0.1
i' = 20.6 +- 0.1
z' = 20.0 +- 0.1
J  = 19.4 +- 0.1
H  = 19.0 +- 0.1
K  = 18.6 +- 0.1

calibrated against the GROND zeropoints and 2MASS field stars, and
uncorrected for the expected mild foreground reddening in the direction
of the burst of E(B-V) = 0.17 (Schlegel et al. 1998).

Galaxy spectral template fitting using the publically available Lephare
code (Arnouts & Ilbert 2009,
http://www.cfht.hawaii.edu/~arnouts/LEPHARE/cfht_lephare/lephare.html)
provides good fits of this very red spectral energy distribution with
starburst or late type galaxies. In this interpretation, a photometric
redshift of z=0.67 (+0.34, -0.12) is indicated. We caution that the error
might be underestimated due to the missing photometric calibration in the
g'r'i'z' bands. We note, however, that at this redshift the absolute
magnitude would be rather bright, M_B ~ -22 mag.