TITLE:   GCN CIRCULAR
NUMBER:  15177
SUBJECT: GRB 130903A: GROND Afterglow Discovery
DATE:    13/09/04 15:02:41 GMT
FROM:    Alexander Kann at TLS Tautenburg  <kann@tls-tautenburg.de>

M. Tanga, V. Sudilovsky, D. A. Kann, and J. Greiner (all MPE Garching)
report on behalf of the GROND team:

We re-observed (Sudilovsky et al., GCN #15170) the field of GRB 130903A
(Mereghetti et al. 2013, GCN #15169) simultaneously in g'r'i'z'JHK with
GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG/ESO
telescope at La Silla Observatory (Chile).

Observations are centered at 09:19 UT on 04 Sep 2013, 35.53 hours after
the GRB trigger, with a total exposure time of 50 min in g'r'i'z' and 40
min in JHK. They were performed at an average seeing of 1".1 and an
average airmass of 1.3.

In the XRT error circle given by Grupe & Holland (GCN #15173), we detect a
source in our first epoch observations (Sudilovsky et al., GCN #15170) at
coordinates

RA (J2000) = 05:28:31.33
Dec. (J2000) = -00:07:38.4

with an error of 0".3 arcseconds.

For the first epoch, we derive preliminary magnitudes (AB system) of:

g' = 22.4 +/- 0.1,
r' = 22.2 +/- 0.1,
i' = 22.0 +/- 0.1,
z' = 21.7 +/- 0.1,
J > 21.6,
H > 20.8, and
K > 20.1.

The source is still detected in g'r'i'z' in the second epoch, but is seen
to have faded by 1 mag in the r' band. We therefore conclude that this is
the afterglow of GRB 130903A.

Magnitudes were obtained against the SDSS catalog in g'r'i'z' and against
the 2MASS catalog in JHK. They have not been corrected for the expected
Galactic extinction of E(B-V) = 0.14 mag in the direction of the burst
(Schlegel et al. 1998).