S. Schmidl (TLS Tautenburg), J. Elliott (MPE Garching), A. Nicuesa
Guelbenzu (TLS Tautenburg), S. Klose (TLS Tautenburg), and J. Greiner
(MPE Garching) report on behalf of the GROND team:
We observed the field of GRB 130315A (Swift trigger 551126; Evans et
al., GCN 14311) simultaneously in g'r'i'z'JHK with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla
First-epoch observations started on March 15, 2013, at 23:51 UT, 11.1
hrs after the GRB trigger. They were performed at an average seeing of
0."7 and at an average airmass of 1.18. No evidence for a fading
afterglow was found.
Second-epoch observations were executed the following night (38.4 hr
after the trigger) under slightly worse observing conditions. A
comparison with the first-epoch images reveales a fading source at the
south-east border of the Swift/XRT error circle (Page et al., GCN 14317)
RA (J2000.0) = 10:30:11.54
Dec (J2000.0) = -51:47:39.8
with an uncertainty of 0."5 in each coordinate.
Based on a total exposure time of 1500 s in r'i'z' at a midtime of
12.8 hrs after the burst, for this source we measure the following
preliminary AB magnitudes:
r' = 22.0 +/- 0.1,
i' = 21.5 +/- 0.1,
z' = 21.1 +/- 0.1.
Between both epochs this object faded by more than 1 magnitude, implying
a decay with alpha > 0.75.
The given magnitudes are calibrated against GROND zeropoints. The
Galactic reddening along the line of sight is E(B-V)= 0.36 mag (Schlegel
et al. 1998).