D. A. Kann (TLS Tautenburg), K. Varela (MPE Garching), S. Schmidl (TLS
Tautenburg), and J. Greiner (MPE Garching) report on behalf of the GROND
We observed the field of GRB 150204A (Swift ground-analysis-detected GRB;
Cummings, GCN 17404) simultaneously in g'r'i'z'JHK with GROND (Greiner et
al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La
Observations started at 01:34 UT on February 05, 2015, 19 hrs after the
GRB trigger. They were performed at an average seeing of 1".0 and at an
average airmass of 1.5. We obtained a second epoch of imaging the
following night, 47 hours after the trigger, with an average seeing of
1".2 and at an average airmass of 1.2.
We detect five faint sources in the 5".0 error circle of "Source 2"
reported by Melandri et al. (GCN 17409), confirmed to be the X-ray
afterglow by Sbarufatti et al. (GCN 17428). They are located at:
RA (J2000) = 10:40:57.43
Dec. (J2000) = -64:02:20.3
RA (J2000) = 10:40:57.18
Dec. (J2000) = -64:02:22.7
RA (J2000) = 10:40:56.91
Dec. (J2000) = -64:02:23.1
RA (J2000) = 10:40:56.73
Dec. (J2000) = -64:02:21.9
RA (J2000) = 10:40:56.38
Dec. (J2000) = -64:02:19.2
with typical errors of 0".5 in each coordinate.
None of these source vary between 20 and 48 hours after the GRB, which we
also confirm via image subtraction; therefore they are likely not
associated with the GRB. The field is crowded, and especially in the NIR,
a part of the error circle is affected by scattered light from a nearby
bright star to the east.
Based on total exposures of 125 minutes in g'r'i'z' and 100 minutes in
JHK, at a midtime of 20.3 hrs after the burst, we measure the following 3
sigma upper limits (all AB magnitude system):
g' > 23.5 mag,
r' > 23.8 mag,
i' > 23.4 mag,
z' > 23.0 mag,
J > 21.1 mag,
H > 20.9 mag,
K > 19.8 mag.
The magnitudes are calibrated against an SDSS standard star field obtained
immediately after the second-epoch observations (g'r'i'z') and 2MASS field
stars (JHK) and are not corrected for the Galactic foreground extinction
corresponding to a reddening of E_(B-V)= 0.82 mag in the direction of the
burst (Schlegel et al. 1998).