Follow-up observations of MASTER J113122.95-075714.5
ATel #3906; J. Greiner, A. Rau, P. Schady (all MPE Garching)
on 2 Feb 2012; 09:34 UT
Credential Certification: J. Greiner (firstname.lastname@example.org)
Subjects: Infra-Red, Optical, X-ray, Gamma-Ray Burst, Transient
Referred to by ATel #: 3990
The rapid transient MASTER J113122.95-075714.5 was found on two
synchronous images at 17.6 mag (white light) on Jan. 30, 2012 00:05 UT,
with non-detections (fainter than 19.4) about 45 min earlier
(Balanutsa et al. 2012, ATel \#3898).
We observed MASTER J113122.95-075714.5 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 (Chile). The observations were performed
on Feb. 1, 2012, between 7:45-8:00 UT under good conditions. The
total integration time is 460 s in the optical and 480 seconds in the NIR.
We do not detect any source within 5 arcsec of the position as
reported in ATel \#3898, down to limiting magnitudes (all AB):
g' >23.5 mag
r' >23.5 mag
i' >23.3 mag
z' >23.3 mag
J >21.3 mag
H >20.9 mag
K >19.9 mag
Magnitudes were calibrated against GROND zeropoints in g'r'i'z' and 2MASS
field stars in JHK and are not corrected for Galactic foreground extinction
of E(B-V)=0.03 (Schlegel et al. 1998).
The deep optical limits indicate that the transient has faded by >6 mag in
the R-band between the MASTER discovery and the GROND epoch (~2.3 days).
A 2.97 ksec Swift observation was obtained starting on Feb. 1, 2012, at
19:51 UT. No X-ray source (in fact not a single X-ray photon) is detected
at or near the MASTER position, with a conservative upper limit of 1.3E-3 cts/s.
With a fast moving object (satellite) excluded by the point-like shape
in the two synchronous images, and a slow moving object (minor planet,
asteroid) by the non-detection 45 min earlier, such rapid optical
variability could be characteristic for a late-type flare star.
which are numerous and dominate the optical transient sky on short time scales
(Rau et al. 2006, A&A 449, 79; Kulkarni & Rau et al. 2006, ApJ 644, L63).
However, our GROND upper limits in the NIR imply a distance of an assumed M4-M8
star larger than 1.0-2.5 kpc (Galactic Latitude b=50 deg), making this
Alternatively, MASTER J113122.95-075714.5 could be an untriggered
(Balanutsa report no triggers by Swift, Fermi, INTEGRAL, or Wind-KONUS)
or orphan afterglow of a gamma-ray burst (GRB). Conservatively assuming
the putative GRB to have happened at 30 min before the (few minute duration)
MASTER exposure, our GROND limits only imply a decay slope steeper than
t^-1.3, well within the range of typical GRBs. The Swift/XRT non-detection
at 2.4E+05 sec after the event excludes the majority of Swift X-ray afterglows
of long GRBs, but is compatible with the 10-15% faintest afterglows
(Nousek et al. 2006, ApJ 642, 389, Evans et al. 2009, MN 397, 1177).
Based on our GROND images, we note the existence of a clearly extended (~10"
diameter), nearby (z<0.3), blue galaxy just 10" away from the MASTER position.
We are grateful to the Swift team, in particular N. Gehrels
and M. Chester for the rapid approval, scheduling and execution
of the Swift observation.