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 (jcg@mpe.mpg.de)

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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 interpretation borderline.

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.