GROND upper limits for MASTER OT103630.69-003523.8

ATel #3848; Jochen Greiner, Arne Rau, Felipe Olivares E (all MPE Garching)
on 6 Jan 2012; 23:39 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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Following the report of the discovery of MASTER OT103630.69-003523.8 (Lipunov et al. 2012, ATel #3844) we initiated simultaneous g'r'i'zJHK GROND (Greiner et al. 2008, PASP, 120, 405) on January 5th, 2012, at 06:53 UT. We do not detect the source to the following limits (all in AB):

g' > 24.7
r' > 24.5
i' > 23.5
z' > 23.1
J > 18.9
H > 19.5
Ks > 19.4

Magnitudes were calibrated against SDSS in g'r'i'z' and 2MASS field stars in JHK and are not corrected for Galactic foreground extinction of E(B-V)=0.22 (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 (~6.3 days). While this is in principle compatible with the typical decay of a GRB afterglow (slope steeper than t^-1), the lack of reported gamma-ray emission (Lipunov et al. 2012, ATel #3844) makes this scenario less likely.

In contrast, and as argued already by Rau et al. (2006, A&A, 449, 79) and Kulkarni & Rau et al. (2006, ApJ, 644, L63), flares from Galactic M-dwarfs are numerous and dominate the optical transient sky on short time scales. The fast rise (<30 min) and the optical amplitude of >6 mag are in agreement with the M-dwarf scenario. The GROND upper limits are consistent with an M5-8V counterpart within 700 pc.