Swift J1910.2-0546: GROND discovery of a candidate optical/near-IR counterpart

ATel #4144; Arne Rau, Jochen Greiner, Patricia Schady (all MPE)
on 1 Jun 2012; 18:05 UT
Distributed as an Instant Email Notice Transients
Credential Certification: J. Greiner (jcg@mpe.mpg.de)

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We observed the new transient Swift J1910.2-0546 / MAXI1910-057 (Krimm et al. 2012, ATel #4139; Usui et al. 2012, ATel #4140) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) at the 2.2m MPG/ESO telescope at La Silla Observatory (Chile). Observations started on June 1st 08:48 UT and lasted for ~40min. Within the 3 arcmin Swift/BAT error circle (Krimm et al. 2012, ATel #4139) we detect a new bright point source at

 
RA(J2000)  = 19:10:22.79 (287.5949 deg) 
Dec(J2000) =-05:47:46.3  (-5.7990 det) 

with an uncertainty of 0.3" in both coordinates. This source has no 2MASS or DSS counterpart; the outburst amplitude in the optical thus is at least 6 magnitudes. We measure the following magnitudes (all in the AB system):

 
g' = 16.0 +/- 0.1 
r' = 15.7 +/- 0.1 
i' = 15.6 +/- 0.1 
z' = 15.3 +/- 0.1 
J = 15.5 +/- 0.1 
H = 15.5 +/- 0.1 
K = 15.6 +/- 0.1 

These magnitudes were derived by calibrating the images against observations of an SDSS field observed in the same night (g'r'i'z') and 2MASS field stars (JHK).
The extinction-corrected (E(B-V)=0.6; Schlegel et al. 1998) g'-K spectral energy distribution is very blue (F_lambda approx lambda^-3) consistent with an accretion disk spectrum.
We note that the ROSAT source RXS J191024.6-054700 mentioned by Krimm et al. (ATel #4139) is 62 arcsec away form the optical/near-IR source and thus unlikely to be the counterpart for Swift J1910.2-0546/MAXI1910-057.
We encourage optical spectroscopy to identify the nature of this bright transient source.