Swift J1741.5-6548: GROND observation

ATel #4906; D. A. Kann (TLS Tautenburg), R. Filgas (CTU Prague), S. Schmidl, S. Klose (both TLS Tautenburg), M. Tanga, J. Greiner (both MPE Garching)
on 21 Mar 2013; 20:39 UT
Credential Certification: J. Greiner (jcg@mpe.mpg.de)

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We obtained simultaneous imaging in g'r'i'z'JHK of the new transient Swift J1741.5-6548 (Krimm et al., ATel #4902) with GROND (Greiner et al. 2008, PASP, 120, 405) at the 2.2m MPG/ESO telescope at La Silla Observatory (Chile), beginning at 09:11 UT on 2013-03-21, for a total of 920 s in g'r'i'z' and 960 s in JHK, under 1".1 seeing and airmass 1.3.

We clearly detect the bright UVOT source at the edge of the XRT error circle (Krimm et al., ATel #4902) with the following magnitudes:

g' = 18.8 +/- 0.1

r' = 19.2 +/- 0.1

i' = 19.0 +/- 0.1

z' = 19.1 +/- 0.1

J = 19.4 +/- 0.1

H = 19.7 +/- 0.1

K = 20.6 +/- 0.5.

These magnitudes are in the AB system and not corrected for Galactic reddening of E(B-V)=0.06 mag (Schlafly & Finkbeiner 2011, ApJ, 737, 103) along the line of sight. Photometry was obtained against GROND zero points in g'r'i'z' and against 2MASS field stars in JHK.

As stated in Atel #4902, there is a source at this position seen in the DSS. We find this source is only seen in the DSS2 Blue image, and not detected in Red and Infrared. It is not detected in the USNO B1.0 catalog. Comparison with two stars to the east shows the source must have brightened by 1.5 - 2 mag in g'r'. We therefore propose this source to be the optical counterpart of SwiftJ1741.5-6548.

After correcting for reddening, we find the source to be very blue, with a spectral slope (F_nu ~ nu^-beta) beta = 0.69 +/- 0.17. This is suggestive of an accretion disk spectrum.