IGRJ17062-6143: GROND observations of the optical/near-IR counterpart

ATel #4214; Arne Rau, Patricia Schady (both MPE Garching), D. Alexander Kann (TLS Tautenburg), and Jochen Greiner (MPE Garching)
on 26 Jun 2012; 16:10 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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We observed the counterpart of Swift trigger 525148 / IGR J17062-6143 (Barthelmy et al. 2012, GCN #13386) 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 25th 22:52 UT and lasted for ~22min. We clearly detect the optical counterpart already reported by Ricci et al. (2008, ATel #1840) for an earlier outburst in 2008 and confirm the rapid fading found by Ivarsen et al. (2012, GCN #13390). For two 4min observations, with midtimes 13.4 and 27.6min after the Swift trigger time (22:42:31 UT), we measure the following magnitudes (all in the AB system):

midtime = 2012-06-25 22:55.52 UT
g' = 18.0 +/- 0.1
r' = 18.4 +/- 0.1
i' = 18.7 +/- 0.1
z' = 18.8 +/- 0.1
J = 19.1 +/- 0.1
H = 19.6 +/- 0.2
K = >18.8

midtime = 2012-06-25 23:10:06 UT
g' = 18.6 +/- 0.1
r' = 19.0 +/- 0.1
i' = 19.2 +/- 0.1
z' = 19.4 +/- 0.1
J = 19.8 +/- 0.1
H = >19.6
K = >19.0

calibrated against GROND zeropoints in g'r'i'z' and 2MASS field stars in JHK. Upper limits are 4.5sigma.

The extinction-corrected (E(B-V)=0.13mag; Schlafly et al. 2011) spectral energy distribution is very blue (F_lambda approx lambda^-3) consistent with an accretion disk spectrum.

We encourage optical spectroscopy to identify the nature of this transient source.