IGR J19566+0326: GROND optical/near-IR counterpart discovery

ATel #7524; Arne Rau, Rob Yates, Jochen Greiner (all MPE Garching)
on 18 May 2015; 09:59 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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We observed the position of the new INTEGRAL transient IGR J19566+0326 (Cherepashchuk et al., ATel, #7506) on May 15th (07:11 UT) and May 16th (07:41 UT) with GROND (Greiner et al. 2008, PASP, 120, 405) mounted at the 2.2m MPG telescope at ESO's La Silla observatory. Observations were performed simultaneously in g'r'i'z'JHK and with integration times of 7.7min in g'r'i'z' and 8min in JHK in each night.

Within the 3.5" radius XRT error circle (Cherepashchuk et al., ATel, #7506) we find a single source located at

RA(J2000) = 19:56:39.11 (299.16296 deg)
Dec(J2000) = 03:26:43.7 ( 3.44547 deg)

and with an astrometric solution accuracy of 0"2 in both coordinates.

On May 15th we measured the following AB magnitudes

g' = 20.28 +/- 0.15
r' = 20.03 +/- 0.14
i' = 19.64 +/- 0.15
z' = 19.84 +/- 0.14
J = 19.75 +/- 0.14
H = 19.93 +/- 0.23
K = >19.0

calibrated against USNO B1 in g'r'i'z' and 2MASS field stars in JHK and not corrected for the expected Galactic foreground reddening of E(B-V)=0.11mag (Schlafly et al. 2011).

In the second epoch, on May 16th, the source had faded to

r'= 21.2 +/- 0.14.

The quick decay, similar to the one observed in X-rays by Molkov et al. (ATel #7520), together with the blue spectral energy distribution suggests the source to be the optical/near-IR counterpart to IGR J19566+0326.

We note, that the coordinates of the proposed counterpart lie 3.4" from the object listed in the USNO B1 catalog at RA(J2000)=19:56:38.92, Dec(J2000)=03:26:41.8 and proposed by Cherepashchuk et al. (ATel, #7506) as the potential counterpart. There is no object detected in the GROND images at the position of the UNSO B1 source down to a 3-sigma limiting AB magnitude of r'=23.5.