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VVV near-infrared observations of the IGR J17559-2612 field

ATel #4342; A. F. Rojas (PUC, Santiago), N. Masetti (INAF/IASF, Bologna) & D. Minniti (PUC, Santiago)
on 28 Aug 2012; 18:46 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

Subjects: Infra-Red, X-ray, Transient

Following the announcement of the arcsec-sized soft X-ray (ATel #4322) error box of the X-ray transient IGR J17559-2612 (ATel #4309), we searched the archival frames of the NIR VVV survey (vvvsurvey.org; Minniti et al. 2010, New Astron., 15, 433) of the Galactic Bulge and inner arms, which is being obtained with the 4.1m VISTA telescope at Cerro Paranal (Chile), in order to look for a possible quiescent near-infrared (NIR) counterpart to this source.

Initial JHKs images were acquired on 27 March 2010 between 08:35 and 08:42 UT under an average seeing of 0.65 arcsec. Further Ks images were secured on 14 September 2011 at 00:39 UT with a seeing of 0.60 arcsec and on 3 April 2012 at 06:29 UT at a seeing of 0.56 arcsec. Exposure times were 6 s in the J band and 4 s in both H and Ks; the image scale is 0.34 arcsec/pixel for all images.

Besides the two 2MASS objects mentioned in ATel #4322, we find 10 more sources within or at the border of the Swift/XRT X-ray error circle of IGR J17559-2612 (ATel #4322).

Their coordinates and magnitudes are the following:

 
 
 #   RA(J2000)    Dec(J2000)    J(2010)      H(2010)      Ks(2010)     Ks(2011)     Ks(2012) 
 
 1   17 55 57.70  -26 13 50.2   17.3+-0.1    15.8+-0.1    15.0+-0.1    15.2+-0.1    15.1+-0.1 
 2   17 55 57.80  -26 13 46.9  <20.0         18.0+-0.2    17.3+-0.2    17.9+-0.3    18.0+-0.2 
 3   17 55 57.81  -26 13 50.4   18.1+-0.1    16.9+-0.1    16.2+-0.1    16.4+-0.2    16.3+-0.1 
 4   17 55 57.87  -26 13 51.2   19.0+-0.1    17.4+-0.1    16.6+-0.1    16.7+-0.2    16.5+-0.1 
 5   17 55 57.90  -26 13 49.6  <20.0         18.3+-0.2    18.0+-0.3   <18.0         17.4+-0.2 
 6   17 55 57.94  -26 13 47.2  <20.0         18.7+-0.3    17.9+-0.3    17.4+-0.2    17.5+-0.2 
 7   17 55 57.99  -26 13 51.6   19.6+-0.2    17.8+-0.2    17.1+-0.2    17.0+-0.2    17.1+-0.2 
 8   17 55 58.00  -26 13 45.3  <20.0         18.7+-0.3    17.9+-0.3   <18.0         17.9+-0.2 
 9   17 55 58.00  -26 13 47.5  <20.0         18.2+-0.2    17.6+-0.2   <18.0         18.1+-0.3 
10   17 55 58.15  -26 13 47.2   17.14+-0.08  14.99+-0.05  14.01+-0.04  14.04+-0.05  13.94+-0.03 
 
Coordinate errors are +-0.1 arcsec at 1 sigma confidence level; magnitude uncertainties are at 1 sigma confidence level as well.

None of these sources shows significant variability among the three Ks epochs. We note that the brightest one, #10, appears to be very red, being an outlier in the color-color diagram for the VVV sources of this field.

The two 2MASS sources mentioned in ATel #4322 located just outside the XRT error circle also do not show significant variability among the three Ks epochs, their magnitudes being moreover consistent with those in the 2MASS catalogue.

We do not detect any other NIR source within the XRT error circle down to J ~ 20.0; H ~ 19.0; Ks ~ 18.5.

At present we cannot tell which of the above sources (if any) is the quiescent NIR counterpart of IGR J17559-2612. Comparison with NIR imaging acquired during the present outburst or availability of an X-ray position with a subarcsecond error are therefore needed to pinpoint the actual counterpart.