[ Previous | Next | ADS ]

VVV near-infrared observations of the IGR J17177-3656 field

ATel #3275; A. Rojas (PUC, Santiago), N. Masetti (INAF/IASF, Bologna) and D. Minniti (PUC, Santiago)
on 12 Apr 2011; 08:37 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

Subjects: Infra-Red, X-ray, Transient

In order to look for the quiescent longer-wavelength counterpart of the hard X-ray transient source IGR J17177-3656 (ATels #3223, #3226, #3236), we searched the archival frames of the near-infrared (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).

JHKs images were acquired on 23 April 2010 between 07:43 and 07:51 UT under an average seeing of 0.66 arcsec; a further Ks image was secured on 15 October 2010 at 00:43 UT with a seeing of 0.78 arcsec. Exposure times were 6 s for J-band images and 4 s for both H- and Ks-band frames. The image scale is 0.34 arcsec/pixel.

We do not detect any NIR source within either the Chandra X-ray (ATel #3236) or the ATCA radio (ATel #3246) error circles down to the VVV survey limits (J ~ 19.5; H ~ 18.0; K ~ 18.0).

Just outside the border of the Chandra error circle (ATel #3236) two objects are detected (the errors on the coordinates reported below are 0.1 arcsec).

The brighter one, with coordinates (J2000) RA = 17 17 42.70, Dec = -36 56 03.8 and magnitudes J = 15.87+-0.02, H = 13.93+-0.01, Ks(23/4/2010) = 12.93+-0.01 and Ks(15/10/2010) = 12.88+-0.01, is the 2MASS source mentioned in ATels #3226, #3236 and #3241. Its VVV magnitudes are fully consistent with the 2MASS ones (the J-band magnitude of the object reported in the 2MASS survey is actually a lower limit), indicating no substantial long-term variability of the source, although the comparison between the two Ks-band VVV epochs possibly suggests small-scale fluctuations in the NIR emission.

The other object has coordinates (J2000) RA = 17 17 42.66, Dec = -36 56 05.3; its magnitudes are J = 18.09+-0.05, H = 16.94+-0.04, Ks(23/4/2010) = 16.30+-0.08 and Ks(15/10/2010) = 16.28+-0.09. This is the fainter NIR source reported in ATel 3241; it can be noted that, when compared with the photometry reported there, our values appear fainter by about 0.5 mag.

This, despite the fact that this object is slightly outside the Chandra and ATCA error circles, might hint that this latter source is the actual NIR counterpart of IGR J17177-3656.

Further NIR observations are therefore encouraged.