The possible VVV near-infrared counterpart of IGR J17503-2636

ATel #11992; N. Masetti (INAF-OAS, Bologna), T. S. Ferreira, R. K. Saito, R. Kammers (UFSC, Florianopolis) and D. Minniti (UNAB, Santiago)
on 27 Aug 2018; 15:12 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

Tweet

We explored the archival images of the near-infrared (NIR) VVV survey (vvvsurvey.org; Minniti et al. 2010, New Astron., 15, 433) of the Galactic Bulge and inner disk, obtained with the 4.1m VISTA telescope at Cerro Paranal (Chile), containing the soft X-ray error box the transient IGR J17503-2636 (ATel #11952) that was recently detected with INTEGRAL.

JHKs images were acquired on 2010 April 9 between 06:53 and 07:04 UT under an average seeing of 0.9 arcsec. Further ZY photometry was collected on 2011 August 31 between 03:14 and 03:19 UT with a seeing of 1.0 arcsec.
Exposure times were 24 s in the J band, 8 s in both H and Ks, and 20 s in both Z and Y bands; the image scale is 0.34 arcsec/pixel for all frames.

A single NIR object is present inside the 2.2 arcsec-radius Swift/XRT error circle (ATel #11952) at the following coordinates (J2000):

RA = 17:50:18.06
Dec = -26:36:16.7

(coordinate errors are +-0.1 arcsec at 1-sigma confidence level).
This position is 0.9 arcsecs from, thus formally outside, the corrected 0.7 arcsec-radius Chandra localization uncertainty of IGR J17503-2636 (ATels #11990, #11991); however, the two positions are mutually consistent within the respective uncertainties at a confidence level better than 1.5 sigma.
This NIR source has magnitudes

Z > 19.9
Y = 17.90+-0.06
J = 14.23+-0.01
H = 11.79+-0.01
Ks= 10.65+-0.01

(confidence levels are 1-sigma for magnitude uncertainties and 3-sigma for the upper limit). We point out that the shape information flag associated with this object is "-1" (indicating a stellar source) in all the four bands (YJHKs) in which it was detected.

The source shows variability of ~1.2 mag across the 49 Ks frames available for its field in the VVV survey over a time baseline of ~3.5 years, from April 2010 to November 2013. It should be stressed that the object is close to the saturation limit in the Ks-band and therefore the variability may partly be due to spurious effects due to different saturation levels depending on the sky conditions during each pointing; however, this issue alone cannot account for the whole observed variation, which should thus mostly be intrinsic.

We moreover note that, according to Nishiyama (2009, ApJ, 696, 1407), the Galactic Ks-band absorption along the line of sight of this source is ~2 mag, which implies an optical reddening A_V ~ 20 mag and explains why this object is not detected at wavelengths shorter than the Y-band (indeed, optical surveys such as the USNO-A2.0 catalogue do not contain sources at the NIR position).

All the above information is consistent with the NIR object being an heavily reddened OB (super)giant star located beyond the Galactic Center, at ~10 kpc from Earth; this distance implies a peak X-ray luminosity of ~2e36 erg/s according to the data reported in ATel #11952. We thus suggest that this source is the NIR counterpart of IGR J17503-2636 and that this object is a high-mass X-ray binary, possibly of the Supergiant Fast X-ray Transient subclass given its X-ray luminosity at maximum brightness and the rapid decay of its X-ray emission as reported from the Chandra observations (ATel #11990).