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Discovery of the likely optical counterpart of IGR J17329-2731

ATel #10682; David M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Poshak Gandhi (Southampton), Enrico Bozzo (ISDC, Switzerland)
on 30 Aug 2017; 14:38 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Subjects: Optical, X-ray, AGN, Binary, Black Hole, Blazar, Cataclysmic Variable, Neutron Star, Transient, Young Stellar Object

Referred to by ATel #: 10685

On 2017 August 10-13, INTEGRAL detected a new X-ray source designated IGR J17329-2731 (ATel #10644). A Swift observation on August 16 confirmed a faint X-ray source within the INTEGRAL error region, constraining its coordinates and spectrum (ATel #10645). On August 14 HXMT (also known as Insight) observed the source; from preliminary analysis it was faint and not detected (ATel #10653).

Here, we report on new optical observations of the field of IGR J17329-2731 with the 2-m Faulkes telescopes (in Maui, USA and Australia) and 1-m Las Cumbres Observatory (LCO) telescopes (in Chile). We performed imaging observations on August 18,19,20,24,25. SDSS i'-band images were made on all dates, with additional Bessel V, R images on August 18.

Several stars are detected close to the 3.1-arcsec Swift-XRT error circle (ATel #10645). The finding chart and i'-band light curves of these stars can be found at the link below. We calibrated the field using APASS stars in the field for V and i'-bands, and USNO-B1 stars for R-band. Systematic errors could be several tenths of a magnitude, specially in R-band. We find that star 3 is variable at the 10-sigma level. Its magnitude rises from i'=15.40+-0.01 on August 18 (MJD 57983.3) to i'=15.34+-0.01 on August 20 (MJD 57985.3), then fades to i'=15.52+-0.01 on August 25 (MJD 57990.2; errors are relative). There is a hint of variability (at the 3-sigma level) in star 4, which appears to fade from i'~18.6+-0.1 on August 18 to i'=19.4+-0.3 on August 25. However, fainter stars could be contaminating the aperture of star 4 in a varying way depending on the seeing in this crowded field. No other star close to the XRT error circle is variable at the >3.0-sigma level, so we consider star 3 to be the likely optical counterpart to IGR J17329-2731.

On August 18, we measure magnitudes of star 3 of V=19.26+-0.08, R=16.77+-0.14, i'=15.40+-0.01. Star 3 is listed in the USNO-B1 catalogue at coordinates RA, Dec (J2000.0) = 263.211131 deg, -27.500414 deg. Its USNO-B1 magnitudes are R1=16.53 (epoch=1976.8), R2=20.72 (epoch=1991-08-31) and I=13.99. The R-band has therefore brightened by 4 mags from 1991 to 2017. Its near-infrared 2MASS magnitudes are J=9.82+-0.03, H=8.44+-0.03, K=7.78+-0.02 (epoch=1998-07-02). The >10 mag difference between V-band and K-band suggest a very high extinction, as expected based upon the high X-ray obscuring column (ATel #10645).

There are also images of the field in the LCO Science Archive, taken by 1-m LCO telescopes in 2016. These observations were of the object OGLE-2016-BLG-0656 listed here (LCO key project KEY2014A-004; PI Rachel Street). We measure star 3 mags of V>19.5 (3-sigma upper limit) on 2016 April 28 and i'=15.94+-0.02, 15.89+-0.01, 15.90+-0.01 on April 21, May 5 and May 6, respectively. The source has brightened by 0.6 mag since 2016.

The nature of the source is uncertain. The variability amplitude of 4 magnitudes and the location in the Galactic plane suggest it could be a low-mass X-ray binary or CV. However, the infrared magnitudes are very bright (K=7.8), and the source could be a flaring AGN such as a blazar, or a high-mass X-ray binary. From infrared WISE data, Marton et al. (2016) select the same source as a candidate Young Stellar Object. Multiwavelength observations are encouraged to help constrain the nature of IGR J17329-2731. Currently, X-ray observations are planned/ongoing with XMM-Newton and NuSTAR on August 30.

This work makes use of observations from the Las Cumbres Observatory (LCO) Science Archive. A link to the light curves and finding charts of IGR J17329-2731 is included below. We thank Todd Boroson (LCO Observatory Director) for rapid useful feedback regarding the archival data.

IGR J17329-2731 Faulkes Telescope light curve