On the optical counterpart of IGR J17591-2342

ATel #11946; David M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 13 Aug 2018; 13:26 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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On 2018 August 10 to 11, IGR J17591-2342 was discovered as a new X-ray transient by INTEGRAL, confirmed on August 12 with a refined position by Swift (ATel #11941, #11942). We report on optical observations of the field of IGR J17591-2342 taken with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes.

Images were acquired with the 2-m Faulkes Telescope South (FTS; located at Siding Spring Observatory, Australia) in SDSS g', i' and PanSTARRS y-band filters on 2018 Aug 12 at 13:41-13:57 UT (less than 4 hours after the announcement of the discovery in ATel #11941). Images in SDSS r', i' and PanSTARRS y-band filters were taken on 2018 Aug 12 at 21:05-21:23 UT using a 1-m LCO telescope at the South African Astronomical Observatory (SAAO), Sutherland, South Africa. Finally, images in g', i' and y-band filters were taken on 2018 Aug 13 at 6:30-6:46 UT with the 2-m Faulkes Telescope North (FTN; at Haleakala Observatory, Maui, Hawaii, USA).

We flux calibrate our images using several close, isolated stars listed in the PanSTARRS catalogue. We also aligned and combined the FTS and FTN images taken in g', i' and y-bands to produce deeper images in these three filters (finding charts are linked below). No stars are detected within the 3.6 arcsec radius Swift XRT error circle of the X-ray source (ATel #11942), although one star is clearly detected on the edge of the error circle in all our images (labelled '1' in the i'-band finding chart below), which is also listed in the PanSTARRS and 2MASS catalogues. Its PanSTARRS coordinates (J2000) are RA = 17 59 02.693; Dec = -23 43 12.04 (3.7" from the centre of the XRT circle) and catalogue magnitudes are g' = 25.468 +- ? (no error given); r' = 19.767 +- 0.036; i' = 17.737 +- 0.011; y = 15.505 +- 0.013. We derive magnitudes of this star from each of our images; all our r', i' and y mags agree with the catalogue values within < 0.1 +- 0.1 mag. From the combined FTS and FTN g'-band image, we measure g' = 23.0 +- 0.2 for this star - significantly brighter than the PanSTARRS value, although we suspect this apparent brightening may not be real, and could be due to the unknown error on the PanStarrs catalogue g'-band magnitude, and/or the low S/N of the star in our image.

The next three closest PanSTARRS stars are located 5.5" to 6.2" from the centre of the XRT error circle (labelled 2 to 4 in the i'-band finding chart). Star 2 is detected in y-band only; its magnitudes agree within 0.2 +- 0.2 mags of PanSTARRS. Star 3 is not detected in any filter. Star 4 (at RA = 17 59 02.505; Dec = -23 43 07.93) has magnitudes that are brighter than the catalogue values by 0.15-0.19 mag in all i' and y images (it is not significantly detected in g' and r'). Within the 3.6" XRT error circle, there are no additional sources detected in the combined images down to magnitude (3-sigma) limits of g' = > 22.7; i' > 21.8; y > 18.4. We will continue to image the location with the LCO telescopes while the X-ray flux increases, to see if any star brightens.

The Galactic neutral hydrogen column density in the direction of IGR J17591-2342 is n_H ~ 1.4E22 cm^-2 (Kalberla et al. 2005, A&A, 440, 775; Dickey & Lockman, 1990, ARAA, 28, 215). The absorption column density from the Swift XRT spectrum was measured to be n_H = (4.2+/-0.8)x1E22 cm^-2, exceeding the Galactic value. Given these two values and adopting the relation of Guver & Ozel (2009, MNRAS, 400, 2050), the optical extinction is likely to lie in the range Av ~ 6 - 23. Infrared and longer wavelength observations (e.g. radio) are therefore strongly encouraged.

The LCO observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO).

IGR J17591-2342 LCO optical finding charts