Candidate optical counterparts of XTE J1728-295 = IGR J17285-2922

ATel #2827; D. M. Russell, Y. J. Yang, A. Patruno, N. Degenaar, D. Altamirano, R. Wijnands (University of Amsterdam), F. Lewis (Open University, University of Glamorgan)
on 2 Sep 2010; 22:14 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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Following the refined position of the faint X-ray transient XTE J1728-295 (ATel #2824) also named IGR J17285-2922 and currently in outburst (ATel #2823, #2825), we report a search for the optical counterpart using the Faulkes Telescope South (located at Siding Spring, Australia). We performed imaging of the field centred on the Swift XRT position in V (200 sec on source), R (200 sec), I (three images totalling 520 sec) and H-alpha (200 sec) filters on 2010-09-01 (MJD 55440.4) and V (200 sec), R (200 sec) and I (two images totalling 400 sec) on 2010-09-02 (MJD 55441.4). Seeing was reasonable, ~ 1.3 arcsec on both dates.

The source lies in a densely populated field in the galactic plane and we detect three faint sources within the 3.5 arcsec XRT error circle in the I-band images, with a few more stars near the edge of the error circle. The brighter stars are seen in archival digital sky survey images. Observations of the Landolt standard star field Mark A at a similar airmass on 2010-09-02 were used for flux calibration. Although our estimated magnitudes of the brighter stars are consistent with those listed in the USNO and GSC 2.2 catalogs, conditions were not photometric, so the magnitudes given below should be taken with caution.

Since the X-ray transient is currently in outburst, we may expect the optical counterpart to be blue, variable on short timescales and/or show prominent H-alpha (6563 Angstom) emission. Only the brighter of the stars are detected in the H-alpha image (stars 1, 2 and 6 listed below; see finding chart linked below). The H-alpha - R values of these three stars are the same to within 0.12 magnitudes suggesting that none of them have a bright H-alpha emission line. Star 3, at the centre of the error circle appears to be relatively blue (the instrumental V-I value is 0.5 mag bluer than the other faint stars), making it a good candidate. Using the five I-band images spanning 24 hours, we find that none of the stars detected in or around the XRT error circle are variable by more than ~0.1 mag (variability is measured at the < 2 sigma confidence level for all). There is also no evidence for variability of the stars between the two dates in R and V bands. Below is a link to the finding charts. From the combined images of both dates, the magnitudes of the candidate optical counterparts numbered in the finding charts are:

Star 1: R ~ 14.0, I ~ 13.6
Star 2: R ~ 17.8, I ~ 16.3
Star 3: R ~ 19.2, I ~ 18.6
Star 4: R ~ 19.6, I ~ 18.2
Star 5: too faint to measure
Star 6: R ~ 18.3, I ~ 17.0
Star 7: R ~ 20.7, I ~ 19.3

The measured NH of 5.4 +/- 1.2 x 10^21 cm^-2 (ATel #2824) may correspond to an optical extinction towards the source of Av ~ 2.4; A_I ~ 1.2 (Guver & Ozel 2009). This is fairly low for an X-ray transient in the galactic plane, so we speculate that one of the above candidates may indeed be the optical counterpart. We aim to continue monitoring this field in the coming weeks to identify optical variability as the X-ray outburst progresses.

The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). DMR acknowledges support from a Netherlands Organisation for Scientific Research (NWO) Veni Fellowship. FL acknowledges support from the Dill Faulkes Educational Trust.

Optical finding charts for XTE J1728-295