Monitoring of the likely optical counterpart of XTE J1728-295

ATel #2997; D. M. Russell, Y. J. Yang, N. Degenaar, R. Wijnands, A. Patruno, R. Kaur, M. Armas Padilla (Univ. of Amsterdam), F. Lewis (Faulkes Telescope Project, Open Univ., Univ. of Glamorgan), J. D. Armstrong (Univ. of Hawai'i)
on 1 Nov 2010; 18:53 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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We have been monitoring the position of the very faint X-ray transient XTE J1728-295 (= IGR J17285-2922, ATel #2823, #2824, #2825) with the Faulkes Telescopes North and South throughout its outburst. Data were acquired on thirteen dates from 1st September until 28th October, mostly in I-band and R-band.

Of the original seven stars detected with the Faulkes Telescope South within and around the 3.5 arcsec Swift XRT error circle, only star 3, a blue star, is consistent with the refined 0.6 arcsec Chandra position (ATel #2827, #2869). Torres et al. (ATel #2870) performed high resolution, deep optical imaging with the Magellan Clay telescope and found two additional faint point sources consistent with the Chandra X-ray position. We were able to monitor the magnitude of star 3 in R and I-bands over the last two months with the Faulkes Telescopes North and South.

All of the five stars originally considered as candidate counterparts to the X-ray source are consistent with being of constant flux, within errors, except star 3 (some are fainter and some are brighter than star 3). Star 3 appears to brighten from I ~ 18.65 +- 0.10 when monitoring began on 1st September (MJD 55440.4; ATel #2827), to I ~ 18.24 +- 0.07 two days later (MJD 55442.3), then it faded again only recently, between 28th September and 20th October to I ~ 18.54 +- 0.10. The X-ray flux has also decreased over the last month (ATel #2991).

Star 3 lies just 3.5 arcsec from a bright (I ~ 13.6) star. Although we exclude data with poor seeing, we also repeated the photometry adopting a smaller aperture, in case flux from the bright star was contaminating the larger aperture. Similar results were found, confirming the variability of star 3. The apparent magnitude of star 3 is also not well correlated with the seeing. From R-band photometry, star 3 is also the only apparently variable star in R-band out of the same five stars considered. However, the R-band light curve morphology is somewhat different, with an apparent recent brightening between 28th September and 20th October.

We conclude that star 3 is likely to be the optical counterpart of XTE J1728-295 due to its probable long-term variability of ~ 0.4 mag and its blue V-I colour compared to the other stars close to the X-ray position (ATel #2827). Further imaging towards quiescence is required to confirm or discard the association of star 3 with the X-ray source.

The Faulkes Telescope Project is an educational and research partner of the Las Cumbres Observatory Global Telescope Network (LCOGT). We thank the teachers and pupils of West Monmouth School, Wales and Freeport School District, New York, for observing XTE J1728-295 on 8th and 30th September, respectively. DMR acknowledges support from a Netherlands Organisation for Scientific Research (NWO) Veni Fellowship. FL acknowledges support from the Dill Faulkes Educational Trust.

XTE J1728-295 optical light curves and finding charts