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ATEL # 2775; D. M. Russell (University of Amsterdam), T. Muņoz Darias (INAF - Brera Observatory), F. Lewis (Open University, University of Glamorgan), P. Soleri (Groningen)
on 6 Aug 2010; 15:30 UT
Password Certification: David M. Russell (D.M.Russell@uva.nl)
Subjects: Infra-Red, Optical, X-ray, Binaries, Black Holes, Transients
Referred to by ATEL #: 2804, 2805, 2818
We have been regularly monitoring the outburst decay of the black hole candidate X-ray binary XTE J1752-223 (discovered by RXTE; ATel #2258) with the 2-m Faulkes Telescopes North and South (located at Haleakala on Maui and Siding Spring, Australia, respectively). Exposures in B, V, R and i'-bands (mostly 100-sec exposures each) were taken every ~ 3 days since 2010-03-22 (MJD 55277). The optical counterpart (ATels #2263, #2268, #2424) is detected up until 2010-07-15 (MJD 55392), after which detections become ambiguous due to close field stars in this crowded region of the Galactic plane. Our light curves are linked below.
XTE J1752-223 initially faded slowly from B ~ 19.7, V ~ 18.2, R ~ 17.2, i' ~ 16.2 at the start of our campaign, dropping by ~ 0.8 mag in two months, with evidence for variability on day-timescales (changes up to ~ 0.4 mag in a few days). During this time the source made a transition from the soft state to the hard state (ATels #2518, #2530, #2549) and the synchrotron jet may thereafter make a contribution to the optical and infrared flux. XTE J1752-223 then maintained a ~ constant flux for 1.5 months until ~ 2010-06-27, then faded rapidly (by ~ 2 mag in 18 days), finally reaching V = 21.2 +- 0.3, R = 19.8 +- 0.2, i' ~ 18.9 +- 0.2 by 2010-07-15 (and B = 20.9 +- 0.2 on 2010-07-02, after which it was not detected in B-band). On 2010-07-15 the seeing was 0.9 arcsec, and we are able to resolve the counterpart from a nearby faint star just 0.8 arcsec to the north. Finding charts are linked below. It is uncertain whether the source had reached its quiescent flux level by 2010-07-15 or continued to fade, although the magnitudes a few days later on 2010-07-19 were slightly brighter and images on 2010-08-04 and 2010-08-05 show some faint residual flux at the position of the counterpart, but which includes flux from the close faint star.
The quiescent magnitudes of XTE J1752-223 are therefore B >= 20.9 +- 0.2; V >= 21.2 +- 0.3; R >= 19.8 +- 0.2; i' >= 18.9 +- 0.2. This is > 4.2 mag fainter than the V-band magnitude measured near the peak of the outburst, V ~ 16.7 (Curran et al. 2010).
On 2010-07-01 we observed XTE J1752-223 in the near-infrared with the Long-slit Intermediate Resolution Infrared Spectrograph (LIRIS) in imaging polarimetry mode on the 4.2-m William Herschel Telescope (WHT) at the Observatorio del Roque de los Muchachos, La Palma, Spain. Conditions were excellent with a seeing of 0.6 - 0.8 arcsec. Images were reduced and combined; the total on source exposure times were 240 sec in H-band and 360 sec in Ks-band. The source was detected and the faint star 0.8 arcsec to the north was resolved from XTE J1752-223 (see finding charts linked below). Magnitudes were estimated using 2MASS stars in the field, yielding H = 15.6 +- 0.1; Ks = 15.2 +- 0.1 for XTE J1752-223.
The Faulkes Telescope observations are part of an ongoing monitoring campaign of ~ 30 low-mass X-ray binaries (Lewis et al. 2008, arXiv:0712.2751). The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). The William Herschel Telescope is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. DMR acknowledges support from a Netherlands Organisation for Scientific Research (NWO) Veni Fellowship. FL acknowledges support from the Dill Faulkes Educational Trust.
Light curves and finding charts of XTE J1752-223