Abstract
We present Swift broad-band observations of the recently discovered black hole candidate, X-ray transient, XTE J1752−223, obtained over the period of outburst from 2009 October to 2010 June. From Swift–Ultraviolet/Optical Telescope (UVOT) data we confirm the presence of an optical counterpart which displays variability correlated, in the soft state, to the X-ray emission observed by Swift–X-ray Telescope (XRT). The optical counterpart also displays hysteretical behaviour between the states not normally observed in the optical bands, suggesting a possible contribution from a synchrotron-emitting jet to the optical emission in the rising hard state. We offer a purely phenomenological treatment of the spectra as an indication of the canonical spectral state of the source during different periods of the outburst. We find that the high-energy hardness–intensity diagrams over two separate bands follow the canonical behaviour, confirming the spectral states. Our XRT timing analysis shows that in the hard state there is significant variability below 10 Hz which is more pronounced at low energies, while during the soft state the level of variability is consistent with being minimal. These properties of XTE J1752−223 support its candidacy as a black hole in the Galactic Centre region.
Highlights
Low mass X-ray binaries are for the majority of the time in a state of quiescensce with faint or non-detected X-ray emission, though optical or near-infrared counterparts may be visible due to emission from the donor star, or possibly the jet, hot spot, or outer accretion disk
The source was observed to have reverted to a hard state at the end of March 2010 (MJD ∼ 55280; Munoz-Darias et al 2010a), after the low energy light curve decreased to trace once more the high energy light curve; the thermal component was no longer dominant
XTE J1752-223 was observed by Swift in a relatively low intensity, hard state from 2009-10-26 to 2009-11-03 (MJD 55131-55138) which was signified by hard power law energy spectra, and high levels (∼ 50 per cent) of RMS variability in the X-ray light curve
Summary
Low mass X-ray binaries are for the majority of the time in a state of quiescensce with faint or non-detected X-ray emission, though optical or near-infrared (nIR) counterparts may be visible due to emission from the donor star, or possibly the jet, hot spot, or outer accretion disk. In this article we present the Swift – Burst Alert Telescope, X-ray Telescope and Ultraviolet/Optical Telescope – monitoring observations of XTE J1752-223, obtained over the period of the outburst Based on these data, we identify the periods of the various states and compare the behavior of the major photometric, spectral and timing parameters during these states to those expected from black hole X-ray binaries. All sequences were combined in an individual fits file, using fappend, which was again summed using uvotimsum to obtain a deep image of the field in each filter From these images, two possible optical counterparts of XTE J1752-223 are identified, consistent with the X-ray position (Markwardt et al 2009a); source A, consistent with the counterpart proposed by Torres et al (2009a), is within 1σ and source B is within 2σ of the X-ray position. Taking this value as an upper limit to the extinction of source A and using the effective wavelengths of the Swift filters (Poole et al 2008) and the parameterization of Pei (1992), the extinctions is the Swift bands are: Av ≤ 4.7, Ab ≤ 6.1, Au ≤ 7.6, Auvw1 ≤ 10.5, Auvw2 ≤ 12.5, Auvm2 ≤ 15.0, though these should be treated with caution as estimates of the extinction so close to the Galactic plane (< 5 deg) are unreliable
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