Abstract
We present optical monitoring of the black hole candidate XTE J1752-223 during its 2009 - 2010 outburst and decay to quiescence. The optical light curve can be described by an exponential decay followed by a plateau, then a more rapid fade towards quiescence. The plateau appears to be due to an extra component of optical emission that brightens and then fades over ~ 40 days. We show evidence for the origin of this optical 'flare' to be the synchrotron jet during the decaying hard state, and we identify and isolate both disc and jet components in the spectral energy distributions. The optical flare has the same morphology and amplitude as a contemporaneous X-ray rebrightening. This suggests a common origin, but no firm conclusions can be made favouring or disfavouring the jet producing the X-ray flare. The quiescent optical magnitudes are B >= 20.6, V >= 21.1, R >= 19.5, i' >= 19.2. From the optical outburst amplitude we estimate a likely orbital period of < 22 h. We also present near-infrared (NIR) photometry and polarimetry and rare mid-infrared imaging (8 - 12 microns) when the source is nearing quiescence. The fading jet component, and possibly the companion star may contribute to the NIR flux. We derive deep mid-IR flux upper limits and NIR linear polarization upper limits. With the inclusion of radio data, we measure an almost flat jet spectral index between radio and optical; F_nu ~ nu^(~ +0.05). The data favour the jet break to optically thin emission to reside in the infrared, but may shift to frequencies as high as the optical or UV during the peak of the flare.
Highlights
Since the first detections of X-ray transients in the 1960s, monitoring the X-ray evolution of their outbursts has been an active field of research (e.g. Evans, Belian & Conner 1970; Chen, Shrader & Livio 1997)
The optical outburst morphology can be described by a gradual exponential decay followed by a plateau, a faster fade towards quiescence with evidence for reflares
Optical exponential decays followed by a plateau and a rapid fade were seen in A0620−00 and XTE J1550−564 (Kuulkers 1998; Jain et al 2001), and prominent reflares were seen in GRO J0422+32 and GRS 1009−45 (Callanan et al 1995; Chen et al 1997)
Summary
Since the first detections of X-ray transients in the 1960s, monitoring the X-ray evolution of their outbursts has been an active field of research (e.g. Evans, Belian & Conner 1970; Chen, Shrader & Livio 1997). In the last few years it has been shown that as well as the radio behaviour, the OIR properties (flux and colour) of some sources can be predicted from the X-ray properties (and vice versa; Coriat et al 2009; Russell et al 2011b). These correlations between the different components of the system help us to understand the process of accretion on to compact objects and jet formation in BHXBs. XTE J1752−223 is an X-ray transient discovered by the Rossi X-ray Timing Explorer (RXTE) and Swift satellites in 2009 October (Markwardt et al 2009).
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