Swift monitoring observations of 1H 1743-322 and its evolution towards a state transition

ATel #8800; Zhen Yan, Jie Lin, Wenfei Yu, Wenda Zhang, Hui Zhang and Dongming Mao (Shanghai Astronomical Observatory)
on 11 Mar 2016; 06:10 UT
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

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Following the report of the new outburst of black hole X-ray binary H1743-322 (ATel #8751), we requested a series of Swift ToO observations to monitor the X-ray temporal and spectral evolution and potential jet contribution to the UV flux during the outburst. The latest two Swift observations were taken on Mar 7th and Mar 10th, when the hard X-ray (15-50 keV) flux seen from Swift/BAT started to drop. The 0.5-10 keV X-ray spectra of the two observations are quite similar. Both spectra can be fitted well by a power-law model with indices of 1.58+-0.04 and 1.62+-0.05, respectively. Both Fourier power spectra obviously show Type-C low frequency quasi-periodic oscillations (QPOs). The central frequency of the QPO increased from 0.44+-0.03 to 0.75+-0.01 Hz, and the fractional root mean square (RMS) between 0.1 and 10 Hz had dropped from 0.39+-0.02 to 0.32+-0.02.

We also investigated the Swift/XRT observations of several previous outbursts of H1743-322 observed with Swift/XRT. In the last two outbursts in 2014 and 2015, H1743-322 stayed in the low-hard state throughout the entire outbursts. Therefore compare the current outburst with the previous two outbursts will tell if the current outburst will be the same low hard state outburst. The Swift/XRT spectrum near the previous outburst peak was also well-fitted by a power-law model with an index ~ 1.55, and the central frequency of the low frequency QPO was about 0.2-0.3 Hz. However, the central frequency of the QPO exponentially evolved from less than 1 Hz to about 3-5 Hz within one week during the hard-to-soft transition of the normal outbursts, at the same time the fractional RMS quickly dropped (Debnath et al. 2013, Zhou et al. 2013).

Based on the decreasing trend of the hard X-ray flux and the RMS variability, the increasing trend of the QPO frequency, and the higher QPO frequency during the current outburst, we suspect that a hard-to-soft state transition is likely to start in the next few days. The Swift will continue monitor the source and we encourage multi-wavelength observations in X-ray, radio, optical and infrared wavelengths in the next week due to jet activities during the X-ray state transition expected to occur (e.g. Miller-Jones et al. 2012). We thank the Swift PI, Neil Gehrels, for approving these observations and the Swift team for scheduling our observations.