GX 339-4 has entered the intermediate state

ATel #7649; Zhen Yan, Wenda Zhang, Jie Lin and Wenfei Yu (Shanghai Astronomical Observatory)
on 17 Jun 2015; 09:45 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Zhen Yan (zyan@shao.ac.cn)

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The recent outburst of GX 339-4 started on around 2014-10-30 (ATel #6649) and the hard-to-soft state transition was detected around 2015-01-19 (ATel #6960). Since then the source had stayed in the soft state until recently. The latest Swift/XRT observation performed on 2015-06-16 at 11:27:59 UT shows that the 0.5-10 keV X-ray spectrum is well-fitted by disk component plus a power-law component, where the inner disk temperature was 0.70+-0.01, and the photon index was about 4.3+-0.7. We find the addition of a power-law component provides a statistically significant improvement (F-test = 99.8%) to the spectral fit. The disk flux was about 67% of the total flux in the 0.5-10 keV band. The power spectrum density (PSD) obviously shows a band limit noise (BLN) and a quasi-periodic oscillation (QPO). The central frequency of the QPO was about 60.2+-0.8 mHz. This indicates that the source has left the soft state and entered the intermediate state. The dominance of the disk component in the X-ray spectrum is probably due to the narrow energy range of Swift/XRT. On the other hand, the state transition time determined from timing usually precedes that determined from the X-ray spectra (Kalemci et al. 2013). We encourage monitoring this source at other wavelength, especially at radio, optical and infrared wavelengths due to expected jet activities during the transition to the X-ray hard state. We thank the Swift PI, Neil Gehrels, for approving these observations and the Swift team for scheduling our observations in the next two weeks to monitor its UV flux variations.