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GX 339-4 in transition back to the hard state after a long outburst

ATel #7962; John A. Tomsick (SSL/UCB), Felix Fuerst (Caltech), and Tomaso Belloni (INAF-OAB)
on 28 Aug 2015; 00:39 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 7977, 10797

The black hole transient GX 339-4 has been in outburst since 2014 October (ATEL 6649), starting off in the hard state and making a transition to the soft state in 2015 January (ATELs 6090 and 7009). After being in soft or intermediate states for the past seven months (ATELs 7201, 7434, 7649), the source finally appears to be headed back to the hard state. Based on a falling soft X-ray flux measured by MAXI and a rising hard X-ray flux measured by Swift/BAT, we requested a Swift/XRT observation to assess the source state. The observation was carried out on 2015 Aug 27, 12.3-14.3 h UT, during which 1.845 ks of exposure time was obtained.

Fitting the 0.5-10 keV XRT energy spectrum with an absorbed power-law gives a photon index of Gamma = 2.12+/-0.02 (90% confidence errors), nH = (5.4+/-0.2)e21 cm-2 (using wilm abundances), and an unabsorbed 0.5-10 keV flux of 1.5e-9 erg/cm2/s. The reduced chi2 is 1.21 for 348 degrees of freedom, and the fit can be improved by adding a low temperature thermal component or a broad emission line near 6 keV (presumably related to iron). The XRT bandpass is not sufficient to constrain all three components.

Fitting the 0.01-100 Hz XRT power spectrum with a zero-centered Lorentzian indicates a fractional rms amplitude of 24%+/-1%. The peak frequency is 0.7+/-0.1 Hz, and the reduced chi2 is 1.23 for 92 degrees of freedom.

Based on studies of spectral states for GX 339-4 specifically (e.g., Stiele et al. 2011, MNRAS, 418, 1746; Dincer et al. 2012, ApJ, 753, 55; Kalemci et al. 2007, ATEL 1074), the fractional rms of 24% and the photon index of 2.1 are typical when the source is approaching the hard state. When the source actually reaches the hard state, it would not be surprising to see a fractional rms as high as 40% (Stiele et al. 2011; Dincer et al. 2012) and a photon index as hard as 1.6 (Fuerst et al. 2015, ApJ, 808, 122). A second Swift observation and a series of XMM and NuSTAR observations are planned to continue to follow the source. Multi-wavelength observations are encouraged, especially because it is not clear whether a compact jet has turned on yet. We thank Neil Gehrels for approving this Swift observation and the Swift operations team for carrying it out.