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GX 339-4 variability evolution observed by RXTE suggests the start of a spectral transition

ATel #2523; Nikolai Shaposhnikov (UMD/CRESST/GSFC) and John A. Tomsick (SSL/UC Berkeley)
on 30 Mar 2010; 00:37 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

Subjects: X-ray, Request for Observations, Black Hole, Transient

Referred to by ATel #: 2525, 2573, 3191

Since the beginning of the current outburst, the black hole transient GX 339-4 has been in a steadily brightening low-hard state (ATELs #2380 and #2384). Fourier power spectra for the last two Rossi X-ray Timing Explorer (RXTE) observations on March 26 (13:57 UT at start) and March 28 (03:02 UT at start) show quasi-periodic oscillations (QPOs) at 0.12 and 0.16 Hz, respectively, on top of broad band noise. The 3-25 keV energy spectrum has remained consistent with having an absorbed power-law continuum with an iron Kalpha emission line and edge. However, over the past few weeks, the power-law photon index has gradually softened from 1.503+/-0.006 on March 5 to 1.632+/-0.007 on March 28 (90% confidence errors are given). The evolution of the low frequency QPO strongly suggests that the source is starting a transition towards the high-soft state. The RXTE team plans to continue monitoring the source. Coverage at other wavelengths is encouraged.