State transition of GX 339-4

ATel #455; D. M. Smith (University of California, Santa Cruz (UCSC)), Jeroen Homan (MIT), Nathan Bezayiff (UCSC)
on 7 Apr 2005; 22:09 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: David M. Smith (dsmith@ssl.berkeley.edu)

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Twice-weekly monitoring with the Proportional Counter Array (PCA) of the Rossi X-ray Timing Explorer shows that GX 339-4 is coming out of an extended period spent in the soft state. The most recent observation, made on 6 April 2005 from 18:03:00 to 18:20:00, shows spectral and timing characteristics of an intermediate state.

A disk blackbody component of best-fit maximum temperature (0.57 +/- 0.02) keV carries 2/3 of the energy from 2.5 to 25 keV, and a power law of best-fit photon index (2.21 +/- 0.09) carries most of the remainder. A broadened and redshifted iron fluorescence line (Gaussian center energy (5.4 +/- 0.2) keV, sigma = (1.3 +/- 0.2) keV) must also be included to achieve a good spectral fit.

The spectral changes have been gradual and subtle, but the power spectrum shows rapid and clear evolution over the last few pointings (see Table). The most recent observation is the first to show a quasi-periodic oscillation. The QPO, at (3.50 +/- 0.04 Hz), is strong, with integrated rms of (10.5 +/- 0.6)%, and well defined, with Q = (3.6 +/- 0.7).

We note that the transition to the hard state seems to be occurring at approximately the same luminosity as during decline of the last outburst, in February 2003, lending further evidence to the idea that the soft-to-hard transition near the end of transient outbursts occurs at a nearly constant fraction of the Eddington luminosity (Maccarone 2003, A&A 409, 697), in strong contrast to the early hard-to-soft transition, which usually occurs near the peak luminosity of the outburst, whatever that peak happens to be. In the current case, the transition is accompanied by a small increase in luminosity, and it is possible that the current transition could represent the very beginning of a new rebrightening phase rather than the ordinary transition during decline. In either case, the system is certainly in a rare and short-lived state, and observations at all wavelengths are encouraged. Within two weeks, when the source is likely to become even harder, we expect an increase in the radio and IR brightness of GX 339-4.

The URL below links to figures showing 1) the evolution of the PCA count rate and hardness during the current outburst and 2) the QPO in the most recent observation.

GX 339-4 outburst evolution and recent QPO