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Confirmation of the Transition to the Very High State in GX 339-4

ATel #322; D. M. Smith, S. K. Bushart (University of California, Santa Cruz)
on 20 Aug 2004; 01:09 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: David M. Smith (dsmith@ssl.berkeley.edu)

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

Referred to by ATel #: 323, 707

Homan (2004, ATEL #318) recently predicted that black-hole candidate GX 339-4 would make a transition to the Very High State around August 14, and we find, also using data from the Rossi X-ray Timing Explorer, that this transition has occurred.

The photon spectral index had averaged around 1.50 for the past month in the hard state; the recent evolution has been: 1.54 at 6h on 5 August, 1.58 at 10h on 8 August, 1.81 at 0h on 12 August, 2.39 at 23h on 13 August, and 2.83 at 14h on 17 August. These values are taken from fits that included both the power law component and a disk blackbody component. It is notable that the J band luminosity of the system had already dropped by the better part of a magnitude on 8 August (Buxton and Bailyn, 2004, ATEL #316) at a time when the spectral softening in the x-rays had barely begun.

Despite the very soft spectral index, the power law still energetically dominates the thermal component as of 17 August. Furthermore, the power spectrum resembles that of the canonical hard state, not the soft state, being flat-topped at low frequencies and showing a prominent quasi-periodic oscillation at about 4 Hz. The total rms variability is still high, at 23%. This is therefore a Very High or Intermediate state, the difference between the two being somewhat unclear, particularly considering the relatively low luminosity at which this transition has occurred (Homan, 2004, ATEL #318).

The first plot linked below shows the RXTE PCA count spectra which give the power-law indices above, with the colors white, red, green, dark blue, and light blue corresponding to the chronological order above. The second plot shows the history of the current outburst, giving the count rate (in counts per second per RXTE PCU) and a simple measure of spectral softness (counts between 2.5 and 6.0 keV)/(counts between 8.0 and 25.0 keV). Day 0 on this plot is 13 February 2004. The preliminary, purely hard outburst may be part of the same event as the current activity, or it may be more or less independent. Data from the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory (Harmon et al. 2002, ApJS 138, 149) and from the RXTE All Sky Monitor (Levine et al. 1996, ApJ 469, L33) show that there was a long period of variable hard-state activity before the decisive soft-state outburst of 1998 (Zdziarski et al. 2004, MNRAS 351, 791).

Observations of this rare state at all wavelengths are encouraged.

Spectrum and Lightcurve Plots