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