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State Changes in GX 339-4

ATel #95; D. M. Smith, Space Sciences Laboratory, U. C. Berkeley; T. Belloni, Brera Observatory, Italy; W. A. Heindl and E. Kalemci, Center for Astrophysics and Space Science, U. C. San Diego; R. Remillard and M. Nowak, MIT; J. H. Swank, NASA Goddard Space Flight Center; and S. Corbel, Universite Paris VII and CEA Saclay
on 20 May 2002; 18:20 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: David M. Smith (dsmith@ssl.berkeley.edu)

Subjects: X-ray, Gamma Ray, Binary, Black Hole, Transient

Referred to by ATel #: 196

Using recent data from the Rossi X-ray Timing Explorer, we find that the black hole candidate GX 339-4, in outburst since March 26 (see ATEL #85), entered the Very High State (VHS) around May 6, a rare state of x-ray emission last seen in this object in 1988 (Miyamoto et al. 1991, ApJ 383, 784). The VHS may be the same as the "Intermediate State" (IS) identified by some authors.

The energy spectrum is roughly consistent with the sum of a power law of photon index -2.5 and a disk blackbody with maximum temperature 1 keV. The contribution of the thermal component has been increasing: the 2-50 keV luminosity of 2x10-8 ergs/cm2/s was equally split between these components on May 16, with the thermal component carrying about 2/3 of the luminosity by May 18.

The power spectrum from May 6-16, similar to the 1988 Ginga observations, showed flat-topped noise with a break around 1 Hz and a strong quasi-periodic oscillation that moved from 0.6 to 9 Hz as the state has evolved. The most recent observations have shown a greater variety of behavior, including a strong amount of red noise in one observation on May 17 and a short pointing on May 18 which showed almost no variability at all (rms 1.7%). This may indicate that the state has left the VHS/IS entirely and entered the high/soft state (HS), or it may simply be part of the unpredictable shifts in behavior which often characterize the VHS.

Prompt observations at all wavelengths are encouraged, whether the current state is still VHS or is now HS. Since the VHS in other black-hole candidates has show a wide range of behaviors, frequent observing could be very rewarding.