GX 339-4 leaving the hard X-ray state - radio flare expected

ATel #318; Jeroen Homan (MIT)
on 10 Aug 2004; 21:57 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Jeroen Homan (homan@merate.mi.astro.it)

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Recent Rossi X-ray Timing Explorer observations of the black hole transient GX 339-4 show that the source is leaving the bright hard state. The evolution of the spectral and variability properties is very similar to the behavior seen during the outburst of GX 339-4 in 2002: the spectral power-law component has started to steepen and QPO frequencies are increasing. Observations in the optical/IR (see ATEL #316) further strengthen the similarities. If the analogy with the 2002 outburst holds and the time scale for the transition is the same, we expect the source to enter the very high state around August 14. In 2002 a large radio flare that lasted for a few hours was observed 1-2 days before the transition to the very high state, which would correspond to August 12-13 for the current transition. Radio, optical/IR and X-ray observations of the source for the next few days are therefore strongly encouraged. We note that the current transition is occurring at a luminosity that is lower by a factor of ~4 than in 2002, showing that these changes are probably not determined by the instantaneous mass accretion rate. Radio, optical/IR and X-ray coverage of such a transition at a different mass accretion rate could help to uncover the role of this parameter in the states of black hole X-ray binaries.