Recurrent black hole transient GX 339-4 in transition to the hard state

ATel #1074; E. Kalemci (Sabanci Univ.), J. A. Tomsick (SSL / UC Berkeley), S. Migliari, K. Pottschmidt (CASS / UC San Diego), S. Corbel (CEA Saclay & Univ. Paris 7), M. Buxton (Columbia Un.), C. Bailyn (Yale), P. Kaaret (Un. of Iowa), T. Belloni (INAF-OAB)
on 15 May 2007; 22:33 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Emrah Kalemci (ekalemci@sabanciuniv.edu)

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The recurrent black hole transient GX 339-4 has been declining in luminosity since mid February as observed by the RXTE. It was in the soft state on April 5 (see ATEL #1050 for INTEGRAL soft state results)

Our group has been monitoring the source since April 20th with PCA on RXTE. The source showed on-and-off behavior in terms of the level of timing noise between April 20th and May 12th. Some days it was off with less than 3% rms amplitude of variability, and some days it showed a flat-topped power spectrum with 8-11% rms amplitude of variability in 3-25 keV band. (The PSDs were fitted with Lorentzians, and the rms amplitudes were calculated by integrating the Lorentzians from 0 Hz to infinity.) There were no QPOs, and the variability never persisted for more than 3 days. The energy spectrum (fitted with absorption, smeared iron edge, power-law and disk blackbody) indicated a soft intermediate state (Homan & Belloni 2005) with photon index between 2.5 and 2.8 and disk temperature between 0.47 and 0.51 keV.

Recently, we observed spectral and temporal changes indicating that the source has entered a hard intermediate state. On May 12th, a well defined peak at 2 Hz with 9% rms amplitude of variability was observed, along with hardening of the energy spectrum. On May 13th, the PSD showed peaked structure and the rms amplitude was stronger, 13%. Finally, on May 14th, the rms amplitude jumped to 21% and a QPO appeared at 4 Hz. The photon index decreased to 2.20+-0.02 and the inner disk temperature was 0.376+-0.005 keV. On May 15th, the trend continued. The PSD showed 22% rms amplitude and a QPO at 3.25 Hz. The photon index was 2.07+-0.03, and the inner disk temperature was 0.32 keV. The 3-25 keV energy spectrum is dominated by the power-law component. The current 3-25 keV flux is 1.23e-9 ergs/cm2/s.

Based on the past behavior of this source, and other black hole transients, a hardening of the energy spectra and radio and infrared activity due to a compact jet are expected within 1-2 weeks (Kalemci et al. 2005). Multiwavelength observations are highly encouraged.