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Multiple fast state transitions in GX 339-4

ATEL # 2593; S. Motta, T. Belloni, T. Muņoz-Darias (INAF - Brera Observatory), J. Homan (MIT)
on 2 May 2010; 17:51 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Tomaso Belloni (belloni@merate.mi.astro.it)

Subjects: X-ray, Request for Observations, Binaries, Black Holes, Transients

The daily RXTE monitoring of the black-hole candidate GX 339-4 showed in the past few days evidence for frequent state transitions. After the main transition to the soft state (HSS), between April 18 and April 20 (ATel #2577), GX 339-4 remained in the HSS but showed a few instances of type-B QPOs, indicative of a back-transition to the soft intermediate state (SIMS). These SIMS observations were made on April 22 23:36-00:00UT, April 29 16:32-17:34UT and April 30 02:45-03:06UT. These three observations corresponded to an X-ray color (defined as the ratio of PCA counts in the 5.7-9.5 keV band over those in the 2.9-5.7 keV band) in the range expected for the SIMS (0.24-0.28). The light curves of the April 29-30 observations were dominated by dips (18% of the count rate in the 3-33 keV range). The dips have a sharp drop and slower recovery shape and a typical duration of 10-20, separated by 20-50 seconds. A hardness analysis shows that the dips correspond to a slight hardening of the spectrum, possibly indicative of absorption effects. During the dips, the type-B QPO is visible only in the high-energy channels. The latest RXTE/PCA observation (on 2010 May 2, 10:3211:00UT, ID 95409-01-17-02) clearly showed that the source has moved back to the hard intermediate state (HIMS). The power-density spectrum is dominated by three broad-band components with characteristic frequencies around 0, 0.3 and 4.1 Hz respectively, over which which a type C QPO with centroid 6.7 Hz is visible. The total 0.1-64 Hz fractional rms was 14% and the X-ray color was at 0.3 (HIMS). In comparison, the previous observation (2010 May 1, 02:14-02:38UT, ID 95409-01-17-01) showed a total 0.1-64 Hz fractional rms of 4% and an X-ray color around 0.2 (HSS). The power density spectrum was dominated by a power law component and no QPO was observed. A preliminary fit to the PCA energy spectrum of the HIMS observation with a power law, a disk blackbody and a Gaussian emission line (plus interstellar absorption) yields a power law photon index of 2.3 and a disk inner temperature of ~0.9 keV. A new softening is expected in the next few days, based on the usual outburst evolution. In particular, this sequence of intermediate states might lead to a (new) jet ejection and variable radio emission. Multi-wavelength observations covering these transitions are encouraged.

Light curve and hardness-intensity diagram for the current GX 339-4 outburst


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