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Transition to the soft state in GX 339-4

ATel #2577; T. Belloni, S. Motta, T. Muñoz Darias (INAF - Brera Observatory)
on 20 Apr 2010; 08:31 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Tomaso Belloni (belloni@merate.mi.astro.it)

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

Referred to by ATel #: 2593, 2745, 3117, 3191, 3383, 5252

The daily RXTE pointed monitoring of the black-hole candidate GX 339-4 showed clear indications of a transition to the soft intermediate state and then to the soft state. On 2010 April 17 14:49UT the source was in the hard intermediate state, displaying a type-C QPO at 5.71+/- 0.03 Hz in the power density spectrum, together with band limited noise. The total 0.1-64 Hz fractional rms was 14%. The source spectrum was already softening considerably since a few days (see ATel # 2573) and its 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) was at 0.36. The next RXTE observation was made on 2010 April 18 17:04-17:36UT. The X-ray color was now 0.27, a value which is the previous recorded outburst corresponded to transitions to the SIMS. The power density spectrum shows a clear type-B QPO at 5.73 +/- 0.01 Hz, Gaussian shaped (6.5% fractional rms) and with very little associated noise in form of a power law. Total fractional rms was 12%. The following observation on April 20 02:45-03:04 showed a much softer spectrum, with a color of 0.14 and a featureless power density spectrum with a 4% fractional rms, typical of the soft state. The total count rate also dropped starting from this observation, after 96 days of increase. The transition took place exactly where it was expected on the basis of the previous outbursts.

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