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H 1743-322 moves towards a state transition

ATel #3301; T. Belloni, H. Stiele, S. Motta, T. Munoz-Darias (INAF - Brera Observatory, Italy)
on 22 Apr 2011; 14:27 UT
Credential Certification: Tomaso Belloni (belloni@merate.mi.astro.it)

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

The black-hole candidate H 1743-322, currently in outburst (ATel #3277,#3280,#3295) in the Hard Intermediate State, was observed by RXTE on April 21 20:13-20:32UT. Compared with the previous observation on April 20 12:22-13:22UT, the source has softened considerably and its fast-variability characteristic frequencies have increased. The hardness (defined as the ratio of PCA counts in the 6.1-10.2 keV band over those in the 3.3-6.1 keV band) dropped from 0.794 +/- 0.004 to 0.644 +/- 0.004. The centroid frequency of the type-C QPO increased from 1.83 +/- 0.01 Hz to 3.60 +/- 0.02 Hz, while the 0.01-64 Hz fractional rms decreased from 25.2% to 21.9%. The energy spectrum on April 21 can be fitted with an absorbed Comptonization model including disk reflection (pexrav) plus an emission line. The best fit photon index of 2.34 +/- 0.02 is very similar to the values observed in 2009 at similar hardness. Only the flux is slightly lower.

The hardness evolution is very similar to that of the previous outburst in 2009 (Motta, Munoz-Darias & Belloni 2010, MNRAS, 408, 1796, see also http://web.me.com/tbelloni/BlackHoleTransients/H1743.html ), where a transition to the SIMS took place. If the similarity with the 2009 outburst holds, by extrapolating the time evolution of the hardness we can estimate that the transition will take place between April 23 and April 24. However, in 2008, no such transition was observed after a hardness value comparable to the current one (Capitanio et al. 2009, MNRAS, 398, 1194).

Observations at all wavelengths are strongly encouraged to follow the source into transition.