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H 1743-322 (= IGR J17464-3213) Transition to the Hard State

ATel #198; John A. Tomsick (UC San Diego) & Emrah Kalemci (UC Berkeley)
on 20 Oct 2003; 20:23 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: John A. Tomsick (jtomsick@ucsd.edu)

Subjects: X-ray, Request for Observations, Black Hole, Transient

Referred to by ATel #: 304

The black hole candidate X-ray transient H 1743-322 (= IGR J1746-3213 = XTE J17464-3213) that began its current outburst on 2003 March 21 (ATEL#132) appears to be in the process of making a transition to the hard state. We have been monitoring this source several times per week with RXTE since 2003 Sept. 24 UT, a couple weeks after hard X-ray flux was detected from the source by INTEGRAL (ATEL#189). During our monitoring observations, the 3-25 keV PCA count rate dropped by a factor of 3, while the energy spectra are well-described by the canonical disk-blackbody plus power-law model with an inner disk temperature of 0.75-0.90 keV and a power-law photon index of 2.0-2.4. Although the spectral parameters have not shown major changes, the ratio of the power-law flux to the total flux (both in the 3-25 keV band) gradually increased from 0.34 on Oct. 15 to 0.79 during our most recent observation on Oct. 20, 2-3 hours UT. We measured a 3-25 keV absorbed flux of 1.65E-9 erg cm^-2 s^-1 on Oct. 20. In addition, we detected timing noise for the first time at an rms level of 7% on Oct. 18, increasing to an rms level of 11.0% +/- 0.1% for the continuum component on Oct. 20. We also detected a 7.8 Hz QPO and its harmonic during the most recent observation. These changes indicate that the source is making the transition to the hard state, and, based on the behavior of other systems, we predict that H 1743-322 will reach the hard state in a time period of a few days to a couple weeks. Black hole X-ray transients typically become bright at radio wavelengths in the hard state, and multi-wavelength observations are strongly encouraged. More RXTE observations are scheduled.