The Black Hole Candidate 4U 1630-47 in the Midst of a Transition to the Hard State
ATel #247; J. A. Tomsick
on 8 Mar 2004; 06:10 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
Monitoring observations by the Rossi X-ray Timing Explorer indicate that the recurrent black hole candidate X-ray transient 4U 1630-47 is undergoing a transition to the Hard state. The energy spectrum, which is well-described by the canonical disk-blackbody plus power-law model (along with interstellar absorption and an iron absorption edge), has been evolving over the past few weeks with the most significant evolution being a gradual drop in the inner disk temperature from about 1 keV to 0.49 +/- 0.03 keV during the most recent observation on UT 2004 March 7. The power-law photon index has also hardened and has been between 1.9 and 2.0 during four observations over the past 5 days
(March 2-7). The 3-25 keV absorbed flux was 6.5 x 10^-10 erg/cm2/s on March 7, and this is close to the flux where transitions to the Hard state have been seen during previous 4U 1630-47 outbursts. The source also showed a significant increase in the level of timing noise (to 10-15% rms) over the past 5 days, and the power spectrum shows strong, 6-10% rms, QPOs at 4-6 Hz. Although 4U 1630-47 has been in outburst for the past 1.5 years, these are the first indications of Hard state activity during this outburst. Hard state activity in other black hole sources is often accompanied by an increase in IR and radio flux, and multi-wavelength observations are encouraged. IR observations would be especially useful as they could lead to a confirmation of the probable IR counterpart suggested by Augusteijn et al. (2001, A&A, 375, 447). RXTE monitoring will continue.
