Hard X-ray monitoring observations by INTEGRAL of the Norma Arm region show that the X-ray binary and black-hole candidate 4U 1630-47 has brightened in the hard X-rays (>18 keV) which could indicate the start of a possible state transition since its X-ray outburst of 2011 December (ATel #3830).
During INTEGRAL revolution 1143 (2012 Feb. 22), 4U 1630-47 was detected by both JEM-X (~56 sigma) and ISGRI (~ 9 sigma) at fluxes of 350+-6 mCrab (3-10 keV), 58+-10 mCrab (10-25 keV), and 14+-1 mCrab (18-40 keV). The source was not detected in the 40-100 keV band. Around one week later, in rev. 1145 (2012 Feb 28), the source was again detected by both instruments: 347+-4 mCrab (3-10 keV), 45+-6 mCrab (10-25 keV), and 14+-2 mCrab (18-40 keV).
For comparison, in rev. 1139 (2012 Feb. 10), 4U 1630-47 was not detected by ISGRI (3-sigma upper limit of ~ 6 mCrab in 18-40 keV) even though the JEM-X fluxes were similar (332+-10 mCrab and 46+-20 mCrab in 3-10 keV and 10-25 keV, respectively).
Fitting an absorbed disk-blackbody + power-law model to the JEM-X and ISGRI spectra from rev. 1145 yields Gamma = 3.3(-0.9+1.0) with a disk-blackbody temperature of 1.4+-0.2 keV (Chi_nu^2/dof=0.9/11). The absorbing column is nH~11x10^22 /cm2, but it is poorly constrained. The source flux is (6+-1)x10^-9 erg/cm2/s over the energy range of the fit (3-80 keV).
These spectral parameters are close to the values listed in Table 4.4 of McClintock & Remillard (2006) for 4U 1630-47 in the thermal-dominant state. The disk-blackbody component contributes 74\% of the continuum flux between 2 and 20 keV. Given the new detections in the hard X-ray band with ISGRI, the flux of the power-law component has increased since rev. 1139.
Monitoring observations of 4U 1630-47 by other X-ray telescopes confirm the recent increase in hard X-rays. The MAXI light curve (http://maxi.riken.jp/top/index.php?cid=1&jname=J1634-473) indicates that the source softened considerably after its 2011 December outburst, remained in the soft state for ~2 months, and is now trending harder. The rise in hard X-ray flux (15-50 keV) is also visible in the Swift-BAT light curve of the source (http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/4U1630-472/).
Taken together, these observations suggest that 4U 1630-47 is still formally consistent with being in the thermal-dominant state, but it might be heading toward an intermediate state or the steep power-law state. We will continue to monitor this source, since its field will be observed again by INTEGRAL in the coming days and weeks.
We take this opportunity to announce our INTEGRAL Spiral Arms (ISA) program, in which INTEGRAL will regularly monitor the Norma/Perseus Arms and Scutum/Sagittarius Arms throughout the year. Images of the fields and light curves for the sources they contain can be found at the following URL:
|Publication date||29 Feb 2012|
|Publication status||Published - 29 Feb 2012|
|Series||The Astronomer's telegram|
- Black hole