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Evidence for Hard State Activity from the Black Hole Transient 4U 1630-47

ATEL # 2794; John A. Tomsick (UCB/SSL) and Kazutaka Yamaoka (Aoyama Gaukin Univ.)
on 17 Aug 2010; 7:15 UT
Password Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

Subjects: X-ray, Black Holes, Transients

Swift and RXTE observations provide strong evidence that 4U 1630-47 is currently in the hard state. This black hole transient had an outburst that started in 2009 December (ATELs #2363 and #2365). RXTE monitored the source until 2010 July 24 when it became too faint to observe with RXTE, but the transition to the hard state that is usually seen for black hole transients during outburst decay was never observed.

However, over the past 10 days (starting around 2010 August 7), the RXTE Galactic bulge scans show evidence for activity from 4U 1630-47 (see http://lheawww.gsfc.nasa.gov/users/craigm/galscan/html/4U_1630-472.html). In addition, a pointed RXTE observation of the source taken on 2010 August 14 indicates the presence of variability in the 0.01-10 Hz band at an rms level of more than 8% (3-25 keV). Although a higher rms level (than 8%) is often seen in the hard state, the RXTE/PCA observations are contaminated by a nearby source (IGR J16320-4751), so the rms level seen on August 14 is likely considerably higher.

On 2010 August 17, we obtained a 700 second Swift observation to check the flux of the source. The Swift observation verifies that the flux level from 4U 1630-47 has indeed increased, and the 1-10 keV Swift/XRT spectrum is consistent with an absorbed power-law with a photon index of 2.0+/-0.6 (90% confidence errors) and a column density of (7+/-3)x10^22 cm^-2 using interstellar abundances from Wilms et al. (2000) and cross-sections from Balucinska-Church & McCammon (1998). The spectrum also indicates an unabsorbed 2-10 keV flux level of 1.0x10^-10 ergs cm^-2 s^-1. Thus, the Swift observation confirms the enhanced flux suggested by RXTE, and the variability seen by RXTE along with the hard spectrum measured by Swift provide evidence for hard state activity.


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R. E. Rutledge , Editor
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