Swift detects an increase in the hard X-ray emission from the BHC 4U 1630-47

ATel #4077; P. Romano, V. Mangano (INAF-IASF Palermo), S. D. Barthelmy (GSFC), M. M. Chester (PSU), J. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), D. M. Palmer (LANL), M. Siegel (PSU), A. Bodaghee (UC Berkeley), E. Bozzo (ISDC), E. Kuulkers (ESAC/ESA), J. A. Tomsick (UC Berkeley)
on 29 Apr 2012; 17:02 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Pat Romano (romano@ifc.inaf.it)

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The X-ray binary and black-hole candidate 4U 1630-47, which has been in outburst since 2011 December (ATel #3830), triggered the Swift Burst Alert Telescope (BAT) on 2012-04-28 at 15:50:31 UT (trigger=521085, GCN 13270). Swift slewed immediately to the source. Using the data set from T-239 to T+963 s from recent telemetry downlinks, the BAT ground-calculated position is RA, Dec = 248.498, -47.435 deg, which equivalent to:
RA(J2000) = 16h 33m 59.6s
Dec(J2000) = -47d 26' 05.6"
with an uncertainty of 3.6 arcmin (radius, sys+stat, 90% containment). The partial coding was 100%. The ground-processed mask-weighted light curve shows roughly constant emission of 0.025 ph/cm2/s from the source at T-80 s, when the source location came into the BAT FoV during a pre-planned slew, out past T+1380 s, when it went out of the FoV due to an observing constraint. The time-averaged spectrum from T-239 to T+963 s is best fit by a simple power-law model with a photon index of 2.6+-0.4 and a 15-50 keV flux of 1E-09 ergs/cm2/s. All the quoted errors are at the 90% confidence level.

The XRT began observing the field at 15:57:07.4 UT, 395.6 seconds after the BAT trigger. The XRT on-board position is RA, Dec = 248.5063, -47.3934 deg, which is equivalent to:
RA(J2000) = +16h 34m 1.51s
Dec(J2000) = -47d 23' 36.2"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This position is 1.8 arcseconds from the radio position for 4U 1630-47. The light curve averages around about 100 counts/s and shows bright flares that reached about 130 counts/s during the first orbit of data and 160 counts/s during the second and third orbits. The spectrum of the first orbit of data (T+402 to T+1745 s) can be fit with an absorbed power law with a photon index of 3.3+-0.4 and an absorbing column of (12.1+-0.2)E22 cm-2. The observed (unabsorbed) 2-10 keV flux is 8E-9 (3E-8) ergs/cm2/s.

By combining the strictly simultaneous BAT event data and XRT data (T+402 to T+963 s) and fitting with an absorbed power-law plus disk-blackbody model we obtain (Chi_nu^2/dof=1.3/675) a column density of 8.8(-0.2+0.4)E22 cm-2, gamma=2.4+-0.3, a disk-blackbody temperature =1.5+-0.1 keV and an observed 2-50 keV flux of ~2E-8 ergs/cm2/s. These results are compatible with those reported by Bodaghee et al (ATel #3945).

UVOT began observing the transient 405 s after the BAT trigger. No optical/UV counterpart has been found near the location of the XRT source. The 3-sigma upper limit at the XRT position in the first white finding chart is 20.2 mag; 3-sigma upper limits in summed images in the white, v, b, u, uvw1, uvm2, uvw2 bands are 20.4, 18.0, 19.7, 18.8, 18.9, 18.4, and 19.1 mag, respectively, in a total exposure time of 1.4 ks. No correction has been made for the large, but uncertain extinction expected.

The long outburst of this source and the recent increase in flux is also recorded by the BAT Transient Monitor (http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/4U1630-472/) and by MAXI. Nakahira et al. (ATel #4075) propose the source may be undergoing a state transition, possibly into the very high state.

Scaled Map Transient Analysis for 4U 1630-472