A DELAYED TRANSITION TO THE HARD STATE FOR 4U 1630-47 AT THE END OF ITS 2010 OUTBURST

Here we report on Swift and Suzaku observations near the end of an outburst from the black hole transient 4U 1630- 47 and Chandra observations when the source was in quiescence. 4U 1630- 47 made a transition from a soft state to the hard state 50 days after the main outburst ended. During this unusual delay, the flux continued to drop, and one Swift measurement found the source with a soft spectrum at a 2- 10 keV luminosity of L = 1.07x1035 erg s-1 for an estimated distance of 10 kpc. While such transients usually make a transition to the hard state at L/LEdd = 0.3%- 3%, where LEdd is the Eddington luminosity, the 4U 1630- 47 spectrum remained soft at L/LEdd = 0.008 M- 1 10 % (as measured in the 2- 10 keV band), where M10 is the mass of the black hole in units of 10M. An estimate of the luminosity in the broader 0.5- 200 keV bandpass gives L/LEdd = 0.03 M- 1 10 %, which is still an order of magnitude lower than typical. We also measured an exponential decay of the X- ray flux in the hard state with an e- folding time of 3.39 0.06 days, which is much less than previous measurements of 12- 15 days during decays by 4U 1630- 47 in the soft state. With the 100 ks Suzaku observation, we do not see evidence for a reflection component, and the 90% confidence limits on the equivalent width of a narrow iron Ka emission line are < 40 eV for a narrow line and < 100 eV for a line of any width, which is consistent with a change of geometry (either a truncated accretion disk or a change in the location of the hard X- ray source) in the hard state. Finally, we report a 0.5- 8 keV luminosity upper limit of < 2 x 1032 erg s-1 in quiescence, which is the lowest value measured for 4U 1630- 47 to date.