A Delayed Transition to the Hard State for 4U 1630-47 at the End of its 2010 Outburst

Abstract

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 = 1.07 × 10³⁵ erg s^-1 for an estimated distance of 10 kpc. While such transients usually make a transition to the hard state at L/L _Edd = 0.3%-3%, where L _Edd is the Eddington luminosity, the 4U 1630-47 spectrum remained soft at L/L _Edd = 0.008 M₁₀^-1% (as measured in the 2-10 keV band), where M ₁₀ is the mass of the black hole in units of 10 M _⊙. An estimate of the luminosity in the broader 0.5-200 keV bandpass gives L/L _Edd = 0.03 M₁₀^-1%, 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 Kα 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 × 10³² erg s^-1 in quiescence, which is the lowest value measured for 4U 1630-47 to date.