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Description
The black hole X-ray binary 4U 1630-472 shows outbursts repeatedly with an interval of 600 days. We studied the 2012 outburst, which was one of the biggest outbursts of the source. We found unusual X-ray flux drops by ~50% with a duration of ~1 day at its brightest phase, which is much shorter than the timescale of accretion but longer than the dynamical timescale in the inner region of the accretion disk. To understand the cause of the drops, we compared the MAXI and Swift spectra obtained during and outside the drops. We found that, during the drops, the source decreased its flux without significantly changing the spectral shape below 10 keV. We consider two possible interpretations for the observed spectral variation: (1) fully ionized, Compton-thick gas passed through our line of sight and reduced the apparent X-ray flux, and (2) the Comptonized corona developed with the decrease of the inner disk temperature and the disk luminosity and the spectral hardening by strong Comptonization compensated the spectral softening.
