The 1998 outburst of the X-ray transient 4U 1630-47 observed with it BeppoSAX

We report results of five pointed BeppoSAXobserva- tions of the black hole candidate 4U 1630-47 during part of its 1998 outburst when the 2-10 keV source luminosity decreased from 2:1 10 38 erg s 1 (for an assumed distance of 10 kpc) by a factor of 7. The 2-200 keV spectrum can be modeled by a soft multi-temperature disk blackbody and a hard power-law. During four of the observations, there is evidence for devia- tions from a pure power-law shape >20 keV which may be due to reflection. As the outburst decayed a number of spectral trends are evident: (1) the amount of photoelectric absorption decreased, (2) the spectrum became harder (i.e. the power-law index became smaller), (3) the temperature of the disk black- body decreased and (4) the inferred inner disk radius increased by a factor of 4. The change in the accretion disk inner radius during the outburst is in contrast to results from other soft X-ray transients. We find that the disk blackbody temperature depends on the inner disk radius roughly as T / r 3=4 . For a standard Shakura-Sunyaev disk model this implies that the inferred mass accretion rate at the inner disk radius did not change appreciably during the part of the outburst observed by BeppoSAX.