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

We report results of five pointed BeppoSAX observations of the black hole c andidate 4U 1630-47 during part of its 1998 outburst when the 2-10 keV sour ce luminosity decreased from 2.1 x 10(38) erg s(-1) (for an assumed distanc e of 10 kpc) by a factor of similar to 7. The 2-200 keV spectrum can be mod eled by a soft multi-temperature disk blackbody and a hard power-law. Durin g four of the observations, there is evidence for deviations from a pure po wer-law shape greater than or similar to 20 keV which may be due to reflect ion. As the outburst decayed a number of spectral trends are evident: (1) t he amount of photoelectric absorption decreased, (2) the spectrum became ha rder (i.e. the power-law index became smaller), (3) the temperature of the disk blackbody decreased and (4) the inferred inner disk radius increased b y a factor of similar to 4. The change in the accretion disk inner radius d uring the outburst is in contrast to results from other soft X-ray transien ts. We find that the disk blackbody temperature depends on the inner disk r adius roughly as T proportional to 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 obse rved by BeppoSAX.