abstract
- We study the spectral properties of the first 14 observations of the rise to outburst phase of the X-ray transient J1550-564. Using the PCA and HEXTE instruments, we find that the 3-200 keV spectra smoothly pass from a standard low/hard state to a very high state. The classic high state is never encountered possibly indicating that it is not a phenomenon of the rise phase. We find that the individual PCA spectra can be fitted adequately by a disk black body and a thermal Comptonization model which includes reflection. Once the very high state is reached there is clear spectral curvature of the continuum which possibly indicates the presence of a composite thermal/non-thermal plasma. Detailed modelling of the reflection parameters shows a sharp increase in mean ionization at the onset of the transition between the low state and very high state. There is a related variability in the reflected fraction but its exact value depends on the continuum model used. We can constrain the inner radius using relativistic smearing and while there are large uncertainties, the data are incompatible with a disk extending to the last stable orbit in either state. Since the system is on the rise to outburst, the disk instability models strongly imply that there is no standard inner disk at the time the low state spectrum is observed. This is compatible with a truncated disk, filled by an X-ray hot, advection dominated accretion flow. However, magnetic flares above the outbursting disk can also match the observed spectra once the effects of either outflow and/or strong photoionization of the surface of the disk are included.