Mapping the inner accretion disc of the Galactic black hole J1550–564 through its rise to outburst

We study the spectral properties of the first 14 observations of the rise to outburst phase of the X-ray transient J1550–564. Using both the PCA and HEXTE instruments, we find that the 3–200 keV spectra pass smoothly 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 disc blackbody 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. Our 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. The reflected fraction varies around values of Ω/2π∼0.1, and is never consistent with Ω/2π = 1. We can constrain the inner radius using relativistic smearing and, while there are large uncertainties, the data are incompatible with a disc extending to the last stable orbit (6 Rg) in either state. As the system is on the rise to outburst, the disc instability models (and observed increasing QPO frequency) strongly imply that there is no standard inner disc at the time the low-state spectrum is observed. This is compatible with a truncated disc, filled by an X-ray hot, advection-dominated accretion flow. However, magnetic flares above the outbursting disc can also match the observed spectra, once the effects of either outflow and/or strong photoionization of the surface of the disc are included. We clearly see strong ionization of the reflector in the very high state. This is probably from collisional ionization, as the disc surface temperature is ∼0.7 keV. This can strongly suppress reflection from the inner disc.