The increasing availability of good quality broad-band X-ray spectral data has been proving essential for constraining the properties of the innermost accretion flow in BH-accreting systems. However, current measurements are mostly based on the use of a single technique, mostly the fit of reflection models to time-averaged X-ray spectra. While relevant, the self-consistency of the obtained results is rarely checked with independent methods.
Within the BHmapping project this problem was addressed in two ways. On the one hand, two spectral methods were developed to obtain independent estimates of the inner radius of the disc. The application of these methods reveals inconsistencies in several measurements reported in the literature. On the other hand, X-ray spectral-timing techniques were used to independently constrain the geometry of the inner accretion flow in BHXRBs. This allows accounting for the strong spectral variability of the source, an important aspect neglected in time-averaged spectral analysis. Overall, the results obtained are consistent with a scenario where strong evolution of the disc truncation radius occurs throughout an outburst, with the inner X-ray source showing spectral inhomogeneities usually not accounted for in time-averaged spectral fits.
In the BHmapping project, X-ray spectral-timing techniques were also used to independently constrain the properties of outflows. In particular, these were applied, for the first time, to the newly discovered X-ray obscurers in AGN. This allowed investigating variability timescales normally inaccessible to standard techniques. Most notably, this study proved the great potential of such approach, especially in view of the upcoming generation of high collective area and high spectral resolution calorimeters onboard XRISM and Athena. Finally, these techniques proved powerful in constraining jet-accretion flow coupling mechanisms, demonstrating the importance of simultaneous multi wavelength, high-time resolution monitorings of BHXRBs.
