2023MNRAS.522.2869S

Spectral energy distribution (SED) of the broad-band continuum emission from black-hole accretion discs can serve as a tool to measure parameters of the central body and constrain the geometry of the inner accretion flow. We focus on the case of an active galactic nucleus (AGN), with an accretion disc dominating the UV/optical bands. We parametrize the changes in the thermal and power-law components, which can reveal the diminution of the emissivity. To this end we explore the effects of gaps in the accretion disc and the emerging SED that can be caused by the presence of either (i) the inner, optically thin, radiatively inefficient hot flow; (ii) a secondary black hole embedded within the accretion disc; or (iii) a combination of both components. We suggest that the resulting changes in the SED of the underlying continuum can help us understand some departures from the standard-disc scenario. We estimate that the data required for such a project must be sampled in detail over the far-UV to soft X-ray bands during the interval of about a month corresponding to the characteristic variability time-scale of an AGN. Detecting a gap at intermediate radii of a few 100 gravitational radii would require quality photometry with uncertainties up to $\sim 1~{{\ \rm per\ cent}}$. The presence of the central cavity in the standard disc can be recovered in UV photometric data with an accuracy of 5 per cent and better. We show the effect of the intrinsic reddening of the source and demonstrate when it can be disentangled.