Lamp-post with an outflow and the hard state of Cyg X-1

TL;DR

This study models relativistic reflection in Cyg X-1's hard state, considering a moving X-ray source on the black hole's axis, and finds that outflowing sources or truncated discs better explain observed spectra.

Contribution

The paper introduces an improved model, reflkerrV, for analyzing relativistic reflection accounting for source outflow and applies it to Cyg X-1 data, exploring geometries of the accretion disc.

Findings

Good fit with an untruncated disc and outflow at 0.36c

Better fit with a truncated disc and hot inner flow

Outflow or disc truncation explains weak reflection features

Abstract

Relativistic reflection observed in the hard states of accreting black holes often shows a weak amplitude relative to the main Comptonization component, which may result from either a disc truncation or a non-isotropy of the X-ray source, e.g. due to a motion away from the reflector. We investigate here the latter case, assuming that the X-ray source is located on the symmetry axis of the Kerr black hole. We discuss effects relevant to a proper computation of the reflected radiation and we implement them in the model for data analysis, reflkerrV. We apply it to the simultaneous Suzaku and NuSTAR observation of Cyg X-1 in the hard state and we find a good fit for an untruncated disc irradiated by the source moving away from it at 0.36c. However, we find a slightly better solution in a geometry closely approximating the truncated disc irradiated by an inner hot flow. In this solution we either still need a subrelativistic outflow or the source opposite to the observer must contribute to the directly observed radiation. We also discuss differences between the implementation of the outflow effect in reflkerrV and in relxilllpCp.