Relativistic X-Ray Reverberation from Super-Eddington Accretion Flow

Abstract

<jats:title>Abstract</jats:title> <jats:p>X-ray reverberation is a powerful technique that uses the echoes of the coronal emission reflected by a black hole (BH) accretion disk to map out the inner disk structure. While the theory of X-ray reverberation has been developed almost exclusively for standard thin disks, reverberation lags have recently been observed from likely super-Eddington accretion sources such as the jetted tidal disruption event Swift J1644+57. In this paper, we extend X-ray reverberation studies into the regime of super-Eddington accretion with a focus on investigating the lags in the fluorescent Fe K<jats:italic>α</jats:italic> line region. We find that the coronal photons are mostly reflected by the fast and optically thick winds launched from the super-Eddington accretion flow, and this funnel-like reflection geometry produces lag–frequency and lag–energy spectra with unique observable characteristics. The lag–frequency spectrum exhibits a step-function-like decline near the first zero-crossing point. As a result, the magnitude of the lag scales linearly with the BH mass for a large parameter space, and the shape of the lag–energy spectrum remains almost independent of the choice of frequency bands. Not only can these features be used to distinguish super-Eddington accretion systems from sub-Eddington systems, but they are also key for constraining the reflection geometry and extracting parameters from the observed lags. When fitting the observed reverberation lag of Swift J1644+57 to our modeling, we find that the super-Eddington disk geometry is slightly preferred over the thin disk geometry, and we obtain a BH mass of 5–6 × 10<jats:sup>6</jats:sup> <jats:italic>M</jats:italic> <jats:sub>☉</jats:sub> and a coronal height around 10 <jats:italic>R<jats:sub>g</jats:sub> </jats:italic>.</jats:p>