A truncated inner disc in the Seyfert 1 galaxy WKK 4438

TL;DR

This study analyzes X-ray data from WKK 4438 to determine if its accretion disc extends to the ISCO, finding evidence for a truncated disc at about 10 gravitational radii and a high black hole spin.

Contribution

It provides the first evidence supporting a truncated inner disc in WKK 4438, challenging assumptions of continuous discs reaching the ISCO in such galaxies.

Findings

Disc may be truncated at ~10 rg

Black hole likely has high spin (a* ~ 0.998)

Models with truncated disc fit data well

Abstract

Understanding if and when the accretion disc extends down to the innermost stable circular orbit (ISCO) is important since it is the fundamental assumption behind measuring black hole spin. Here, we examine the 2013 and 2018 NuSTAR and Swift data (0.5 - 50 keV) of the narrow-line Seyfert 1 galaxy, WKK 4438. The X-ray emission can be fitted well with models depicting a corona and blurred reflection originating from a disc around a low spin (a* ~ 0) black hole. However, such models result in unconventional values for some of the parameters (e.g. inverted emissivity profile and high coronal height). Alternatively, equally good fits can be achieved if the disc is truncated at ~10 rg and the black hole is spinning at the Thorne limit (a* = 0.998). In these cases, the model parameters are consistent with the interpretation that the corona is centrally located close to the black hole and illuminating the disc at a larger distance.