A truncated inner disc in the Seyfert 1 galaxy WKK 4438

Understanding whether and when the accretion disc extends down to the innermost stable circular orbit is important since it is the fundamental assumption behind measuring black hole spin. Here, we examine the 2013 and 2018 NuSTAR and Swift data ($0.5\!-\!50{\rm \, keV}$) 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 $\sim 10{\, r_{\rm g}}$ 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.