Broad iron line as a relativistic reflection from warm corona in AGNs

Context. We present that the broad feature usually observed in X-ray spectra at around 6.4 keV can be explained by ray-traced emission from the two-slab system containing a dissipative, warm corona on the top of an accretion disk in an active galactic nucleus (AGN). Such an accretion flow is externally illuminated by X-ray radiation from a lamp located above a central supermassive black hole (SMBH). Thermal lines from highly ionized iron ions (FeXXV and FeXXVI) caused by both internal heating and reflection from the warm corona, can be integrated into the observed broad line profile due to the close vicinity to the SMBH. Aims. We investigate the dependence of the total broad line profile on the variations in black hole spin parameter, viewing angle, lamp height, and dissipation factor. Our results introduce a new method to probe properties of warm corona using high-resolution spectroscopic measurements with current XRISM and future NewATHENA X-ray missions. Methods. We use photoionization code <ns0:monospace>TITAN</ns0:monospace> to compute local ion population and emission line profiles, and ray-tracing code <ns0:monospace>GYOTO</ns0:monospace> to include relativistic effects on the outgoing X-ray spectrum. Results. In our models, the temperature of the inner atmosphere covering the disk can reach values of 107 − 108 K due to warm corona dissipation and external illumination, which is adequate for generating highly ionized iron lines. These lines can undergo significant gravitational redshift near the black hole, leading to a prominent spectral feature centered around 6.4 keV. Conclusions. For all computed models, relativistic corrections shift highly ionized iron lines to the 6.4 keV region, usually attributed to fluorescent emission from the illuminated skin of an accretion disk. Hence, for a warm corona that covers the inner disk regions, the resulting theoretical line profile under strong gravity is a sum of different iron line transitions, with highly ionized iron contributing the most to the total line profile observed in an AGN.