Stacked X-ray spectra of eROSITA AGN

With Xstack, we stacked 17929 AGNs ("spec-z" sample, total exposure ~23Ms) with similar X-ray loudness, {alpha}_ox_, and UV luminosity, LUV, and 4159 AGNs ("BH-mass" sample, ~3Ms) with similar Eddington ratios, {lambda}_Edd_, and black hole masses, MBH. We analyzed the resulting stacked X-ray spectra with a phenomenological model for both samples. We further fit the stacked optical-UV X-ray SED with the physical AGNSED model on a 3x3M_BH_-{lambda}_Edd_ grid.

We observed that the soft excess strength rises strongly with increasing {alpha}_ox_ and {lambda}_Edd_ binning (by a factor of five), while the hard X-ray spectral shape remains largely unchanged, consistent with the interpretation that soft excess is primarily driven by the warm corona rather than reflection. The trends are weaker with LUV binning and reversed for MBH binning. The analysis of the optical-UV X-ray SEDs with AGNSED revealed that the warm corona radius (in units of Rg) generally increases with {lambda}_Edd_ and decreases with MBH, or equivalently the disk-to-warm-corona transition consistently occurs near ~1x10^4^K. The hot corona contracts with {lambda}_Edd_, and the radius remains independent of MBH, aligning with disk evaporation predictions.

The soft excess is likely warm-corona dominated, with the disk-to-warm-corona transition potentially linked to hydrogen ionization instability at ~1x10^4^K, which is consistent with previous work utilizing eFEDS-HSC stacked data. Our work highlights the power of spectral stacking for revealing the AGN disk-corona connection.