Systematic broad-band X-ray study of super-Eddington accretion on to supermassive black holes - I. X-ray continuum

Abstract

We present the first systematic broad-band X-ray study of super-Eddington accretion onto supermassive black holes with simultaneous NuSTAR and XMM-Newton or Swift/XRT observations of a sample of eight super-Eddington accreting Active Galactic Nuclei (AGNs) with Eddington ratio 1 < λ _Edd < 426. We find that the super-Eddington accreting massive black holes (SEAMBHs) show a steep primary continuum slope as expected for sources accreting in the super-Eddington regime, mostly dominated by relativistic reflection. The iron K α emission lines of the sources of our sample show relativistic broadening. In addition, the equivalent widths of the narrow components of the iron K α lines follow the 'X-ray Baldwin' effect, also known as the 'Iwasawa-Taniguchi' effect. We found a statistically significant correlation between the photon index of the primary power law and the Eddington ratio, consistent with past studies. Moreover, as expected for super-Eddington sources, the median value of the reflection fraction of the sources we analysed is a factor ~ 2 higher than the median reflection fraction value of the type 1 AGN from the BASS sample. We are able to estimate the coronal temperature for three sources of our sample: Mrk 382 (kT_e = 7.75 keV), PG 0026+129 (kT_e = 39 keV), and IRAS 04416+1215 (kT_e = 2.95 keV). Looking at the position of the SEAMBHs sources of our sample in the compactness-temperature diagram it appears that in super-Eddington AGN, as for lower Eddington ratio AGN, the X-ray corona is controlled by pair production and annihilation.