X-ray emission from active galactic nuclei with intermediate-mass black holes

Abstract

We present a systematic X-ray study of eight active galactic nuclei (AGNs) with intermediate-mass black holes (M_BH~ 8-95 × 10⁴ M_⊙) based on 12 XMM-Newton observations. The sample includes the two prototype AGNs in this class-NGC 4395 and POX 52 and six other AGNs discovered with the Sloan Digitized Sky Survey. These AGNs show some of the strongest X-ray variability, with the normalized excess variances being the largest and the power density break timescales being the shortest observed among radio-quiet AGNs. The excess-variance-luminosity correlation appears to depend on both the BH mass and the Eddington luminosity ratio. The break timescale-black hole mass relations for AGN with IMBHs are consistent with that observed for massive AGNs. We find that the FWHM of the Hβ /Hα line is uncorrelated with the BH mass, but shows strong anticorrelation with the Eddington luminosity ratio. Four AGNs show clear evidence for soft X-ray excess emission (kT_in ~ 150-200 eV). X-ray spectra of three other AGNs are consistent with the presence of the soft excess emission. NGC 4395 with lowest L/L_Edd lacks the soft excess emission. Evidently small black mass is not the primary driver of strong soft X-ray excess emission from AGNs. The X-ray spectral properties and optical-to-X-ray spectral energy distributions of these AGNs are similar to those of Seyfert 1 galaxies. The observed X-ray/UV properties of AGNs with IMBHs are consistent with these AGNs being low-mass extensions of more massive AGNs, those with high Eddington luminosity ratio looking more like narrow-line Seyfert 1 s and those with low L/L_Edd looking more like broad-line Seyfert 1 galaxies.