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 (MBH ∼ 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 time scales 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 time scale – 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 (kTin ∼ 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/LEdd 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 extension of more massive AGNs; those with high Eddington luminosity ratio looking more like narrow-line Seyfert 1s while those with low L/LEdd looking more like broad-line Seyfert 1s.