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A statistical relation between the X-ray spectral index and Eddington ratio of active galactic nuclei in deep surveys.

Brightman, M. and Silverman, J.D. and Mainieri, V. and Ueda, Y. and Schramm, M. and Matsuoka, K. and Nagao, T. and Steinhardt, C. and Kartaltepe, J. and Sanders, D.B. and Treister, E. and Shemmer, O. and Brandt, W.N. and Brusa, M. and Comastri, A. and Ho, L.C. and Lanzuisi, G. and Lusso, E. and Nandra, K. and Salvato, M. and Zamorani, G. and Akiyama, M. and Alexander, D.M. and Bongiorno, A. and Capak, P. and Civano, F. and Del Moro, A. and Doi, A. and Elvis, M. and Hasinger, G. and Laird, E.S. and Masters, D. and Mignoli, M. and Ohta, K. and Schawinski, K. and Taniguchi, Y. (2013) 'A statistical relation between the X-ray spectral index and Eddington ratio of active galactic nuclei in deep surveys.', Monthly notices of the Royal Astronomical Society., 433 (3). pp. 2485-2496.


We present an investigation into how well the properties of the accretion flow on to a supermassive black hole may be coupled to those of the overlying hot corona. To do so, we specifically measure the characteristic spectral index, Γ, of a power-law energy distribution, over an energy range of 2–10 keV, for X-ray selected, broad-lined radio-quiet active galactic nuclei (AGN) up to z ∼ 2 in Cosmic Evolution Survey (COSMOS) and Extended Chandra Deep Field South (E-CDF-S). We test the previously reported dependence between Γ and black hole mass, full width at half-maximum (FWHM) and Eddington ratio using a sample of AGN covering a broad range in these parameters based on both the Mg II and Hα emission lines with the later afforded by recent near-infrared spectroscopic observations using Subaru/Fibre Multi Object Spectrograph. We calculate the Eddington ratios, λEdd, for sources where a bolometric luminosity (LBol) has been presented in the literature, based on spectral energy distribution fitting, or, for sources where these data do not exist, we calculate LBol using a bolometric correction to the X-ray luminosity, derived from a relationship between the bolometric correction and LX/L3000. From a sample of 69 X-ray bright sources (>250 counts), where Γ can be measured with greatest precision, with an estimate of LBol, we find a statistically significant correlation between Γ and λEdd, which is highly significant with a chance probability of 6.59× 10−8. A statistically significant correlation between Γ and the FWHM of the optical lines is confirmed, but at lower significance than with λEdd indicating that λEdd is the key parameter driving conditions in the corona. Linear regression analysis reveals that Γ = (0.32 ± 0.05) log10λEdd + (2.27 ± 0.06) and Γ = (−0.69 ± 0.11) log10(FWHM/km s−1) + (4.44 ± 0.42). Our results on Γ–λEdd are in very good agreement with previous results. While the Γ–λEdd relationship means that X-ray spectroscopy may be used to estimate black hole accretion rate, considerable dispersion in the correlation does not make this viable for single sources, however could be valuable for large X-ray spectral samples, such as those to be produced by eROSITA.

Item Type:Article
Keywords:Accretion, Accretion discs, Galaxies: active, Galaxies: nuclei, Quasars: emission lines, Quasars: general, X-rays: galaxies.
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Publisher statement:This article has been accepted for publication in Monthly notices of the Royal Astronomical Society © 2013 The Authors Published by Oxford University Press on behalf of Royal Astronomical Society. All rights reserved.
Date accepted:No date available
Date deposited:30 June 2014
Date of first online publication:August 2013
Date first made open access:No date available

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