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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 Capak, P. (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. ISSN 0035-8711. doi:10.1093/mnras/stt920.

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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, λ_(Edd), for sources where a bolometric luminosity (L_(Bol)) has been presented in the literature, based on spectral energy distribution fitting, or, for sources where these data do not exist, we calculate L_(Bol) using a bolometric correction to the X-ray luminosity, derived from a relationship between the bolometric correction and L_X/L_(3000). From a sample of 69 X-ray bright sources (>250 counts), where Γ can be measured with greatest precision, with an estimate of L_(Bol), we find a statistically significant correlation between Γ and λ_(Edd), 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 λ_(Edd) indicating that λ_(Edd) is the key parameter driving conditions in the corona. Linear regression analysis reveals that Γ = (0.32 ± 0.05) log_(10)λ_(Edd) + (2.27 ± 0.06) and Γ = (−0.69 ± 0.11) log_(10)(FWHM/km s^(−1)) + (4.44 ± 0.42). Our results on Γ–λ_(Edd) are in very good agreement with previous results. While the Γ–λ_(Edd) 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
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URLURL TypeDescription Paper DOIArticle
Brightman, M.0000-0002-8147-2602
Capak, P.0000-0003-3578-6843
Additional Information:© 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2013 May 22. Received 2013 May 21; in original form 2013 April. First published online: June 17, 2013. The authors would like to thank the anonymous referee for the careful reading and constructive criticism of our manuscript. The authors would also like to thank the builders and operators of XMM–Newton and Chandra and all those involved in the COSMOS and E-CDF-S surveys. MB would like to acknowledge the financial support at the start of this project from the Japan Society for the Promotion of Science (JSPS).
Funding AgencyGrant Number
Japan Society for the Promotion of Science (JSPS)UNSPECIFIED
Subject Keywords:accretion; accretion discs; galaxies: active; galaxies: nuclei; quasars: emission lines; quasars: general; X-rays: galaxies
Issue or Number:3
Record Number:CaltechAUTHORS:20130904-085052683
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41075
Deposited By: Tony Diaz
Deposited On:18 Sep 2013 17:23
Last Modified:10 Nov 2021 04:25

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