A statistical relation between the X-ray spectral index and Eddington ratio of active galactic nuclei in deep surveys
By M. Brightman, J. Silverman, V. Mainieri, Y. Ueda, M. Schramm, K. Matsuoka, Tohru Nagao, C. Steinhardt, J. Kartaltepe, D. Sanders and 26 more. 2013
154 citations recorded by the source corpus.
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.
Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics
View Paper: https://www.semanticscholar.org/paper/5dc57a68f395aef00ce01f7a51ee5c0aff957c20
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Citations in the corpus
- X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue (2014, background): “The ultimate source of power is accretion onto the black hole, and thus Γ is thought to be related to physical AGN properties such as the Eddington rate (see Brightman et al. 2013, for a recent discussion).”
- Cosmic X-ray surveys of distant active galaxies (2015, unknown): “…have now been confirmed via both targeted (e.g., Shemmer et al 2006, 2008) and X-ray survey-based (e.g., Risaliti et al 2009; Jin et al 2012a; Brightman et al 2013; Fanali et al 2013) studies, which find clear Γ − LBol/LEdd correlations across a broad range of luminosity and redshift (and no…”