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

August 2013 • 2013MNRAS.433.2485B

Authors • Brightman, M. • Silverman, J. D. • Mainieri, V. • Ueda, Y. • Schramm, M. • Matsuoka, K. • Nagao, T. • Steinhardt, C. • Kartaltepe, J. • Sanders, D. B. • Treister, E. • Shemmer, O. • Brandt, W. N. • Brusa, M. • Comastri, A. • Ho, L. C. • Lanzuisi, G. • Lusso, E. • Nandra, K. • Salvato, M. • Zamorani, G. • Akiyama, M. • Alexander, D. M. • Bongiorno, A. • Capak, P. • Civano, F. • Del Moro, A. • Doi, A. • Elvis, M. • Hasinger, G. • Laird, E. S. • Masters, D. • Mignoli, M. • Ohta, K. • Schawinski, K. • Taniguchi, Y.

Abstract • 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/L₃₀₀₀. 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) log₁₀λ_Edd + (2.27 ± 0.06) and Γ = (-0.69 ± 0.11) log₁₀(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.

Links

• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2013MNRAS.433.2485B
• PREPRINT http://arxiv.org/abs/1305.3917
• DATA http://archive.eso.org/bin/ads2eso?2013MNRAS.433.2485B
• DATA https://cda.harvard.edu/chaser?obsid=441,581,582,1431,1672,2239,2312,2313,2405,2406,2409,5015,5016,5017,5018,5019,5020,5021,5022,6164,8591,8592,8593,8594,8595,8596,8597,9575,9578,9593,9596,9718,12043,12044,12045,12046,12047,12048,12049,12050,12051,12052,12053,12054,12055,12123,12128,12129,12135,12137,12138,12213,12218,12219,12220,12222,12223,12227,12230,12231,12232,12233,12234
• DATA https://nxsa.esac.esa.int/nxsa-web/#bibcode=2013MNRAS.433.2485B
• ELECTR https://doi.org/10.1093%2Fmnras%2Fstt920