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BAT AGN Spectroscopic Survey - XII. The relation between coronal properties of active galactic nuclei and the Eddington ratio
Ricci, C.; Ho, L. C.; Fabian, A. C.; Trakhtenbrot, B.; Koss, M. J.; Ueda, Y.; Lohfink, A.; Shimizu, T.; Bauer, F. E.; Mushotzky, R.; Schawinski, K.; Paltani, S.; Lamperti, I.; Treister, E.; Oh, K.
AA(Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile; Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China; Chinese Academy of Sciences South America Center for Astronomy, Camino El Observatorio 1515, Las Condes, Santiago, Chile 0000-0001-5231-2645), AB(Chinese Academy of Sciences South America Center for Astronomy, Camino El Observatorio 1515, Las Condes, Santiago, Chile; Department of Astronomy, School of Physics, Peking University, Beijing 100871, China), AC(Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK 0000-0002-9378-4072), AD(Department of Physics, ETH Zurich, Wolfgang-Pauli-Str. 27, CH-8093 Zurich, Switzerland 0000-0002-3683-7297), AE(Eureka Scientific Inc., 2452 Delmer St. Suite 100, Oakland, CA 94602, USA), AF(Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan), AG(Department of Physics, Montana State University, Bozeman, MT 59717-3840, USA), AH(Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, D-85741 Garching, Germany 0000-0002-2125-4670), AI(Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile; Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA; Millenium Institute of Astrophysics, Santiago, Chile), AJ(Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA), AK(Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Str. 27, CH-8093 Zurich, Switzerland), AL(Department of Astronomy, University of Geneva, ch. d'Ecogia 16, CH-1290 Versoix, Switzerland), AM(Astrophysics Group, Department of Physics and Astronomy, University College London, 132 Hampstead Road, London NW1 2PS, UK 0000-0003-3336-5498), AN(Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile), AO(Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan)
Monthly Notices of the Royal Astronomical Society, Volume 480, Issue 2, p.1819-1830 (MNRAS Homepage)
Publication Date:
Astronomy Keywords:
galaxies: active, galaxies: Seyfert, quasars: general, quasars: supermassive black holes, X-rays: general
Abstract Copyright:
2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
Bibliographic Code:


The bulk of the X-ray emission in active galactic nuclei (AGNs) is produced very close to the accreting supermassive black hole (SMBH), in a corona of hot electrons which up scatters optical and ultraviolet photons from the accretion flow. The cut-off energy (EC) of the primary X-ray continuum emission carries important information on the physical characteristics of the X-ray emitting plasma, but little is currently known about its potential relation with the properties of accreting SMBHs. Using the largest broad-band (0.3-150 keV) X-ray spectroscopic study available to date, we investigate how the corona is related to the AGN luminosity, black hole mass and Eddington ratio (lambdaEdd). Assuming a slab corona the median values of the temperature and optical depth of the Comptonizing plasma are kTe = 105 ± 18 keV and tau = 0.25 ± 0.06, respectively. When we properly account for the large number of EC lower limits, we find a statistically significant dependence of the cut-off energy on the Eddington ratio. In particular, objects with lambdaEdd > 0.1 have a significantly lower median cut-off energy (EC = 160 ± 41 keV) than those with lambdaEdd <= 0.1 (EC = 370 ± 51 keV). This is consistent with the idea that radiatively compact coronae are also cooler, because they tend to avoid the region in the temperature-compactness parameter space where runaway pair production would dominate. We show that this behaviour could also straightforwardly explain the suggested positive correlation between the photon index (Gamma) and the Eddington ratio, being able to reproduce the observed slope of the Gamma-lambdaEdd trend.
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