Dust and the intrinsic spectral index of quasar variations: hints of finite stress at the innermost stable circular orbit

Author:

Weaver John R123ORCID,Horne Keith3

Affiliation:

1. Cosmic Dawn Center (DAWN)

2. Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen, Denmark

3. SUPA School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK

Abstract

ABSTRACT We present a study of 9 242 spectroscopically confirmed quasars with multiepoch ugriz photometry from the SDSS Southern Survey. By fitting a separable linear model to each quasar’s spectral variations, we decompose their five-band spectral energy distributions into variable (disc) and non-variable (host galaxy) components. In modelling the disc spectra, we include attenuation by dust on the line of sight through the host galaxy to its nucleus. We consider five commonly used attenuation laws, and find that the best description is by dust similar to that of the Small Magellanic Cloud, inferring a lack of carbonaceous grains from the relatively weak 2175-Å absorption feature. We go on to construct a composite spectrum for the quasar variations spanning 700–8000 Å. By varying the assumed power-law Lν ∝ να spectral slope, we find a best-fitting value α = 0.71 ± 0.02, excluding at high confidence the canonical Lν ∝ ν1/3 prediction for a steady-state accretion disc with a T ∝ r−3/4 temperature profile. The bluer spectral index of the observed quasar variations instead supports the model of Agol & Krolik, and Mummery & Balbus, in which a steeper temperature profile, T ∝ r−7/8, develops as a result of finite magnetically induced stress at the innermost stable circular orbit extracting energy and angular momentum from the black hole spin.

Funder

Danish National Research Foundation

European Research Council

STFC

Alfred P. Sloan Foundation

National Science Foundation

U.S. Department of Energy

National Aeronautics and Space Administration

Max Planck Society

Higher Education Funding Council for England

American Museum of Natural History

University of Basel

Case Western Reserve University

University of Chicago

Drexel University

Institute for Advanced Study

Johns Hopkins University

Los Alamos National Laboratory

New Mexico State University

Ohio State University

University of Pittsburgh

University of Portsmouth

Princeton University

United States Naval Observatory

University of Washington

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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