EDP Sciences
Press Release
Free access
Issue A&A
Volume 490, Number 3, November II 2008
Page(s) 933 - 943
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:200810729
Published online 18 September 2008

A&A 490, 933-943 (2008)
DOI: 10.1051/0004-6361:200810729

Microlensing variability in the gravitationally lensed quasar QSO 2237+0305 $\mathsf{\equiv}$ the Einstein Cross

II. Energy profile of the accretion disk
A. Eigenbrod1, F. Courbin1, G. Meylan1, E. Agol2, T. Anguita3, R. W. Schmidt3, and J. Wambsganss3

1  Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
    e-mail: alexander.eigenbrod@epfl.ch
2  Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195, USA
3  Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstrasse 12-14, 69120 Heidelberg, Germany

Received 1 August 2008 / Accepted 10 September 2008

We present the continuation of our long-term spectroscopic monitoring of the gravitationally lensed quasar QSO 2237+0305. We investigate the chromatic variations observed in the UV/optical continuum of both quasar images A and B, and compare them with numerical simulations to infer the energy profile of the quasar accretion disk. Our procedure combines the microlensing ray-shooting technique with Bayesian analysis, and derives probability distributions for the source sizes as a function of wavelength. We find that the effective caustic crossing timescale is 4.0 $\pm$ 1.0 months. Using a robust prior on the effective transverse velocity, we find that the source responsible for the UV/optical continuum has an energy profile well reproduced by a power-law $R \propto \lambda^{\zeta}$ with $\zeta=1.2$ $\pm$ 0.3, where R is the source size responsible for the emission at wavelength $\lambda$. This is the first accurate, model-independent determination of the energy profile of a quasar accretion disk on such small scales.

Key words: gravitational lensing -- quasars: individual: QSO 2237+0305 -- accretion, accretion disks

© ESO 2008

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Editor-in-Chief: T. Forveille
Letters Editor-in-Chief: J. Alves
Managing Editor: C. Bertout

ISSN: 0004-6361 ; e-ISSN: 1432-0746
Frequency: 12 volumes per year
Published by: EDP Sciences

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