EDP Sciences
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Volume 430, Number 2, February I 2005
Page(s) 435 - 442
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20041801

A&A 430, 435-442 (2005)
DOI: 10.1051/0004-6361:20041801

X-ray to UV variability correlation in MCG-6-30-15

P. Arévalo1, I. Papadakis2, B. Kuhlbrodt3, 4 and W. Brinkmann1

1  Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
    e-mail: parevalo@mpe.mpg.de
2  Department of Physics, University of Crete, PO Box 2208, 71 003 Heraklion, Crete, Greece
3  Astrophysikalisches Intitut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
4  Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

(Received 5 August 2004 / Accepted 22 September 2004 )

We used a $\sim $300 ks long XMM-Newton observation of the Seyfert 1 galaxy MCG-6-30-15 to study the correlation between the 0.2-10 keV X-ray and the 3000-4000 Å  U bands. We found a significant correlation peak at a time lag of $\tau_{\rm max}\sim 160$ ks where the UV flux variations preceded the variations in the X-ray band. We interpret this result as evidence in favour of Comptonisation models where the observed X-rays are produced through Compton up-scattering of thermal UV seed photons from an accretion disc, as this process naturally predicts the UV variations to precede similar flux variations in the X-rays. The length of the time lag favours models where the observed UV and the seed-photon-emitting regions are connected by perturbations of the accretion flow traveling inwards through the disc, affecting first the main U-band-emitting radii and then the innermost region where the bulk of the seed photons is expected to be produced. Finally, the absence of significant features in the correlation function with X-ray flux variations preceding those in the UV indicates that the observed U-band photons are not mainly produced through reprocessing of hard X-rays in this source.

Key words: accretion, accretion disks -- galaxies: active -- galaxies: individual: MCG-6-30-15 -- X-rays: individual: MCG-6-30-15

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