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

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

Abstract

We used a ∼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  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.