Artículo de revista
Twelve years of X-ray and optical variability in the Seyfert galaxy NGC 4051
Fecha
2009Registro en:
Mon. Not. R. Astron. Soc. 000, 1–17 (2009)
Autor
Breedt, E.
McHardy, I. M.
Arévalo, P.
Uttley, P.
Sergeev, S. G.
Minezaki, T.
Yoshii, Y.
Sakata, Y.
Lira Teillery, Paulina
Chesnok, N. G.
Institución
Resumen
We discuss the origin of the optical variations in the Narrow line Seyfert 1 galaxy
NGC 4051 and present the results of a cross-correlation study using X-ray and optical
light curves spanning more than 12 years. The emission is highly variable in all wave-
bands, and the amplitude of the optical variations is found to be smaller than that of
the X-rays, even after correcting for the contaminating host galaxy flux falling inside
the photometric aperture. The optical power spectrum is best described by an unbro-
ken power law model with slope = 1.4+0.6
−0.2 and displays lower variability power than
the 2–10 keV X-rays on all time-scales probed. We find the light curves to be signifi-
cantly correlated at an optical delay of 1.2+1.0
−0.3 days behind the X-rays. This time-scale
is consistent with the light travel time to the optical emitting region of the accretion
disc, suggesting that the optical variations are driven by X-ray reprocessing.We show,
however, that a model whereby the optical variations arise from reprocessing by a flat
accretion disc cannot account for all the optical variability. There is also a second sig-
nificant peak in the cross-correlation function, at an optical delay of 39+2.7
−8.4 days. The
lag is consistent with the dust sublimation radius in this source, suggesting that there
is a measurable amount of optical flux coming from the dust torus. We discuss the
origin of the additional optical flux in terms of reprocessing of X-rays and reflection
of optical light by the dust.