Frequency-dependent time lags in the X-ray emission of NGC 7469

Nandra, K.$^{1}$, Papadakis, I.E.$^{2}$, Kazanas, D.$^{1}$

$^{1}$LHEA, NASA/GSFC, Greenbelt, MD, USA
$^{2}$Department of Physics, University of Crete, Heraklion, Greece



We report the discovery of time lags in the cross-spectrum of the
Seyfert galaxy NGC 7469. This behavior is common in Galactic black
hole and neutron star binaries and is in the sense that harder X-rays
are delayed with respect to the softer X-rays with a time lag
approximately proportional to the Fourier period. At the longest
period probed by our observation we find a time lag of $\sim 3.5$
hours between the 2-4 and 4-10 keV X-rays. A similar lag and period
dependence is found comparing the 2-4 and 10-15 keV light curves,
albeit with less significance. We find the coherence function of the
light curves to be close to 1 at all frequencies. The data support the
hypothesis that the X-rays in NGC 7469 are produced by inverse Compton
scattering.  If the lag corresponds to a light crossing time, the
implied size of the scattering region is rather large, however, being
tens to hundreds of gravitational radii, depending on the black hole
mass.