Measuring Light Echos in NGC 4051
Date Issued
2017
Author(s)
Abstract
Five archived X-ray observations of NGC 4051, taken using the NuSTAR
observatory, have been analysed, revealing lags between flux variations in
bands covering a wide range of X-ray photon energy. In all pairs of bands
compared, the harder band consistently lags the softer band by at least 1000s,
at temporal frequencies ~5E-5 Hz. In addition, soft-band lags up to 400s are
measured at frequencies ~2E-4 Hz. Light echos from an excess of soft band
emission in the inner accretion disk cannot explain the lags in these data, as
they are seen in cross-correlations with energy bands where the softer band is
expected to have no contribution from reflection. The basic properties of the
time delays have been parameterised by fitting a top hat response function that
varies with photon energy, taking fully into account the covariance between
measured time lag values. The low-frequency hard-band lags and the transition
to soft-band lags are consistent with time lags arising as reverberation delays
from circumnuclear scattering of X-rays, although greater model complexity is
required to explain the entire spectrum of lags. The scattered fraction
increases with increasing photon energy as expected, and the scattered fraction
is high, indicating the reprocessor to have a global covering fraction ~50%
around the continuum source. Circumnuclear material, possibly associated with a
disk wind at a few hundred gravitational radii from the primary X-ray source,
may provide suitable reprocessing.
Volume
467
Issue
4
Start page
3924
Issn Identifier
0035-8711
Ads BibCode
2017MNRAS.467.3924T
Rights
open.access
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