A Chandra X-ray Study of the Nucleus of Cygnus A
may
Andrew J. Young, Andrew S. Wilson , Yuichi Terashima (University of Maryland), Keith A. Arnaud (NASA Goddard Space Flight Center), David A. Smith (University of Maryland)
Abstract
We report sub arc-second resolution X-ray imaging-spectroscopy of the
nucleus
of Cygnus A with the Chandra X-ray observatory and quasi-simultaneous
observations with RXTE. The Chandra image shows an unresolved hard
X-ray source
coincident with the radio core. At lower X-ray energies this source
has a
bipolar morphology extending
kpc towards both the
northwest and
southeast, approximately along the direction of the radio jet and
counter-jet,
respectively. The bipolar morphology may be due, in part, to the
presence of a
dust lane running north-south across the nucleus. The soft X-ray
emission is
well correlated with HST images of [O III] and H
+ [N II]
line
emission.
Cygnus A has a heavily obscured active nucleus. We have extracted a
spectrum of
the nucleus from our Chandra data and, in addition, obtained a hard
X-ray
spectrum with RXTE of the
field centered on Cygnus
A. We have
combined these spectra to determine the power law slope of the
nuclear
emission, and the column density of the obscuring gas. The nucleus is
well
described by a power law of photon index
obscured by
a column
density of
cm-2. The rest-frame 2 - 10
keV
unabsorbed luminosity is
erg
s-1.
The Chandra spectrum of the nucleus shows emission lines from highly
ionized
Ne, ionized Si, and neutral Fe, and a strong Fe absorption edge.
The bipolar nebulosity extending towards the northwest and southeast
is
consistent with being electron-scattered light from the nucleus. The
column
density towards, and intrinsic to, the soft X-ray emission is not
significantly
in excess of the Galactic column density. Therefore, in order to have
a
sufficiently low soft X-ray opacity, the bulk of the scattering
electrons must
be at least mildly ionized with
.
In such a
model approximately 1.2% of the intrinsic nuclear luminosity is
electron-scattered into our line of sight.
CATEGORY: ACTIVE GALAXIES AND QUASARS