537 research outputs found
ROSAT PSPC observations of Cygnus-A : X-ray spectra of the cooling flow and hot spots
We present a {\it ROSAT} Position Sensitive Proportional Counter (PSPC)
observation of the powerful radio galaxy Cygnus-A. The X-ray emission in the
{\it ROSAT} band is dominated by thermal emission from the hot intracluster
medium of the associated cluster. Image deprojection confirms the existence of
a significant cluster cooling flow with total mass deposition rate of and a (Hubble time) cooling radius of \,kpc. Spectral data show the gradient in the emission-weighted mean
temperature with the temperature decreasing towards the centre of the cluster.
We also find signatures of the radio source: in particular, we detect the X-ray
emission from the western radio hot spot previously found by the {\it ROSAT}
High Resolution Imager (HRI). We find the emission from the hot spot to be hard
and discuss the physical implications of this result.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Special relativistic effects on the strength of the fluorescent K-alpha iron line from black hole accretion disks
The broad iron K emission line, commonly seen in the X-ray spectrum
of Seyfert nuclei, is thought to originate when the inner accretion disk is
illuminated by an active disk-corona. We show that relative motion between the
disk and the X-ray emitting material can have an important influence on the
observed equivalent width (EW) of this line via special relativistic aberration
and Doppler effects. We suggest this may be relevant to understanding why the
observed EW often exceeds the prediction of the standard X-ray reflection
model. Several observational tests are suggested that could disentangle these
special relativistic effects from iron abundance effects.Comment: 5 pages, 3 postscript figures. Accepted for publication in MNRAS
(pink pages). Also available at
http://rocinante.Colorado.EDU/~chris/html_papers/special/special.htm
An extended multi-zone model for the MCG-6-30-15 warm absorber
The variable warm absorber seen with {\em ASCA} in the X-ray spectrum of
MCG6-30-15 shows complex time behaviour in which the optical depth of OVIII
anticorrelates with the flux whereas that of OVII is unchanging. The
explanation in terms of a two zone absorber has since been challenged by {\em
BeppoSAX} observations. These present a more complicated behaviour for the
OVIII edge. We demonstrate here that the presence of a third, intermediate,
zone can explain all the observations. In practice, warm absorbers are likely
to be extended, multi-zone regions of which only part causes directly
observable absorption edges at any given time.Comment: 5 pages, 6 figures, accepted for publication in MNRA
Energetic Impact of Jet Inflated Cocoons in Relaxed Galaxy Clusters
Jets from active galactic nuclei (AGN) in the cores of galaxy clusters have
the potential to be a major contributor to the energy budget of the
intracluster medium (ICM). To study the dependence of the interaction between
the AGN jets and the ICM on the parameters of the jets themselves, we present a
parameter survey of two-dimensional (axisymmetric) ideal hydrodynamic models of
back-to-back jets injected into a cluster atmosphere (with varying Mach numbers
and kinetic luminosities). We follow the passive evolution of the resulting
structures for several times longer than the active lifetime of the jet. The
simulations fall into roughly two classes, cocoon-bounded and non-cocoon
bounded sources. We suggest a correspondence between these two classes and the
Faranoff-Riley types. We find that the cocoon-bounded sources inject
significantly more entropy into the core regions of the ICM atmosphere, even
though the efficiency with which energy is thermalized is independent of the
morphological class. In all cases, a large fraction (50--80%) of the energy
injected by the jet ends up as gravitational potential energy due to the
expansion of the atmosphere.Comment: 12 pages, Accepted for publication in Ap
An RXTE study of M87 and the core of the Virgo cluster
We present hard X-ray observations of the nearby radio galaxy M87 and the
core of the Virgo cluster using the Rossi X-ray Timing Explorer. These are the
first hard X-ray observations of M87 not affected by contamination from the
nearby Seyfert 2 galaxy NGC4388. Thermal emission from Virgo's intracluster
medium is clearly detected and has a spectrum indicative of kT=2.5keV plasma
with approximately 25% cosmic abundances. No non-thermal (power-law) emission
from M87 is detected in the hard X-ray band, with fluctuations in the Cosmic
X-ray Background being the limiting factor. Combining with ROSAT data, we infer
that the X-ray spectrum of the M87 core and jet must be steep (Gamma_core>1.90$
and Gamma_jet>1.75), and we discuss the implications of this result. In
particular, these results are consistent with M87 being a mis-aligned BL-Lac
object.Comment: 8 pages, 2 postscript figures. Accepted for publication in Ap
The matter content of the jet in M87: evidence for an electron-positron jet
Recent observations have allowed the geometry and kinematics of the M87 jet
to be tightly constrained. We combine these constraints with historical Very
Long Baseline Interferometry (VLBI) results and the theory of synchrotron
self-absorbed radio cores in order to investigate the physical properties of
the jet. Our results strongly suggest the jet to be dominated by an
electron-positron (pair) plasma. Although our conservative constraints cannot
conclusively dismiss an electron-proton plasma, the viability of this solution
is extremely vulnerable to further tightening of VLBI surface brightness
limits. The arguments presented, coupled with future high-resolution
multi-frequency VLBI studies of the jet core, will be able to firmly
distinguish these two possibilities.Comment: 8 pages, 1 ps figure. Revised and accepted for publication in MNRA
Warm absorbers in active galactic nuclei
Recent {\it ASCA} observations confirm the presence of X-ray absorption due
to partially ionized gas in many Seyfert 1 galaxies; the so-called warm
absorber. Constraints on the location of the warm material are presented with
the conclusion that this material lies at radii coincident with, or just
outside, the broad-line region. The stability of this warm material to isobaric
perturbations under the assumptions of thermal and photoionization equilibrium
is also studied. It is shown that there is a remarkably small range of
ionization parameter, , for which the warm absorber state is stable. The
robustness of this result to changes in the shape of the primary continuum, the
assumed density and optical depth is investigated. Given the constraints on the
location and the stability properties of the material, several models for the
environments of Seyfert nuclei are discussed. These attempt to explain the
presence of significant amounts of partially ionized material. In particular,
various models of the broad-line region are discussed. The simple two-phase
model of the broad-line region proves to be unsatisfactory. A model of the
broad-line region is presented in which a turbulent, hot intercloud medium is
mechanically heated. Turbulent mixing layers could then give rise to warm
absorption features. Finally, a model is discussed in which the warm absorber
is due to a steady state, radiatively driven outflow.Comment: 10 pages (including figures) uuencoded compressed Postscript file.
Accepted for publication in Monthly Notices of the Royal Astronomical Societ
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