1,777 research outputs found
A high-energy view of radio-loud AGN
Seyfert galaxies and quasars were first discovered through optical and radio
techniques, but in recent years high-energy emission, that can penetrate
central gas and dust, has become essentially the defining characteristic of an
AGN. AGNs with extended radio jets are of particular interest, since the jets
signal source orientation. However, the jets extend into the cores, where they
are faster and more compact. Special-relativistic effects then cause jet
brightness and variability time-scales across the electromagnetic spectrum to
be strong functions of jet orientation. Jet X-ray emission is confused, to
varying degrees, with that from the central engine, but can be measured, at
least in a statistical sense, through considerations of the multiwaveband
spectrum and the level of intrinsic absorption. The rich high-energy structures
found in jets which are resolved with Chandra inform our interpretation of the
inner structures. In particular, it is found that shocks are prevalent and
don't necessarily disrupt jets, and that one-zone models of emission near
shocks are an over-simplification.Comment: Review to be published in Proceedings of the workshop "Multiband
Approach to AGN", held on Sep.30-Oct.2 in Bonn. Publication: Memorie della
Societa Astronomica Italiana, v. 26, No.1 (2005). 8 page
Sliding not sloshing in Abell 3744: the influence of radio galaxies NGC 7018 and 7016 on cluster gas
We present new X-ray (Chandra) and radio (JVLA) observations of the nearby
cluster Abell 3744. It hosts two prominent radio galaxies with powers in the
range critical for radio-mode feedback. The radio emission from these galaxies
terminates in buoyant tendrils reaching the cluster's outer edge, and the
radio-emitting plasma clearly influences the cluster's X-ray-emitting
atmosphere. The cluster's average gas temperature, of kT=3.5 keV, is high for
its bolometric luminosity of 3.2 \times 10^{43} ergs s^{-1}, but the 100
kpc-scale cavity carved out by radio-emitting plasma shows evidence of less
than 2 per cent of the excess enthalpy. We suggest instead that a high-velocity
encounter with a galaxy group is responsible for dispersing and increasing the
entropy of the gas in this non-cool-core cluster. We see no evidence for
shocks, or established isobaric gas motions (sloshing), but there is much
sub-structure associated with a dynamically active central region that
encompasses the brightest radio emission. Gas heating is evident in directions
perpendicular to the inferred line of encounter between the infalling group and
cluster. The radio-emitting tendrils run along boundaries between gas of
different temperature, apparently lubricating the gas flows and inhibiting heat
transfer. The first stages of the encounter may have helped trigger the radio
galaxies into their current phase of activity, where we see X-rays from the
nuclei, jets, and hotspots.Comment: Accepted for publication in ApJ (13 pages, 17 figures
The broad-band X-ray spectrum of a QSO sample
A sample of 25 QSOs was used to investigate the average spectrum between the soft X-ray energy band of the Einstein Observatory image proportional counter, and the higher energy band of the HEAO 1 A2 experiment. The spectrum is similar to thoe exhibited by Seyfert galaxies and narrow emission line galaxies above 2 keV. The spectrum is soft enough that if these objects are typical of the higher redshift, more radio-quiet QSOs, then it is possible to exclude QSOs as being the dominant origin of the diffuse X-ray background
The Infrared Jet In 3C66B
We present images of infrared emission from the radio jet in 3C66B. Data at
three wavelengths (4.5, 6.75 and 14.5 microns) were obtained using the Infrared
Space Observatory. The 6.75 micron image clearly shows an extension aligned
with the radio structure. The jet was also detected in the 14.5 micron image,
but not at 4.5 micron. The radio-infrared-optical spectrum of the jet can be
interpreted as synchrotron emission from a population of electrons with a
high-energy break of 4e11 eV. We place upper limits on the IR flux from the
radio counter-jet. A symmetrical, relativistically beamed twin-jet structure is
consistent with our results if the jets consist of multiple components.Comment: 7 pages, 4 figure
Redshift and velocity dispersion of the cluster of galaxies around NGC 326
Redshifts of several galaxies thought to be associated with NGC 326 are
determined. The results confirm the presence of a cluster and find a mean
redshift of z = 0.0477 +/- 0.0007 and a line-of-sight velocity dispersion
sigma_{z} = 599 (+230, -110) km/s. The velocity dispersion and previously
measured X-ray gas temperature of kT ~ 1.9 keV are consistent with the cluster
sigma_{z}/kT relation, and NGC 326 is seen to be a slowly-moving member of the
cluster.Comment: 3 pages, to appear in MNRA
X-ray Emission from Jets in Centrally Obscured AGN
There is good evidence for X-ray emission associated with AGN jets which are
relativistically boosted towards the observer. But to what jet radius does such
X-ray emission persist? To attempt to answer this question one can look at
radio galaxies; their cores are sufficiently X-ray faint that any unbeamed
X-ray emission in the vicinity of the central engine must be obscured. The jets
of such sources are at unfavourable angles for relativistic boosting, and so
their relatively weak X-ray emission must be carefully separated from the
plateau of resolved X-ray emission from a hot interstellar, intragroup, or
intracluster medium on which they are expected to sit. This paper presents
results arguing that jet X-ray emission is generally detected in radio
galaxies, even those of low intrinsic power without hot spots. The levels of
emission suggest an extrapolated radio to soft X-ray spectral index, alpha_rx,
of about 0.85 at parsec to perhaps kiloparsec distances from the cores.Comment: TeX, 6 pages, 8 PostScript figures included, using macros lecproc.cmm
and epsf.sty, included. To appear in the Proceedings of the Conference
``Relativistic Jets in AGNs'', Cracow, May 199
X-ray synchrotron emission from the oblique shock in the jet of the powerful radio galaxy 3C 346
We report the first detection, with Chandra, of X-ray emission from the jet
of the powerful narrow-line radio galaxy 3C 346. X-rays are detected from the
bright radio and optical knot at which the jet apparently bends by about 70
degrees. The Chandra observation also reveals a bright galaxy-scale atmosphere
within the previously-known cluster, and provides a good X-ray spectrum for the
bright core of 3C 346. The X-ray emission from the knot is synchrotron
radiation, as seen in lower-power sources. In common with these sources, there
is evidence of morphological differences between the radio/optical and X-ray
structures, and the spectrum is inconsistent with a one-component
continuous-injection model. We suggest that the X-ray-bright knot is associated
with a strong oblique shock in a moderately relativistic, light jet, at about
20 degrees to the line of sight, and that this shock is caused by the jet
interacting with the wake in the cluster medium behind 3C 346's companion
galaxy. The general jet curvature can result from pressure gradients in the
cluster atmosphere.Comment: Accepted for publication in the MNRAS. 9 page
Unmasking the Active Galactic Nucleus in PKS J2310-437
PKS J2310-437 is an AGN with bright X-ray emission relative to its weak radio
emission and optical continuum. It is believed that its jet lies far enough
from the line of sight that it is not highly relativistically beamed. It thus
provides an extreme test of AGN models. We present new observations aimed at
refining the measurement of the source's properties. In optical photometry with
the NTT we measure a central excess with relatively steep spectrum lying above
the bright elliptical galaxy emission, and we associate the excess wholly or in
part with the AGN. A new full-track radio observation with the ATCA finds that
the core 8.64GHz emission has varied by about 20 per cent over 38 months, and
improves the mapping of the weak jet. With Chandra we measure a
well-constrained power-law spectral index for the X-ray core, uncontaminated by
extended emission from the cluster environment, with a negligible level of
intrinsic absorption. Weak X-ray emission from the resolved radio jet is also
measured. Our analysis suggests that the optical continuum in this radio galaxy
has varied by at least a factor of four over a timescale of about two years,
something that should be testable with further observations. We conclude that
the most likely explanation for the bright central X-ray emission is
synchrotron radiation from high-energy electrons.Comment: 7 pages, 12 figure
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