238 research outputs found
X-ray Evidence for Multiple Absorbing Structures in Seyfert Galaxies
We have used X-ray spectra to measure attenuating columns in a large sample
of Seyfert galaxies. Over 30 of these sources have resolved radio jets,
allowing the relative orientation of the nucleus and host galaxy to be
constrained. We have discovered that the distribution of absorbing columns is
strongly correlated with the relative orientation of the Seyfert structures.
This result is inconsistent with unification models including only a torus and
is instead most readily explained if a second absorber is included: in addition
to a Compton-thick, parsec-scale torus there would also be a larger-scale
absorber with N_H < 10^{23} cm^{-2}. The second absorber is aligned with the
host galactic plane while the torus is arbitrarily misaligned.Comment: 2 pages, 1 figure, to appear in "Multiwavelength AGN Surveys"
(Cozumel, December 8-12 2003), ed. R. Maiolino and R. Mujica, Singapore:
World Scientific, 2004. Additional material may be found at
http://space.mit.edu/home/jonathan/research.htm
Chandra observation of the TeV source HESS J1834-087
Chandra ACIS observed the field of the extended TeV source HESS J1834-087 for
47 ks. A previous XMM-Newton EPIC observation of the same field revealed a
point-like source (XMMU J183435.3-084443) and an offset region of faint
extended emission. In the low-resolution, binned EPIC images the two appear to
be connected. However, the high-resolution Chandra ACIS images do not support
the alleged connection. Instead, in these images XMMU J183435.3-084443 is
resolved into a point source, CXOU~J183434.9--084443 (L=2.5 x 10e33 ergs/s, for
a distance of 4 kpc; photon index 1.1), and a compact (~20'') nebula with an
isotropic morphology and a softer spectrum (L=4.2 x10e33 ergs/s, photon index
2.7). The nature of the nebula is uncertain. We discuss a dust scattering halo
and a pulsar-wind nebula as possible interpretations. Based on our analysis of
the X-ray data, we re-evaluate the previously suggested interpretations of HESS
J1834-087 and discuss a possible connection to the Fermi LAT source 1FGL
J1834.3-0842c. We also obtained an upper limit of 3 x 10e-14 ergs/s cm^2 on the
unabsorbed flux of the SGR J1833--0832 (in quiescence), which happened to be in
the ACIS field of view.Comment: 11 pages, 7 figures, submitted to Ap
The Highly Relativistic Kiloparsec-Scale Jet of the Gamma-Ray Quasar 0827+243
We present Chandra X-ray (0.2-8 keV) and Very Large Array radio (15 and 5
GHz) images of the -ray bright, superluminal quasar 0827+243. The X-ray
jet bends sharply--by ~90 deg., presumably amplified by projection effects--5''
from the core. Only extremely weak radio emission is detected between the
nuclear region and the bend. The X-ray continuum spectrum of the combined
emission of the knots is rather flat, with a slope of , while the
5-15 GHz spectra are steeper for knots detected in the radio. These
characteristics, as well as non-detection of the jet in the optical band by the
Hubble Space Telescope, pose challenges to models for the spectral energy
distributions (SEDs) of the jet features. The SEDs could arise from pure
synchrotron emission from either a single or dual population of relativistic
electrons only if the minimum electron energy per unit mass . Alternatively, the X-ray emission could result from inverse
Compton scattering of the Cosmic Microwave Background photons by electrons with
Lorentz factors as low as . In all models, the bulk Lorentz
factor of the jet flow found on parsec scales must continue
without substantial deceleration out to 800 kpc (deprojected) from the nucleus.
Deceleration does appear to occur at and beyond the sharp bend, such that the
flow could be only mildly relativistic at the end of the jet.Comment: 28 pages with 9 figures. Revised version corrects eq. A1 & A4 (which
incorrectly divided by 4pi) and entries in tables. Minor changes are made in
the text. submitted to Ap
Variable Iron K-alpha Lines in Seyfert 1 Galaxies
We find that variability of the iron K-alpha line is common in Seyfert 1
galaxies. Using data from the ASCA archive for objects that have been observed
more than once during the mission, we study the time-averaged spectra from
individual observations, thereby probing variability on timescales that range
from days to years. Since the statistics of the data do not warrant searches
for line variability in terms of a complex physical model, we use a a simple
Gaussian to model the gross shape of the line, and then use the centroid
energy, intensity and equivalent width as robust indicators of changes in the
line profile. We find that ~70% of Seyfert 1s (ten out of fifteen) show
variability in at least one of these parameters: the centroid energy,
intensity, and equivalent width vary in six, four, and eight sources
respectively. Due to the low S/N, limited sampling and time averaging, we
consider these results to represent lower limits to the rate of incidence of
variability. In most cases changes in the line do not appear to track changes
in the continuum. In particular, we find no evidence for variability of the
line intensity in NGC 4151, suggesting an origin in a region larger than the
putative accretion disk, where most of the iron line has been thought to
originate. Mkn 279 is investigated on short timescales. The time-averaged
effective line energy is 6.5 keV in the galaxy rest frame. As the continuum
flux increases by 20% in a few hours, the Fe K line responds with the effective
line energy increasing by 0.22 keV (~10,500 km s^-1). Problems with the ASCA
and Rosat calibration that affect simultaneous spectral fits are discussed in
an appendix.Comment: 26 pages, 30 figures, accepted for publication in the Astrophysical
Journa
Observations of Magnetic Fields and Relativistic Beaming in Four Quasar Jets
We discuss the physical properties of four quasar jets imaged with the
Chandra X-ray Observatory in the course of a survey for X-ray emission from
radio jets. These objects have sufficient counts to study their spatially
resolved properties, even in the 5 ks survey observations. We have acquired
Australia Telescope Compact Array data with resolution matching Chandra. We
have searched for optical emission with Magellan, with sub-arcsecond
resolution. The radio to X-ray spectral energy distribution for most of the
individual regions indicates against synchrotron radiation from a
single-component electron spectrum. We therefore explore the consequences of
assuming that the X-ray emission is the result of inverse Compton scattering on
the cosmic microwave background. If particles and magnetic fields are near
minimum energy density in the jet rest frames, then the emitting regions must
be relativistically beamed, even at distances of order 500 kpc from the quasar.
We estimate the magnetic field strengths, relativistic Doppler factors, and
kinetic energy flux as a function of distance from the quasar core for two or
three distinct regions along each jet. We develop, for the first time,
estimates in the uncertainties in these parameters, recognizing that they are
dominated by our assumptions in applying the standard synchrotron minimum
energy conditions. The kinetic power is comparable with, or exceeds, the quasar
radiative luminosity, implying that the jets are a significant factor in the
energetics of the accretion process powering the central black hole. The
measured radiative efficiencies of the jets are of order 10^(-4).Comment: Accepted for Publication in the Astrophysical Journal, Part
A survey of X-ray emission from 100 kpc radio jets
We have completed a Chandra snapshot survey of 54 radio jets that are
extended on arcsec scales. These are associated with flat spectrum radio
quasars spanning a redshift range z=0.3 to 2.1. X-ray emission is detected from
the jet of approximately 60% of the sample objects. We assume minimum energy
and apply conditions consistent with the original Felten-Morrison calculations
in order to estimate the Lorentz factors and the apparent Doppler factors. This
allows estimates of the enthalpy fluxes, which turn out to be comparable to the
radiative luminosities.Comment: Conference Proceedings IAU Symposium No. 313, Extragalactic jets from
every angle, pp. 219-224, 4 figure
Continuing a Chandra Survey of Quasar Radio Jets
We are conducting an X-ray survey of flat spectrum radio quasars (FSRQs) with
extended radio structures. We summarize our results from the first stage of our
survey, then we present findings from its continuation.
We have discovered jet X-ray emission from 12 of our first 20 Chandra
targets, establishing that strong 0.5-7.0 keV emission is a common feature of
FSRQ jets. The X-ray morphology is varied, but in general closely matches the
radio structure until the first sharp radio bend. In the sources with optical
data as well as X-ray detections we rule out simple synchrotron models for
X-ray emission, suggesting these systems may instead be dominated by inverse
Compton (IC) scattering. Fitting models of IC scattering of cosmic microwave
background photons suggests that these jets are aligned within a few degrees of
our line of sight, with bulk Lorentz factors of a few to ten and magnetic
fields a bit stronger than G.
In the weeks prior to this meeting, we have discovered two new X-ray jets at
. One (PKS B1055+201) has a dramatic, -long jet. The other (PKS
B1421-490) appears unremarkable at radio frequencies, but at higher frequencies
the jet is uniquely powerful: its optically-dominated, with jet/core flux
ratios of 3.7 at 1 keV and 380 at 480 nm.Comment: 4 pages, 8 figures. To appear in `X-Ray and Radio Connections', ed.
L.O. Sjouwerman and K.K. Dyer (published electronicly at
http://www.aoc.nrao.edu/events/xraydio/). Additional material and higher
resolution figures may be found at http://space.mit.edu/home/jonathan/jets
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