22,957 research outputs found
The Classification of Extragalactic X-ray Jets
The overall classification of X-ray jets has clung to that prevalent in the
radio: FRI vs. FRII (including quasars). Indeed, the common perception is that
X-ray emission from FRI's is synchrotron emission whereas that from FRII's may
be IC/CMB and/or synchrotron. Now that we have a sizable collection of sources
with detected X-ray emission from jets and hotspots, it seems that a more
unbiased study of these objects could yield additional insights on jets and
their X-ray emission. The current contribution is a first step in the process
of analyzing all of the relevant parameters for each detected component for the
sources collected in the XJET website. This initial effort involves measuring
the ratio of X-ray to radio fluxes and evaluating correlations with other jet
parameters. For single zone synchrotron X-ray emission, we anticipate that
larger values of fx/fr should correlate inversely with the average magnetic
field strength (if the acceleration process is limited by loss time equals
acceleration time). Beamed IC/CMB X-rays should produce larger values of fx/fr
for smaller values of the angle between the jet direction and the line of sight
but will also be affected by the low frequency radio spectral index.Comment: 4 pages; to appear in the conference proceedings: "X-Ray Astronomy
2009: Present Status, Multiwavelength Approach and Future Perspectives";
Bologna, Italy, September 2009, Editors: A. Comastri, M. Cappi, L. Angelini,
2010 AIP (in press
VLA Observations of a Complete Sample of Radio Loud Quasars between redshifts 2.5 and 5.28: I. high-redshift sample summary and the radio images
We present high resolution (arcsecond or better) observations made with the
Karl G. Jansky Very Large Array of 123 radio-loud quasars with redshifts in the
range that form a complete flux limited sample ( mJy at 1.4 GHz or 5 GHz). Where possible, we used previous high resolution
VLA observations (mainly A array at 1.4, 5 and 8 GHz) from the NRAO archive and
re-imaged them (43 sources). For the remainder, new observations were made in
the A array at 1.4 and 5 GHz. We show images of the 61 resolved sources, and
list structural properties of all of them. Optical data from the SDSS are
available for nearly every source. This work represents a significant increase
in the number of high redshift quasars with published radio structures, and
will be used to study the properties and evolution of luminous radio sources in
the high redshift universe
A kpc-scale X-ray jet in the BL Lac source S5 2007+777
X-ray jets in AGN are commonly observed in FRII and FRI radio-galaxies, but
rarely in BL Lacs, most probably due to their orientation close to the line of
sight and the ensuing foreshortening effects. Only three BL Lacs are known so
far to contain a kpc-scale X-ray jet. In this paper, we present the evidence
for the existence of a fourth extended X-ray jet in the classical
radio-selected source S5 2007+777, which for its hybrid FRI/II radio morphology
has been classified as a HYMOR (HYbrid MOrphology Radio source). Our Chandra
ACIS-S observations of this source revealed an X-ray counterpart to the
19"-long radio jet. Interestingly, the X-ray properties of the kpc-scale jet in
S5 2007+777 are very similar to those observed in FRII jets. First, the X-ray
morphology closely mirrors the radio one, with the X-rays being concentrated in
the discrete radio knots. Second, the X-ray continuum of the jet/brightest knot
is described by a very hard power law, with photon index Gamma_x~1, although
the uncertainties are large. Third, the optical upper limit from archival HST
data implies a concave radio-to-X-ray SED. If the X-ray emission is attributed
to IC/CMB with equipartition, strong beaming (delta=13) is required, implying a
very large scale (Mpc) jet. The beaming requirement can be somewhat relaxed
assuming a magnetic field lower than equipartition. Alternatively, synchrotron
emission from a second population of very high-energy electrons is viable.
Comparison to other HYMOR jets detected with Chandra is discussed, as well as
general implications for the origin of the FRI/II division.Comment: Accepted for publication in ApJ, 19 pages, 3 figure
A model for the formation of the active region corona driven by magnetic flux emergence
We present the first model that couples the formation of the corona of a
solar active region to a model of the emergence of a sunspot pair. This allows
us to study when, where, and why active region loops form, and how they evolve.
We use a 3D radiation MHD simulation of the emergence of an active region
through the upper convection zone and the photosphere as a lower boundary for a
3D MHD coronal model. The latter accounts for the braiding of the magnetic
fieldlines, which induces currents in the corona heating up the plasma. We
synthesize the coronal emission for a direct comparison to observations.
Starting with a basically field-free atmosphere we follow the filling of the
corona with magnetic field and plasma. Numerous individually identifiable hot
coronal loops form, and reach temperatures well above 1 MK with densities
comparable to observations. The footpoints of these loops are found where small
patches of magnetic flux concentrations move into the sunspots. The loop
formation is triggered by an increase of upwards-directed Poynting flux at
their footpoints in the photosphere. In the synthesized EUV emission these
loops develop within a few minutes. The first EUV loop appears as a thin tube,
then rises and expands significantly in the horizontal direction. Later, the
spatially inhomogeneous heat input leads to a fragmented system of multiple
loops or strands in a growing envelope.Comment: 13 pages, 10 figures, accepted to publication in A&
Magnetic Jam in the Corona of the Sun
The outer solar atmosphere, the corona, contains plasma at temperatures of
more than a million K, more than 100 times hotter that solar surface. How this
gas is heated is a fundamental question tightly interwoven with the structure
of the magnetic field in the upper atmosphere. Conducting numerical experiments
based on magnetohydrodynamics we account for both the evolving
three-dimensional structure of the atmosphere and the complex interaction of
magnetic field and plasma. Together this defines the formation and evolution of
coronal loops, the basic building block prominently seen in X-rays and extreme
ultraviolet (EUV) images. The structures seen as coronal loops in the EUV can
evolve quite differently from the magnetic field. While the magnetic field
continuously expands as new magnetic flux emerges through the solar surface,
the plasma gets heated on successively emerging fieldlines creating an EUV loop
that remains roughly at the same place. For each snapshot the EUV images
outline the magnetic field, but in contrast to the traditional view, the
temporal evolution of the magnetic field and the EUV loops can be different.
Through this we show that the thermal and the magnetic evolution in the outer
atmosphere of a cool star has to be treated together, and cannot be simply
separated as done mostly so far.Comment: Final version published online on 27 April 2015, Nature Physics 12
pages and 8 figure
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