1,210 research outputs found
The origins of X-ray emission from the hotspots of FRII radio sources
We use new and archival Chandra data to investigate the X-ray emission from a
large sample of compact hotspots of FRII radio galaxies and quasars from the 3C
catalogue. We find that only the most luminous hotspots tend to be in good
agreement with the predictions of a synchrotron self-Compton model with
equipartition magnetic fields. At low hotspot luminosities inverse-Compton
predictions are routinely exceeded by several orders of magnitude, but this is
never seen in more luminous hotspots. We argue that an additional synchrotron
component of the X-ray emission is present in low-luminosity hotspots, and that
the hotspot luminosity controls the ability of a given hotspot to produce
synchrotron X-rays, probably by determining the high-energy cutoff of the
electron energy spectrum. It remains plausible that all hotspots are close to
the equipartition condition.Comment: 49 pages, 16 figures. ApJ accepted. Revised version fixes a typo in
one of the Tables and corrects a statement about 3C27
Evaluating preclinical evidence for clinical translation in childhood brain tumours: Guidelines from the CONNECT, PNOC, and ITCC brain networks
Clinical outcomes for many childhood brain tumours remain poor, despite our increasing understanding of the underlying disease biology. Advances in molecular diagnostics have refined our ability to classify tumour types and subtypes, and efforts are underway across multiple international paediatric neuro-oncology consortia to take novel biological insights in the worst prognosis entities into innovative clinical trials. Whilst for the first time we are designing such studies on the basis of disease-specific biological data, the levels of preclincial evidence in appropriate model systems on which these trials are initiated is still widely variable. We have considered these issues between CONNECT, PNOC and ITCC-Brain, and developed a framework in which we can assess novel concepts being brought forward for possible clinical translation. Whilst not intended to be proscriptive for every possible circumstance, these criteria provide a basis for self-assessment of evidence by laboratory scientists, and a platform for discussion and rational decision-making prior to moving forward clinically
A Chandra X-ray Study of Cygnus A - II. The Nucleus
We report Chandra ACIS and quasi-simultaneous RXTE observations of the
nearby, powerful radio galaxy Cygnus A, with the present paper focusing on the
properties of the active nucleus. In the Chandra observation, the hard (> a few
keV) X-ray emission is spatially unresolved with a size \approxlt 1 arcsec (1.5
kpc, H_0 = 50 km s^-1 Mpc^-1) and coincides with the radio and near infrared
nuclei. In contrast, the soft (< 2 keV) emission exhibits a bi-polar nebulosity
that aligns with the optical bi-polar continuum and emission-line structures
and approximately with the radio jet. In particular, the soft X-ray emission
corresponds very well with the [O III] \lambda 5007 and H\alpha + [N II]
\lambda\lambda 6548, 6583 nebulosity imaged with HST. At the location of the
nucleus there is only weak soft X-ray emission, an effect that may be intrinsic
or result from a dust lane that crosses the nucleus perpendicular to the source
axis. The spectra of the various X-ray components have been obtained by
simultaneous fits to the 6 detectors. The compact nucleus is detected to 100
keV and is well described by a heavily absorbed power law spectrum with
\Gamma_h = 1.52^{+0.12}_{-0.12} (similar to other narrow line radio galaxies)
and equivalent hydrogen column N_H (nuc) = 2.0^{+0.1}_{-0.2} \times 10^{23}
cm^-2.
(Abstract truncated).Comment: To be published in the Astrophysical Journal, v564 January 1, 2002
issue; 34 pages, 11 figures (1 color
Sub-Arcsecond Imaging of 3C123:108-GHz Continuum Observations of the Radio Hotspots
We present the results of sub-arcsecond 108 GHz continuum interferometric
observations toward the radio luminous galaxy 3C123. Using multi-array
observations, we utilize the high u,v dynamic range of the BIMA millimeter
array to sample fully spatial scales ranging from 0.5" to 50". This allows us
to make one-to-one comparisons of millimeter-wavelength emission in the radio
lobes and hotspots to VLA centimeter observations at 1.4, 4.9, 8.4, and 15 GHz.
At 108 GHz, the bright, eastern double hotspot in the southern lobe is
resolved. This is only the second time that a multiple hotspot region has been
resolved in the millimeter regime. We model the synchrotron spectra of the
hotspots and radio lobes using simple broken power-law models with high energy
cutoffs, and discuss the hotspot spectra and their implications for models of
multiple hotspot formation.Comment: 16 pages, 3 Figures, ApJ Accepte
The Electron Energy Distribution in the Hotspots of Cygnus A: Filling the Gap with the Spitzer Space Telescope
Here we present Spitzer Space Telescope imaging of Cyg A with the Infrared
Array Camera, resulting in the detection of the high-energy tails or cut-offs
in the synchrotron spectra for all four hotspots of this archetype radio
galaxy. When combined with the other data collected from the literature, our
observations allow for detailed modeling of the broad-band emission for the
brightest spots A and D. We confirm that the X-ray flux detected previously
from these features is consistent with the synchrotron self-Compton radiation
for the magnetic field intensity 170 muG in spot A, and 270 muG in spot D. We
also find that the energy density of the emitting electrons is most likely
larger by a factor of a few than the energy density of the hotspots' magnetic
field. We construct energy spectra of the radiating ultrarelativistic
electrons. We find that for both hotspots A and D these spectra are consistent
with a broken power-law extending from at least 100 MeV up to 100 GeV, and that
the spectral break corresponds almost exactly to the proton rest energy of 1
GeV. We argue that the shape of the electron continuum reflects two different
regimes of the electron acceleration process at mildly relativistic shocks,
rather than resulting from radiative cooling and/or absorption effects. In this
picture the protons' inertia defines the critical energy for the hotspot
electrons above which Fermi-type acceleration processes may play a major role,
but below which the operating acceleration mechanism has to be of a different
type. At energies >100 GeV, the electron spectra cut-off/steepen again, most
likely as a result of spectral aging due to radiative loss effects. We discuss
several implications of the presented analysis for the physics of extragalactic
jets.Comment: 29 pages, 8 figures and 2 tables included. Accepted for publication
in Ap
Herschel imaging of the dust in the Helix Nebula (NGC 7293)
In our series of papers presenting the Herschel imaging of evolved planetary
nebulae, we present images of the dust distribution in the Helix nebula (NGC
7293). Images at 70, 160, 250, 350, and 500 micron were obtained with the PACS
and SPIRE instruments on board the Herschel satellite. The broadband maps show
the dust distribution over the main Helix nebula to be clumpy and predominantly
present in the barrel wall. We determined the spectral energy distribution of
the main nebula in a consistent way using Herschel, IRAS, and Planck flux
values. The emissivity index of 0.99 +/- 0.09, in combination with the carbon
rich molecular chemistry of the nebula, indicates that the dust consists mainly
of amorphous carbon. The dust excess emission from the central star disk is
detected at 70 micron and the flux measurement agree with previous measurement.
We present the temperature and dust column density maps. The total dust mass
across the Helix nebula (without its halo) is determined to be 0.0035 solar
mass at a distance of 216 pc. The temperature map shows dust temperatures
between 22 and 42 K, which is similar to the kinetic temperature of the
molecular gas, strengthening the fact that the dust and gas co-exist in high
density clumps. Archived images are used to compare the location of the dust
emission in the far infrared (Herschel) with the ionized (GALEX, Hbeta) and
molecular hydrogen component. The different emission components are consistent
with the Helix consisting of a thick walled barrel-like structure inclined to
the line of sight. The radiation field decreases rapidly through the barrel
wall.Comment: 8 pages, 9 figures, revised version A&A in pres
The Aquila prestellar core population revealed by Herschel
The origin and possible universality of the stellar initial mass function
(IMF) is a major issue in astrophysics. One of the main objectives of the
Herschel Gould Belt Survey is to clarify the link between the prestellar core
mass function (CMF) and the IMF. We present and discuss the core mass function
derived from Herschel data for the large population of prestellar cores
discovered with SPIRE and PACS in the Aquila Rift cloud complex at d ~ 260 pc.
We detect a total of 541 starless cores in the entire ~11 deg^2 area of the
field imaged at 70-500 micron with SPIRE/PACS. Most of these cores appear to be
gravitationally bound, and thus prestellar in nature. Our Herschel results
confirm that the shape of the prestellar CMF resembles the stellar IMF, with
much higher quality statistics than earlier submillimeter continuum
ground-based surveys
Robotic Finger Assembly
A robotic hand includes a finger with first, second, and third phalanges. A first joint rotatably connects the first phalange to a base structure. A second joint rotatably connects the first phalange to the second phalange. A third joint rotatably connects the third phalange to the second phalange. The second joint and the third joint are kinematically linked such that the position of the third phalange with respect to the second phalange is determined by the position of the second phalange with respect to the first phalange
X-Rays from NGC 3256: High-Energy Emission in Starburst Galaxies and Their Contribution to the Cosmic X-Ray Background
The infrared-luminous galaxy NGC3256 is a classic example of a merger induced
nuclear starburst system. We find here that it is the most X-ray luminous
star-forming galaxy yet detected (~10^42 ergs/s). Long-slit optical
spectroscopy and a deep, high-resolution ROSAT X-ray image show that the
starburst is driving a "superwind" which accounts for ~20% of the observed soft
(kT~0.3 keV) X-ray emission. Our model for the broadband X-ray emission of
NGC3256 contains two additional components: a warm thermal plasma (kT~0.8 keV)
associated with the central starburst, and a hard power-law component with an
energy index of ~0.7. We find that the input of mechanical energy from the
starburst is more than sufficient to sustain the observed level of emission. We
also examine possible origins for the power-law component, concluding that
neither a buried AGN nor the expected population of high-mass X-ray binaries
can account for this emission. Inverse-Compton scattering, involving the
galaxy's copious flux of infrared photons and the relativistic electrons
produced by supernovae, is likely to make a substantial contribution to the
hard X-ray flux. Such a model is consistent with the observed radio and IR
fluxes and the radio and X-ray spectral indices. We explore the role of
X-ray-luminous starbursts in the production of the cosmic X-ray background
radiation. The number counts and spectral index distribution of the faint radio
source population, thought to be dominated by star-forming galaxies, suggest
that a significant fraction of the hard X-ray background could arise from
starbursts at moderate redshift.Comment: 31 pages (tex, epsf), 8 figures (postscript files), accepted for
publication in Part 1 of The Astrophysical Journa
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