905 research outputs found
Non-equilibrium ionization around clouds evaporating in the interstellar medium
It is of prime importance for global models of the interstellar medium to know whether dense clouds do or do not evaporate in the hot coronal gas. The rate of mass exchanges between phases depends very much on that. McKee and Ostriker's model, for instance, assumes that evaporation is important enough to control the expansion of supernova remnants, and that mass loss obeys the law derived by Cowie and McKee. In fact, the geometry of the magnetic field is nearly unknown, and it might totally inhibit evaporation, if the clouds are not regularly connected to the hot gas. Up to now, the only test of the theory is the U.V. observation (by the Copernicus and IUE satellites) of absorption lines of ions such as OVI or NV, that exist at temperatures of a few 100,000 K typical of transition layers around evaporating clouds. Other means of testing the theory are discussed
A Detailed Analysis of a Cygnus Loop Shock-Cloud Interaction
The XA region of the Cygnus Loop is a complex zone of radiative and
nonradiative shocks interacting with interstellar clouds. We combine five far
ultraviolet spectral observations from the Hopkins Ultraviolet Telescope (HUT),
a grid of 24 IUE spectra and a high-resolution longslit Halpha spectrum to
study the spatial emission line variations across the region. These spectral
data are placed in context using ground-based, optical emission line images of
the region and a far-UV image obtained by the Ultraviolet Imaging Telescope
(UIT). The presence of high-ionization ions (OVI, NV, CIV) indicates a shock
velocity near 170 km/s while other diagnostics indicate v_shock=140 km/s. It is
likely that a large range of shock velocities may exist at a spatial scale
smaller than we are able to resolve. By comparing CIV 1550, CIII 977 and CIII]
1909, we explore resonance scattering across the region. We find that a
significant column depth is present at all positions, including those not near
bright optical/UV filaments. Analysis of the OVI doublet ratio suggests an
average optical depth of about unity in that ion while flux measurements of
[SiVIII] 1443 suggest a hot component in the region at just below 10^6K. Given
the brightness of the OVI emission and the age of the interaction, we rule out
the mixing layer interpretation of the UV emission. Furthermore, we formulate a
picture of the XA region as the encounter of the blast wave with a finger of
dense gas protruding inward from the pre-SN cavity.Comment: 21 pages, 9 figures, accepted by the Astronomical Journal, July 2001
Full resolution figures available at http://fuse.pha.jhu.edu/~danforth/xa
The blast wave of Tycho's supernova remnant
We use the Chandra X-ray Observatory to study the region in the Tycho
supernova remnant between the blast wave and the shocked ejecta interface or
contact discontinuity. This zone contains all the history of the shock-heated
gas and cosmic-ray acceleration in the remnant. We present for the first time
evidence for significant spatial variations of the X-ray synchrotron emission
in the form of spectral steepening from a photon index of 2.6 right at the
blast wave to a value of 3.0 several arcseconds behind. We interpret this
result along with the profiles of radio and X-ray intensity using a
self-similar hydrodynamical model including cosmic ray backreaction that
accounts for the observed ratio of radii between the blast wave and contact
discontinuity. Two different assumptions were made about the post-shock
magnetic field evolution: one where the magnetic field (amplified at the shock)
is simply carried by the plasma flow and remains relatively high in the
post-shock region [synchrotron losses limited rim case], and another where the
amplified magnetic field is rapidly damped behind the blast wave [magnetic
damping case]. Both cases fairly well describe the X-ray data, however both
fail to explain the observed radio profile. The projected synchrotron emission
leaves little room for the presence of thermal emission from the shocked
ambient medium. This can only be explained if the pre-shock ambient medium
density in the vicinity of the Tycho supernova remnant is below 0.6 cm-3.Comment: Accepted for publication in ApJ - 61 pages, 17 figure
Fermi Large Area Telescope Fourth Source Catalog Data Release 4 (4FGL-DR4)
We present an incremental version (4FGL-DR4, for Data Release 4) of the
fourth Fermi-LAT catalog of gamma-ray sources. Based on the first 14 years of
science data in the energy range from 50 MeV to 1 TeV, it uses the same
analysis methods as the 4FGL-DR3 catalog did for 12 years of data, with only a
few improvements. The spectral parameters, spectral energy distributions, light
curves and associations are updated for all sources.
We add four new extended sources and modify two existing ones. Among the 6658
4FGL-DR3 sources, we delete 14 and change the localization of 10, while 26 are
newly associated and two associations were changed. We add 546 point sources,
among which 8 are considered identified and 228 have a plausible counterpart at
other wavelengths. Most are just above the detection threshold, and 14 are
transient sources below the detection threshold that can affect the light
curves of nearby sources.Comment: Data files at
https://fermi.gsfc.nasa.gov/ssc/data/access/lat/14yr_catalog/. Refereed paper
is DOI 10.3847/1538-4365/ac675
QualitĂ©s de la viande dâagneaux dâherbe produits en Ă©levage biologique ou conventionnel
La mention "Agriculture Biologique" sur un produit garantit une maniĂšre de produire, mais lâobligation de rĂ©sultats est limitĂ©e Ă la conformation et lâĂ©tat dâengraissement de la carcasse, dâoĂč des questions sur les qualitĂ©s alimentaires du produit viande
XMM-Newton observation of Kepler's supernova remnant
We present the first results coming from the observation of Kepler's
supernova remnant obtained with the EPIC instruments on board the XMM-Newton
satellite. We focus on the images and radial profiles of the emission lines (Si
K, Fe L, Fe K) and of the high energy continuum. Chiefly, the Fe L and Si K
emission-line images are generally consistent with each other and the radial
profiles show that the Si K emission extends to a larger radius than the Fe L
emission (distinctly in the southern part of the remnant). Therefore, in
contrast to Cas A, no inversion of the Si- and Fe-rich ejecta layers is
observed in Kepler. Moreover, the Fe K emission peaks at a smaller radius than
the Fe L emission, which implies that the temperature increases inwards in the
ejecta. The 4-6 keV high energy continuum map shows the same distribution as
the asymmetric emission-line images except in the southeast where there is a
strong additional emission. A two color image of the 4-6 keV and 8-10 keV high
energy continuum illustrates that the hardness variations of the continuum are
weak all along the remnant except in a few knots. The asymmetry in the Fe K
emission-line is not associated with any asymmetry in the Fe K equivalent width
map. The Si K maps lead to the same conclusions. Hence, abundance variations do
not cause the north-south brightness asymmetry. The strong emission in the
north may be due to overdensities in the circumstellar medium. In the
southeastern region of the remnant, the lines have a very low equivalent width
and the X-ray emission is largely nonthermal.Comment: 15 pages, 15 figures, accepted for publication in A&
A cosmic ray current driven instability in partially ionised media
We investigate the growth of hydromagnetic waves driven by streaming cosmic
rays in the precursor environment of a supernova remnant shock. It is known
that transverse waves propagating parallel to the mean magnetic field are
unstable to anisotropies in the cosmic ray distribution, and may provide a
mechanism to substantially amplify the ambient magnetic field. We quantify the
extent to which temperature and ionisation fractions modify this picture. Using
a kinetic description of the plasma we derive the dispersion relation for a
collisionless thermal plasma with a streaming cosmic ray current. Fluid
equations are then used to discuss the effects of neutral-ion collisions. We
calculate the extent to which the environment into which the cosmic rays
propagate influences the growth of the magnetic field, and determines the range
of possible growth rates. If the cosmic ray acceleration is efficient, we find
that very large neutral fractions are required to stabilise the growth of the
non-resonant mode. For typical supernova parameters in our galaxy, thermal
effects do not significantly alter the growth rates. For weakly driven modes,
ion-neutral damping can dominate over the instability at more modest ionisation
fractions. In the case of a supernova shock interacting with a molecular
clouds, such as in RX J1713.7-3946, with high density and low ionisation, the
modes can be rapidly damped.Comment: 5 pages, 2 figures, accepted to A&A. Corrections made. Applications
adde
Turbulent magnetic field amplification driven by cosmic-ray pressure gradients
Observations of non-thermal emission from several supernova remnants suggest
that magnetic fields close to the blastwave are much stronger than would be
naively expected from simple shock compression of the field permeating the
interstellar medium (ISM).
We present a simple model which is capable of achieving sufficient magnetic
field amplification to explain the observations. We propose that the cosmic-ray
pressure gradient acting on the inhomogeneous ISM upstream of the supernova
blastwave induces strong turbulence upstream of the supernova blastwave. The
turbulence is generated through the differential acceleration of the upstream
ISM which occurs as a result of density inhomogeneities in the ISM. This
turbulence then amplifies the pre-existing magnetic field.
Numerical simulations are presented which demonstrate that amplification
factors of 20 or more are easily achievable by this mechanism when reasonable
parameters for the ISM and supernova blastwave are assumed. The length scale
over which this amplification occurs is that of the diffusion length of the
highest energy non-thermal particles.Comment: 13 pages, 4 figures, 1 Table. Accepted for publication in MNRAS,
modified following referee comments and references adde
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