766 research outputs found
TRACE-derived temperature and emission measure profiles along long-lived coronal loops: the role of filamentation
In a recent letter (ApJ 517, L155) Lenz et al. have shown the evidence of
uniform temperature along steady long coronal loops observed by TRACE in two
different passbands (171 A and 195 A filters). We propose that such an evidence
can be explained by the sub-arcsecond structuring of the loops across the
magnetic field lines. In this perspective, we present a model of a bundle of
six thin parallel hydrostatic filaments with temperature stratification
dictated by detailed energy balance and with temperatures at their apex ranging
between 0.8 and 5 MK. If analyzed as a single loop, the bundle would appear
isothermal along most of its length.Comment: 9 pages, 4 figs, LaTeX text, PostScript figure
XMM-Newton Observations of Evolution of Cluster X-Ray Scaling Relations at z=0.4-0.7
We present a spatially-resolved analysis of the temperature and gas density
profiles of galaxy clusters at z=0.4-0.7 observed with XMM-Newton. These data
are used to derive the total cluster mass within the radius r_500 without
assuming isothermality, and also to measure the average temperature and total
X-ray luminosity excluding the cooling cores. We derive the high-redshift M-T
and L-T relations and compare them with the local measurements. The
high-redshift L-T relation has low scatter and evolves as L ~ (1+z)^{1.8\pm0.3}
for a fixed T, in good agreement with several previous Chandra and XMM-Newton
studies (Vikhlinin et al., Lumb et al., Maughan et al.). The observed evolution
of the M-T relation follows M_500 = A T^{3/2} E(z)^{-alpha}, where we measure
alpha=0.88\pm0.23. This is in good agreement with predictions of the
self-similar theory, alpha=1.Comment: ApJ in press, updated to match the accepted versio
A massive warm baryonic halo in the Coma cluster
Several deep PSPC observations of the Coma cluster reveal a very large-scale
halo of soft X-ray emission, substantially in excess of the well known
radiation from the hot intra-cluster medium. The excess emission, previously
reported in the central region of the cluster using lower-sensitivity EUVE and
ROSAT data, is now evident out to a radius of 2.6 Mpc, demonstrating that the
soft excess radiation from clusters is a phenomenon of cosmological
significance. The X-ray spectrum at these large radii cannot be modeled
non-thermally, but is consistent with the original scenario of thermal emission
from warm gas at ~ 10^6 K. The mass of the warm gas is on par with that of the
hot X-ray emitting plasma, and significantly more massive if the warm gas
resides in low-density filamentary structures. Thus the data lend vital support
to current theories of cosmic evolution, which predict that at low redshift
\~30-40 % of the baryons reside in warm filaments converging at clusters of
galaxies.Comment: Astrophysical Journal, in pres
Ion-by-Ion DEM Determination: I. Method
We describe a technique to derive constraints on the differential emission
measure (DEM) distribution, a measure of the temperature distribution, of
collisionally ionized hot plasmas from their X-ray emission line spectra. This
technique involves fitting spectra using a number of components, each of which
is the entire X-ray line emission spectrum for a single ion. It is applicable
to high-resolution X-ray spectra of any collisionally ionized plasma and
particularly useful for spectra in which the emission lines are broadened and
blended such as those of the winds of hot stars. This method does not require
that any explicit assumptions about the form of the DEM distribution be made
and is easily automated.Comment: This paper was split in two. This version is part I. Part II may be
found at astro-ph/050343
Chandra Observation of M84, Radio Lobe Elliptical in Virgo cluster
We analyzed a deep Chandra observation of M84, a bright elliptical galaxy in
the core of the Virgo cluster. We find that the spatial distribution of the
soft X-ray emission is defined by the radio structure of the galaxy. In
particular we find two low density regions associated with the radio lobes and
surrounded by higher density X-ray filaments. In addition to a central AGN and
a population of galactic sources, we find a diffuse hard source filling the
central 10 kpc region. Since the morphology of the hard source appears round
and is different from that seen in the radio or in soft X-rays, we propose that
it is hot gas heated by the central AGN. Finally, we find that the central
elemental abundance in the X-ray gas is comparable to that measured optically.Comment: accepted to ApJ Letters, Oct 2000. 5 pages in emulateap
Origins of the 1/4 keV Soft X-Ray Background
Snowden and coworkers have presented a model for the 1/4 keV soft X-ray
diffuse background in which the observed flux is dominated by a ~ 10^6 K
thermal plasma located in a 100-300 pc diameter bubble surrounding the Sun, but
has significant contributions from a very patchy Galactic halo. Halo emission
provides about 11% of the total observed flux and is responsible for half of
the H I anticorrelation. The remainder of the anticorrelation is presumably
produced by displacement of disk H I by the varying extent of the local hot
bubble (LHB). The ROSAT R1 and R2 bands used for this work had the unique
spatial resolution and statistical precision required for separating the halo
and local components, but provide little spectral information. Some consistency
checks had been made with older observations at lower X-ray energies, but we
have made a careful investigation of the extent to which the model is supported
by existing sounding rocket data in the Be (73-111 eV) and B bands (115-188 eV)
where the sensitivities to the model are qualitatively different from the ROSAT
bands. We conclude that the two-component model is well supported by the
low-energy data. We find that these combined observations of the local
component may be consistent with single-temperature thermal emission models in
collisional ionization equilibrium if depleted abundances are assumed. However,
different model implementations give significantly different results, offering
little support for the conclusion that the astrophysical situation is so
simple.Comment: 17 pages, 6 figures, accepted by the Astrophysical Journa
Smoothed Particle Inference: A Kilo-Parametric Method for X-ray Galaxy Cluster Modeling
We propose an ambitious new method that models the intracluster medium in
clusters of galaxies as a set of X-ray emitting smoothed particles of plasma.
Each smoothed particle is described by a handful of parameters including
temperature, location, size, and elemental abundances. Hundreds to thousands of
these particles are used to construct a model cluster of galaxies, with the
appropriate complexity estimated from the data quality. This model is then
compared iteratively with X-ray data in the form of adaptively binned photon
lists via a two-sample likelihood statistic and iterated via Markov Chain Monte
Carlo. The complex cluster model is propagated through the X-ray instrument
response using direct sampling Monte Carlo methods. Using this approach the
method can reproduce many of the features observed in the X-ray emission in a
less assumption-dependent way that traditional analyses, and it allows for a
more detailed characterization of the density, temperature, and metal abundance
structure of clusters. Multi-instrument X-ray analyses and simultaneous X-ray,
Sunyaev-Zeldovich (SZ), and lensing analyses are a straight-forward extension
of this methodology. Significant challenges still exist in understanding the
degeneracy in these models and the statistical noise induced by the complexity
of the models.Comment: 17 pages, 29 figures, ApJ accepte
An XMM-Newton Observation of the Local Bubble Using a Shadowing Filament in the Southern Galactic Hemisphere
We present an analysis of the X-ray spectrum of the Local Bubble, obtained by
simultaneously analyzing spectra from two XMM-Newton pointings on and off an
absorbing filament in the Southern galactic hemisphere (b ~ -45 deg). We use
the difference in the Galactic column density in these two directions to deduce
the contributions of the unabsorbed foreground emission due to the Local
Bubble, and the absorbed emission from the Galactic halo and the extragalactic
background. We find the Local Bubble emission is consistent with emission from
a plasma in collisional ionization equilibrium with a temperature and an emission measure of 0.018 cm^{-6} pc. Our
measured temperature is in good agreement with values obtained from ROSAT
All-Sky Survey data, but is lower than that measured by other recent XMM-Newton
observations of the Local Bubble, which find
(although for some of these observations it is possible that the foreground
emission is contaminated by non-Local Bubble emission from Loop I). The higher
temperature observed towards other directions is inconsistent with our data,
when combined with a FUSE measurement of the Galactic halo O VI intensity. This
therefore suggests that the Local Bubble is thermally anisotropic.
Our data are unable to rule out a non-equilibrium model in which the plasma
is underionized. However, an overionized recombining plasma model, while
observationally acceptable for certain densities and temperatures, generally
gives an implausibly young age for the Local Bubble (\la 6 \times 10^5 yr).Comment: Accepted for publication in the Astrophysical Journal. 16 pages, 9
figure
Diffuse X-ray emission in spiral galaxies
We compare the soft diffuse X-ray emission from Chandra images of 12 nearby
intermediate inclination spiral galaxies to the morphology seen in Halpha,
molecular gas, and mid-infrared emission. We find that diffuse X-ray emission
is often located along spiral arms in the outer parts of spiral galaxies but
tends to be distributed in a rounder morphology in the center. The X-ray
morphology in the spiral arms matches that seen in the mid-infrared or Halpha
and so implies that the X-ray emission is associated with recent active star
formation. We see no strong evidence for X-ray emission trailing the location
of high mass star formation in spiral arms. However, population synthesis
models predict a high mechanical energy output rate from supernovae for a time
period that is about 10 times longer than the lifetime of massive ionizing
stars, conflicting with the narrow appearance of the arms in X-rays. The
fraction of supernova energy that goes into heating the ISM must depend on
environment and is probably higher near sites of active star formation. The
X-ray estimated emission measures suggest that the volume filling factors and
scale heights are high in the galaxy centers but low in the outer parts of
these galaxies. The differences between the X-ray properties and morphology in
the centers and outer parts of these galaxies suggest that galactic fountains
operate in outer galaxy disks but that winds are primarily driven from galaxy
centers.Comment: 28 pages, 4 figures, to be submitted to Ap
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