661 research outputs found
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
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
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
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
Bandpass Dependence of X-ray Temperatures in Galaxy Clusters
We explore the band dependence of the inferred X-ray temperature of the
intracluster medium (ICM) for 192 well-observed galaxy clusters selected from
the Chandra Data Archive. If the hot ICM is nearly isothermal in the projected
region of interest, the X-ray temperature inferred from a broad-band (0.7-7.0
keV) spectrum should be identical to the X-ray temperature inferred from a
hard-band (2.0-7.0 keV) spectrum. However, if unresolved cool lumps of gas are
contributing soft X-ray emission, the temperature of a best-fit
single-component thermal model will be cooler for the broad-band spectrum than
for the hard-band spectrum. Using this difference as a diagnostic, the ratio of
best-fitting hard-band and broad-band temperatures may indicate the presence of
cooler gas even when the X-ray spectrum itself may not have sufficient
signal-to-noise to resolve multiple temperature components. To test this
possible diagnostic, we extract X-ray spectra from core-excised annular regions
for each cluster in our archival sample. We compare the X-ray temperatures
inferred from single-temperature fits when the energy range of the fit is
0.7-7.0 keV (broad) and when the energy range is 2.0/(1+z)-7.0 keV (hard). We
find that the hard-band temperature is significantly higher, on average, than
the broad-band temperature. Upon further exploration, we find this temperature
ratio is enhanced preferentially for clusters which are known merging systems.
In addition, cool-core clusters tend to have best-fit hard-band temperatures
that are in closer agreement with their best-fit broad-band temperatures. We
show, using simulated spectra, that this diagnostic is sensitive to secondary
cool components (TX = 0.5-3.0 keV) with emission measures >10-30% of the
primary hot component.Comment: Accepted for publication in Ap
The Origin of Soft X-rays in DQ Herculis
DQ Herculis (Nova Herculis 1934) is a deeply eclipsing cataclysmic variable
containing a magnetic white dwarf primary. The accretion disk is thought to
block our line of sight to the white dwarf at all orbital phases due to its
extreme inclination angle. Nevertheless, soft X-rays were detected from DQ Her
with ROSAT PSPC. To probe the origin of these soft X-rays, we have performed
Chandra ACIS observations. We confirm that DQ Her is an X-ray source. The bulk
of the X-rays are from a point-like source and exhibit a shallow partial
eclipse. We interpret this as due to scattering of the unseen central X-ray
source, probably in an accretion disk wind. At the same time, we observe what
appear to be weak extended X-ray features around DQ Her, which we interpret as
an X-ray emitting knot in the nova shell.Comment: 18 pages including 4 figures, accepted for publication in
Astrphyisical Journa
A Simple Scaling Analysis of X-ray Emission and Absorption in Hot-Star Winds
We present a simple analysis of X-ray emission and absorption for hot-star
winds, designed to explore the natural scalings of the observed X-ray
luminosity with wind and sstellar properties. We show that an exospheric
approximation, in which all of the emission above the optical depth unity
radius escapes the wind, reproduces very well the detailed expression for
radiation transport through a spherically symmetric wind. Using this
approximation we find that the X-ray luminosity scales naturally with the
wind density parameter \Mdot/\vinf, obtaining L_x \sim (\Mdot/\vinf)^2 for
optically thin winds, and L_x \sim (\Mdot/\vinf)^{1+s} for optically thick
winds with an X-ray filling factor that varies in radius as . These
scalings with wind density contrast with the commonly inferred empirical
scalings of X-ray luminosity with bolometric luminosity . The
empirically derived linear scaling of for thick winds can
however be reproduced, through a delicate cancellation of emission and
absorption, if one assumes modest radial fall-off in the X-ray filling factor
( or , depending on details of the secondary
scaling of wind density with luminosity). We also explore the nature of the
X-ray spectral energy distribution in the context of this model, and find that
the spectrum is divided into a soft, optically thick part and a hard, optically
thin part. Finally, we conclude that the energy-dependent emissivity must have
a high-energy cut-off, corresponding to the maximum shock energy, in order to
reproduce the general trends seen in X-ray spectral energy distributions of hot
stars.Comment: 16 pages, 2 figures, requiress aaspp4.sty, accepted by Astrophysical
Journal, to appear in the Aug 10, 1999 issue. Several minor changes have been
made at the suggestion of the referee. We have added an appendix in which we
consider winds with beta-velocity laws, rather than simply constant
velocitie
Unveiling the nature of RX J0002+6246 with XMM-Newton
The X-ray source RX J0002+6246 was discovered close to the supernova remnant
CTB1 in a ROSAT observation performed in 1992. The source phenomenology (soft
spectrum, apparent lack of counterparts, possible pulsations at 242 ms, hints
for surrounding diffuse emission) led to interpret it as an isolated neutron
star in a new supernova remnant. We have analysed an archival XMM-Newton
observation performed in 2001. The source coordinates, as computed on the
XMM-Newton images, coincide with those of a bright source listed in optical and
infrared catalogues. The X-ray spectrum is well described by an optically thin
plasma model. No fast pulsations are seen, nor clear evidence of a supernova
remnant associated to the source. Thus, we conclude that RX J0002+6246 is not
an isolated neutron star, but the X-ray counterpart of the bright
optical/infrared source, most likely a F7 spectral class star located at about
0.2 kpc.Comment: 5 pages, 2 figures and 1 table. Accepted for publication in Monthly
Notices of the Royal Astronomical Society Main Journa
Shocks and sonic booms in the intracluster medium: X-ray shells and radio galaxy activity
Motivated by hydrodynamic simulations, we discuss the X-ray appearance of
radio galaxies embedded in the intracluster medium (ICM) of a galaxy cluster.
We distinguish three regimes. In the early life of a powerful source, the
entire radio cocoon is expanding supersonically and hence drives a strong shock
into the ICM. Eventually, the sides of the cocoon become subsonic and the ICM
is disturbed by the sonic booms of the jet's working surface. In both of these
regimes, X-ray observations would find an X-ray shell. In the strong shock
regime, this shell will be hot and relatively thin. However, in the weak shock
(sonic-boom) regime, the shell will be approximately the same temperature as
the undisturbed ICM. If a cooling flow is present, the observed shell may even
be cooler than the undisturbed ICM due to the lifting of cooler material into
the shell from the inner (cooler) regions of the cluster. In the third and
final regime, the cocoon has collapsed and no well-defined X-ray shell will be
seen. We discuss ways of estimating the power and age of the source once its
regime of behavior has been determined.Comment: 4 pages, submitted for publication in Astrophysical Journal. Full
paper (including figure) can be obtained from
http://rocinante.Colorado.EDU/~chris/papers/xray_hydro.p
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