3,557 research outputs found
The Composition of the Interstellar Medium towards the Lockman Hole. HI, UV and X-ray observations
The Lockman Hole is well known as the region with the lowest neutral atomic
hydrogen colum density on the entire sky. We present an analysis of the soft
X-ray background radiation towards the Lockman Hole using ROSAT all-sky survey
data. This data is correlated with the Leiden/Dwingeloo survey (Galactic HI
21cm-line emission) in order to model the soft X-ray background by using
radiative transfer calculations for four ROSAT energy bands simultaneously. It
turns out, that an important gas fraction, ranging between 20-50%,of the X-ray
absorbing material is not entirely traced by the HI but is in the form of
ionized hydrogen. Far-ultraviolet absorption line measurements by FUSE are
consistent with this finding and support an ionized hydrogen component towards
the Lockman Hole.Comment: 11 pages, 11 figures, Accepted for Publication in Astronomy &
Astrophysics, For full resolution images, see
http://www.astro.uni-bonn.de/~mkappes/pub/ms3506.pd
The harmonic power spectrum of the soft X-ray background I. The data analysis
Fluctuations of the soft X-ray background are investigated using harmonic
analysis. A section of the ROSAT All-Sky Survey around the north galactic pole
is used. The flux distribution is expanded into a set of harmonic functions and
the power spectrum is determined. Several subsamples of the RASS have been used
and the spectra for different regions and energies are presented. The effects
of the data binning in pixels are assessed and taken into account. The spectra
of the analyzed samples reflect both small scale effects generated by strong
discrete sources and the large scale gradients of the XRB distribution. Our
results show that the power spectrum technique can be effectively used to
investigate anisotropy of the XRB at various scales. This statistics will
become a useful tool in the investigation of various XRB components.Comment: 12 pages, A&A accepte
Missing baryons and the soft X-ray background
The X-ray background intensity around Lick count galaxies and rich clusters
of galaxies is investigated in three ROSAT energy bands. It is found that the
X-ray enhancements surrounding concentrations of galaxies exhibit significantly
softer spectrum than the standard cluster emission and the average
extragalactic background. The diffuse soft emission accompanying the galaxies
is consistent with the thermal emission of the hot gas postulated first by the
Cen & Ostriker hydrodynamic simulations. Our estimates of the gas temperature -
although subject to large uncertainties - averaged over several Mpc scales are
below 1 keV, which is substantially below the temperature of the intra-cluster
gas, but consistent with temperatures predicted for the local intergalactic
medium. It is pointed out that the planned ROSITA mission would be essential
for our understanding of the diffuse thermal component of the background.Comment: AA accepted, 6 pages, incl. 4 figure
Groups and the Entropy Floor- XMM-Newton Observations of Two Groups
Using XMM-Newton spatially resolved X-ray imaging spectroscopy we obtain the
temperature, density, entropy, gas mass, and total mass profiles for two groups
of galaxies out to ~0.3 Rvir (Rvir, the virial radius). Our density profiles
agree well with those derived previously, and the temperature data are broadly
consistent with previous results but are considerably more precise. Both of
these groups are at the mass scale of 2x10^13 Msolar but have rather different
properties. They have considerably lower gas mass fractions at r<0.3 Rvir than
the rich clusters. NGC2563, one of the least luminous groups for its X-ray
temperature, has a very low gas mass fraction of ~0.004 inside 0.1 Rvir, which
rises with radius. NGC4325, one of the most luminous groups at the same average
temperature, has a higher gas mass fraction of 0.02. The entropy profiles and
the absolute values of the entropy as a function of virial radius also differ,
with NGC4325 having a value of ~100 keV cm-2 and NGC2563 a value of ~300 keV
cm-2 at r~0.1 Rvir. For both groups the profiles rise monotonically with radius
and there is no sign of an entropy "floor". These results are inconsistent with
pre-heating scenarios which have been developed to explain the entropy floor in
groups but are broadly consistent with models of structure formation which
include the effects of heating and/or the cooling of the gas. The total entropy
in these systems provides a strong constraint on all models of galaxy and group
formation, and on the poorly defined feedback process which controls the
transformation of gas into stars and thus the formation of structure in the
universe.Comment: 22 pages, 2 figure
X-ray observations of PKS 0745-191 at the virial radius: Are we there yet?
We wish to reassess the properties of the ICM at large radii in the galaxy
cluster PKS 0745-191 in light of the recent Suzaku measurements. We analyze an
archival 10.5 ksec ROSAT/PSPC observation to extract the surface-brightness
profile of PKS 0745-191 and infer the deprojected density profile. We then
compare the ROSAT surface-brightness profile with the Suzaku result. We perform
a mass analysis combining the ROSAT density profile and the published
temperature profiles from different instruments. We find that the ROSAT
surface-brightness profile is statistically inconsistent (7.7 sigma) with the
Suzaku result around and beyond the value of r200 estimated by Suzaku. We argue
that, thanks to its large field of view and low background, ROSAT/PSPC is to
the present day the most sensitive instrument to low surface-brightness X-ray
emission in the 0.4-2.0 keV band. We also note that the Suzaku temperature and
mass profiles are at odds with the results from at least two other satellites
(XMM-Newton and Swift). The difference in surface brightness between ROSAT and
Suzaku is most likely explained by the existence of additional foreground
components at the low Galactic latitude of the source, which were not taken
into account in the Suzaku background modeling. In light of our mass analysis,
we conclude that any estimate of the fraction of the virial radius reached by
X-ray measures is affected by systematic errors of the order of 25%. As a
result, the properties of the ICM at the virial radius are still uncertain, and
the Suzaku results should be considered with caution.Comment: 6 pages, 5 figures, accepted for publication in A&
The Negro in Classical Italy
Originally published in the American Journal of Philolog
Greek Tragedy in France and America
Originally published in The Classical Journa
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