522 research outputs found
X-Ray IGM in the Local Group
Recent observations with the dispersive X-ray spectrometers aboard Chandra
and Newton Observatory have begun to probe the properties of the X-ray
intergalactic medium (IGM) at small redshifts. Using large quantities (~950
ksec) of spectroscopic data acquired using the RGS aboard Newton Observatory,
we investigated the intervening material toward three low redshift, high
Galactic latitude AGNs with nominally featureless spectra: Mrk421, PKS2155-304
and 3C273. Each spectrum provides clear evidence for what appears to be a local
(z~0), highly ionized absorbing medium betrayed by the OVII 1s-2p resonance
transition feature seen at 21.6A (N[OVII] ~ 1E16 cm-2). Measurements are also
made for the Lyman alpha transition of the adjacent ionization state, (OVIII;
18.97A), which potentially constrains the absorber's temperature. Finally, in a
collisional equilibrium approximation, upper limits to diffuse emission
intensities place upper limits on the electron density (ne < 2E-4 cm-3), lower
limits on the scale length of the absorber (L > 140 kpc) and lower limits on
its mass (M > 5E10 M[sun]). Limits on the absorber's scale length and its
velocity distribution lead us to identify it with the Local Group.Comment: 6 pages, 1 table and 2 figs. Latex. To appear in "The IGM/Galaxy
Connection: The Distribution of Baryons at z=0" to be published by Kluwer
Academic Publishing. Resubmitted with 2 typos corrected, page 5 and figure 2.
Thanks to Masao Sako for pointing these ou
EUVE Spectroscopy of the Accretion Region in AM Herculis
Progress in determining the gravitational energy conversion balance in accreting white dwarf stars is reported. Detailed photospheric spectra of the white dwarf stars in the brightest magnetic cataclysmic binaries, in the region where most of the energy is radiated by the stellar atmosphere - in the extreme ultraviolet band is obtained
X-ray Spectroscopy of Interstellar Carbon: Evidence for Scattering by Carbon-Bearing Material in the Spectrum of 1ES 1553+113
Molecules and particles make up of carbon in the
interstellar medium, yet the exact chemical structure of these constituents
remains unknown. We present carbon K-shell absorption spectroscopy of the
Galactic Interstellar Medium obtained with the Low Energy Transmission Grating
Spectrometer on the {\it Chandra} Observatory, that directly addresses this
question. We probe several lines of sight, using bright AGN as backlighters. We
make our measurements differentially with respect to the bright source Mrk 421,
in order to take the significant carbon K absorption in the instrument into
account. In the spectrum of the blazar 1ES 1553+113 we find evidence for a
novel feature: strong extinction on the low-energy side of the neutral C
resonance, which is indicative of scattering by graphite particles. We
find evidence for characteristic particle radii of order m. If
this explanation for the feature is correct, limits on the mass of the
available carbon along the line of sight may imply that the grains are
partially aligned, and the X-rays from the source may have intrinsic
polarization.Comment: 11 pages, 9 figures, 4 table
X-ray spectroscopy of galaxy clusters: beyond the CIE modeling
X-ray spectra of galaxy clusters are dominated by the thermal emission from
the hot intracluster medium. In some cases, besides the thermal component,
spectral models require additional components associated, e.g., with resonant
scattering and charge exchange. The latter produces mostly underluminous fine
spectral features. Detection of the extra components therefore requires high
spectral resolution. The upcoming X-ray missions will provide such high
resolution, and will allow spectroscopic diagnostics of clusters beyond the
current simple thermal modeling. A representative science case is resonant
scattering, which produces spectral distortions of the emission lines from the
dominant thermal component. Accounting for the resonant scattering is essential
for accurate abundance and gas motion measurements of the ICM. The high
resolution spectroscopy might also reveal/corroborate a number of new spectral
components, including the excitation by non-thermal electrons, the deviation
from ionization equilibrium, and charge exchange from surface of cold gas
clouds in clusters. Apart from detecting new features, future high resolution
spectroscopy will also enable a much better measurement of the thermal
component. Accurate atomic database and appropriate modeling of the thermal
spectrum are therefore needed for interpreting the data.Comment: published in Space Science Review
A Direct Upper Limit on the Density of Cosmological Dust from the Absence of an X-ray Scattering Halo around the z=4.3 QSO 1508+5714
We report on the results of a search for an intergalactic X-ray dust
scattering halo in a deep observation of the bright, high-redshift quasar QSO
1508+5714 with the Chandra X-ray Observatory. We do not detect such a halo. Our
result implies an upper limit on the density of diffuse, large-grained
intergalactic dust of Omega_ dust < 2 x 10^-6, assuming a characteristic grain
size of 1micron. The result demonstrates the sensitivity of this technique for
detecting very small amounts of intergalactic dust which are very hard to
detect otherwise. This will allow us to put important constraints on systematic
effects induced by extinction on the interpretation of the SN Ia Hubble
Diagram, as well as on the amount and properties of cosmological dust being
expelled into the intergalactic medium at early z~2 times.Comment: 14 pages, 2 figures. to appear in ApJ, vol. 651, Nov. 200
A possible dearth of hot gas in galaxy groups at intermediate redshift
We examine the X-ray luminosity of galaxy groups in the CNOC2 survey, at
redshifts 0.1 < z < 0.6. Previous work examining the gravitational lensing
signal of the CNOC2 groups has shown that they are likely to be genuine,
gravitationally bound objects. Of the 21 groups in the field of view of the
EPIC-PN camera on XMM-Newton, not one was visible in over 100 ksec of
observation, even though three of the them have velocity dispersions high
enough that they would easily be visible if their luminosities scaled with
their velocity dispersions in the same way as nearby groups' luminosities
scale. We consider the possibility that this is due to the reported velocity
dispersions being erroneously high, and conclude that this is unlikely. We
therefore find tentative evidence that groups at intermediate redshift are
underluminous relative to their local cousins.Comment: 16 pages, 5 figures, reference added in section 1, typos corrected,
published in Ap
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