658 research outputs found
What X-ray source counts can tell about large-scale matter distribution
Sources generating most of the X-ray background (XRB) are dispersed over a
wide range of redshifts. Thus, statistical characteristics of the source
distribution carry information on matter distribution on very large scales. We
test the possibility of detecting the variation in the X-ray source number
counts over the celestial sphere. A large number of Chandra pointings spread
over both galactic hemispheres are investigated. We searched for all the
point-like sources in the soft band of 0.5 - 2 keV and statistically assessed
the population of sources below the detection threshold. A homogeneous sample
of the number counts at fluxes above ~10^{-15} erg s^{-1} cm^{-2} was
constructed for more than 300 ACIS fields. The sources were counted within a
circular area of 15 arcmin diameter. The count correlations between overlapping
fields were used to assess the accuracy of the computational methods used in
the analysis. The average number of sources in the investigated sample amounts
to 46 per field. It is shown that the source number counts vary between fields
at a level exceeding the fluctuation amplitude expected for the random
(Poissonian) distribution. The excess fluctuations are attributed to the cosmic
variance generated by the large-scale structures. The rms variations of the
source counts due to the cosmic variance within the 15$ arcmin circle reach 8%
of the average number counts. An amplitude of the potential correlations of the
source counts on angular scales larger than the size of a single pointing
remains below the noise level.Comment: 8 pages, 4 figures; expansion of observational material resulted in
substantial changes; accepted for publication in A&
Hydrodynamic Simulation of the Cosmological X-ray Background
(Abridged) We use a hydrodynamic simulation of a LambdaCDM model to predict
the extragalactic X-ray background (XRB), focussing on emission from the
intergalactic medium (IGM). We also include X-rays from point sources
associated with galaxies in the simulation, and make maps of the angular
distribution of the emission. We find that filaments in the maps are not
evident, being diluted by projection. In the soft (0.5-2 keV) band, the mean
intensity of radiation from intergalactic and cluster gas is 2.3*10^-12
ergdeg^-2cm^-2s^-1, 35% of the total soft band emission. This is compatible at
the ~1 sigma level with estimates of the unresolved soft background from ROSAT
and {\it Chandra}. Only 4% of the hard (2-10 keV) emission is associated with
the IGM. Relative to AGN flux, the IGM component peaks at a lower redshift
(median z~0.45) so its clustering makes an important contribution to that of
the total XRB. The angular correlations on 0.1-10 arcmin scales are
significant, with an amplitude roughly consistent with an extrapolation of
recent ROSAT results to small scales. A cross-correlation of the XRB against
nearby galaxies taken from a simulated redshift survey also yields a strong
signal from the IGM. Although some recent papers have argued that the expected
soft band intensity from gas in galaxy, group, and cluster halos would exceed
XRB limits unless much of the gas is expelled by supernova feedback, we obtain
reasonable compatibility with current observations in a simulation that
incorporates cooling, star formation, and only modest feedback. A prediction of
our model is that the unresolved portion of the soft XRB will remain mostly
unresolved.Comment: Improved referencing of related papers. Submitted to ApJ, 19 pages,
17 postscript figures, most reduced in resolution, emulateapj.sty, for full
resolution version, see http://cfa-www.harvard.edu/~rcroft/xray.ps.g
The AGN-starburst connection, Galactic superwinds, and M_BH - sigma
Recent observations of young galaxies at redshifts z ~ 3 have revealed
simultaneous AGN and starburst activity, as well as galaxy-wide superwinds. I
show that there is probably a close connection between these phenomena by
extending an earlier treatment of the M_BH - sigma relation (King, 2003). As
the black hole grows, an outflow drives a shell into the surrounding gas. This
stalls after a dynamical time at a size determined by the hole's current mass
and thereafter grows on the Salpeter timescale. The gas trapped inside this
bubble cools and forms stars and is recycled as accretion and outflow. The
consequent high metallicity agrees with that commonly observed in AGN
accretion. Once the hole reaches a critical mass this region attains a size
such that the gas can no longer cool efficiently. The resulting energy-driven
flow expels the remaining gas as a superwind, fixing both the M_BH - sigma
relation and the total stellar bulge mass at values in good agreement with
observation. Black hole growth thus produces starbursts and ultimately a
superwind.Comment: ApJ, in press, 4 page
X-Ray Emission from the Warm Hot Intergalactic Medium
The number of detected baryons in the Universe at z<0.5 is much smaller than
predicted by standard big bang nucleosynthesis and by the detailed observation
of the Lyman alpha forest at red-shift z=2. Hydrodynamical simulations indicate
that a large fraction of the baryons today is expected to be in a ``warm-hot''
(10^5-10^7K) filamentary gas, distributed in the intergalactic medium. This
gas, if it exists, should be observable only in the soft X-ray and UV bands.
Using the predictions of a particular hydrodynamic model, we simulated the
expected X-ray flux as a function of energy in the 0.1-2 keV band due to the
Warm-Hot Intergalactic Medium (WHIM), and compared it with the flux from local
and high red-shift diffuse components. Our results show that as much as 20% of
the total diffuse X-ray background (DXB) in the energy range 0.37-0.925keV
could be due to X-ray flux from the WHIM, 70% of which comes from filaments at
redshift z between 0.1 and 0.6. Simulations done using a FOV of 3', comparable
with that of Suzaku and Constellation-X, show that in more than 20% of the
observations we expect the WHIM flux to contribute to more than 20% of the DXB.
These simulations also show that in about 10% of all the observations a single
bright filament in the FOV accounts, alone, for more than 20% of the DXB flux.
Red-shifted oxygen lines should be clearly visible in these observations.Comment: 19 pages, 6 figure
Comparison of four methods for isolation of Yersinia enterocolitica from raw and pasteurized milk from northern Iran
Four methods for isolation of Yersinia enterocolitica from raw and pasteurized milk from northern Iran were compared. Three hundred and ten raw milk samples were collected from tanks on their arrival at various central dairies, and 40 pasteurized milk samples were collected from tanks on their arrival at a manufacturing plant. Each sample was examined for the presence of Y. enterocolitica by (1) direct culture; (2) enrichment in double-strength buffered peptone water at 4°C for 1 month; (3) enrichment in modified Rappaport medium at room temperature for 72 h after a preenrichment in double-strength peptone water at 4°C for 1 month; and (4) enrichment in a medium containing sucrose, tris (hydroxymethyl) aminomethane, sodium azide, and ampicillin at 28°C for 48 h after a preenrichment in double-strength peptone water at 4°C for 1 month. All samples and enrichments were spread on MacConkey agar plus calcium chloride and Tween 80, Yersinia selective agar, and Hektoen medium plus ampicillin. Five samples (1.6%) of raw milk but no pasteurized milk samples were positive for Y. enterocolitica. No Y. enterocolitica were recovered by methods 1 or 2. Y. enterocolitica were recovered from 2 samples by method 3 followed by culture on Yersinia selective agar, and from 5 samples by method 4 followed by culture on Hektoen medium plus ampicillin. The isolates were biotype 1A or 1B, serotype O:7-13 or O:9 and phage type Xo or Xz. All isolates were resistant to ampicillin and amoxicillin, and sensitive to tetracycline, streptomycin, chloramphenicol, and trimethoprim-sulfamethoxazole. © 2004 Elsevier B.V. All rights reserved
The redshift-dependence of gamma-ray absorption in the environments of strong-line AGN
The case of gamma-ray absorption due to photon-photon pair production of jet
photons in the external photon environment like accretion disk and broad-line
region radiation field of gamma-ray loud active galactic nuclei (AGN) that
exhibit strong emission lines is considered. I demonstrate that this ''local
opacity'', if detected, will almost unavoidably be redshift-dependent in the
sub-TeV range. This introduces non-negligible biases, and complicates
approaches for studying the evolution of the extragalactic background light
with contemporary GeV instruments like e.g. the Gamma-ray Large Area Space
Telescope (GLAST), etc., where the gamma-ray horizon is probed by means of
statistical analysis of absorption features (e.g. Fazio-Stecker relation, etc.)
in AGN spectra at various redshifts. It particularly applies to strong-line
quasars where external photon fields are potentially involved in gamma-ray
production.Comment: 19 pages, 5 figures; accepted for publication in Ap
Towards the use of the most massive black hole candidates in AGN to test the Kerr paradigm
The super-massive objects in galactic nuclei are thought to be the Kerr black
holes predicted by General Relativity, although a definite proof of their
actual nature is still lacking. The most massive objects in AGN () seem to have a high radiative efficiency () and a
moderate mass accretion rate (). The high
radiative efficiency could suggest they are very rapidly-rotating black holes.
The moderate luminosity could indicate that their accretion disk is
geometrically thin. If so, these objects could be excellent candidates to test
the Kerr black hole hypothesis. An accurate measurement of the radiative
efficiency of an individual AGN may probe the geometry of the space-time around
the black hole candidate with a precision comparable to the one achievable with
future space-based gravitational-wave detectors like LISA. A robust evidence of
the existence of a black hole candidate with and accreting from a
thin disk may be interpreted as an indication of new physics. For the time
being, there are several issues to address before using AGN to test the Kerr
paradigm, but the approach seems to be promising and capable of providing
interesting results before the advent of gravitational wave astronomy.Comment: 12 pages, 6 figures. v2: some typos correcte
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