859 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
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