74 research outputs found
The large-scale bias of the hard X-ray background
Recent deep X-ray surveys combined with spectroscopic identification of the
sources have allowed the determination of the rest-frame 2-8 keV luminosity as
a function of redshift. In addition, an analysis of the HEAO1 A2 2-10 keV
full-sky map of the X-ray background (XRB) reveals clustering on the scale of
several degrees. Combining these two results in the context of the currently
favored Lambda-CDM cosmological model implies an average X-ray bias factor,
b_x, of b_x^2 = 1.12 +- 0.33, i.e., b_x = 1.06 +- 0.16. These error estimates
include only statistical error; the systematic error sources, while comparable,
appear to be sub-dominant. This result is in contrast to the large biases of
some previous estimates and is more in line with current estimates of the
optical bias of L* galaxies.Comment: 6 pages, 3 eps figures, accepted for ApJ, vol. 612, 10 September 200
The Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest: The X-ray Catalog
We present the X-ray catalog and basic results from our Chandra Large Area
Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest field. Our 9
ACIS-I fields cover a contiguous solid angle of ~0.4 sq. deg. and reach fluxes
of 5E-16 cgs (0.4-2 keV) and 3E-15 cgs (2-8keV). Our survey bridges the gap
between ultradeep pencil-beam surveys, such as the Chandra Deep Fields (CDFs),
and shallower, large area surveys, allowing a better probe of the X-ray sources
that contribute most of the 2-10 keV cosmic X-ray background (CXB). We find a
total of 525 X-ray point sources and 4 extended sources. At ~10E-14 cgs 2-8
keV, our number counts are significantly higher than those of several
non-contiguous, large area surveys. On the other hand, the integrated flux from
the CLASXS field, combined with ASCA and Chandra ultradeep surveys, is
consistent with results from other large area surveys, within the variance of
the CXB. Spectral evolution is seen in the hardening of the sources at fluxes
below 1E-14 cgs Above 4E1-14 cgs(0.4-8 keV), ~60 of the sources are variable.
Four extended sources in CLASXS is consistent with the previously measured
LogN-LogS of galaxy clusters. We report the discovery of a gravitational
lensing arc associated with one of these sources. (Abridged)Comment: 67 pages, 26 figures, accepted for publication in the Astronomical
Journa
Absolute measurement of the unresolved cosmic X-ray background in the 0.5-8 keV band with Chandra
We present the absolute measurement of the unresolved 0.5-8 keV cosmic X-ray
background (CXB) in the Chandra Deep Fields (CDFs) North and South, the longest
observations with Chandra (2 Ms and 1 Ms, respectively). We measure the
unresolved CXB intensity by extracting spectra of the sky, removing all point
and extended sources detected in the CDF. To model and subtract the
instrumental background, we use observations obtained with ACIS in stowed
position, not exposed to the sky. The unresolved signal in the 0.5-1 keV band
is dominated by diffuse Galactic and local thermal-like emission. In the 1-8
keV band, the unresolved spectrum is adequately described by a power law with a
photon index 1.5. We find unresolved CXB intensities of (1.04+/-0.14)x10^-12
ergs cm^-2 s^-1 deg^-2 for the 1-2 keV band and (3.4+/-1.7)x10^-12 ergs cm^-2
s^-1 deg^-2 for the 2-8 keV band. Our detected unresolved intensities in these
bands significantly exceed the expected flux from sources below the CDF
detection limits, if one extrapolates the logN/logS curve to zero flux. Thus
these background intensities imply either a genuine diffuse component, or a
steepening of the logN/logS curve at low fluxes, most significantly for
energies <2 keV. Adding the unresolved intensity to the total contribution from
sources detected in these fields and wider-field surveys, we obtain a total
intensity of the extragalactic CXB of (4.6+/-0.3)x10^-12 ergs cm^-2 s^-1 deg^-2
for 1-2 keV and (1.7+/-0.2)x10^-11 ergs cm^-2 s^-1 deg^-2 for 2-8 keV. These
totals correspond to a CXB power law normalization (for photon index 1.4) of
10.9 photons cm^-2 s^-1 keV^-1 sr^-1 at 1 keV. This corresponds to resolved
fracations of 77+/-3% and 80+/-8% for 1-2 and 2-8 keV, respectively.Comment: 23 emulateapj pages, accepted for publication in ApJ. Minor
revisions, most notably a new summary of the error analysi
Connecting Galaxy Evolution, Star Formation and the X-ray Background
As a result of deep hard X-ray observations by Chandra and XMM-Newton a
significant fraction of the cosmic X-ray background (CXRB) has been resolved
into individual sources. These objects are almost all active galactic nuclei
(AGN) and optical followup observations find that they are mostly obscured Type
2 AGN, have Seyfert-like X-ray luminosities (i.e., L_X ~ 10^{43-44} ergs
s^{-1}), and peak in redshift at z~0.7. Since this redshift is similar to the
peak in the cosmic star-formation rate, this paper proposes that the obscuring
material required for AGN unification is regulated by star-formation within the
host galaxy. We test this idea by computing CXRB synthesis models with a ratio
of Type 2/Type 1 AGN that is a function of both z and 2-10 keV X-ray
luminosity, L_X. The evolutionary models are constrained by parameterizing the
observed Type 1 AGN fractions from the recent work by Barger et al. The
parameterization which simultaneously best accounts for Barger's data, the CXRB
spectrum and the X-ray number counts has a local, low-L_X Type 2/Type 1 ratio
of 4, and predicts a Type 2 AGN fraction which evolves as (1+z)^{0.3}. Models
with no redshift evolution yielded much poorer fits to the Barger Type 1 AGN
fractions. This particular evolution predicts a Type 2/Type 1 ratio of 1-2 for
log L_X > 44, and thus the deep X-ray surveys are missing about half the
obscured AGN with these luminosities. These objects are likely to be Compton
thick. Overall, these calculations show that the current data strongly supports
a change to the AGN unification scenario where the obscuration is connected
with star formation in the host galaxy rather than a molecular torus alone. The
evolution of the obscuration implies a close relationship between star
formation and AGN fueling, most likely due to minor mergers or interactions.Comment: 36 pages, 8 figures, ApJ in press. Minor changes to match published
versio
The resolved fraction of the Cosmic X-ray Background
We present the X-ray source number counts in two energy bands (0.5-2 and 2-10
keV) from a very large source sample: we combine data of six different surveys,
both shallow wide field and deep pencil beam, performed with three different
satellites (ROSAT, Chandra and XMM-Newton). The sample covers with good
statistics the largest possible flux range so far: [2.4*10^-17 - 10^-11] cgs in
the soft band and [2.1*10^-16 - 8*10^{-12}]cgs in the hard band. Integrating
the flux distributions over this range and taking into account the (small)
contribution of the brightest sources we derive the flux density generated by
discrete sources in both bands. After a critical review of the literature
values of the total Cosmic X--Ray Background (CXB) we conclude that, with the
present data, the 94.3%, and 88.8% of the soft and hard CXB can be ascribed to
discrete source emission. If we extrapolate the analytical form of the Log
N--Log S distribution beyond the flux limit of our catalog in the soft band we
find that the flux from discrete sources at ~3*10^-18 cgs is consistent with
the entire CXB, whereas in the hard band it accounts for only 93% of the total
CXB at most, hinting for a faint and obscured population to arise at even
fainter fluxes.Comment: Accepted for publication in Ap
The Extended Chandra Deep Field-South Survey: X-ray Point-Source Catalog
The Extended Chandra Deep Field-South (ECDFS) survey consists of 4 Chandra
ACIS-I pointings and covers 1100 square arcminutes ( 0.3
deg) centered on the original CDF-S field to a depth of approximately 228
ks. This is the largest Chandra survey ever conducted at such depth, and only
one XMM-Newton survey reaches a lower flux limit in the hard 2.0--8.0 keV band.
We detect 651 unique sources -- 587 using a conservative source detection
threshold and 64 using a lower source detection threshold. These are presented
as two separate catalogs. Of the 651 total sources, 561 are detected in the
full 0.5--8.0 keV band, 529 in the soft 0.5--2.0 keV band, and 335 in the hard
2.0--8.0 keV band. For point sources near the aim point, the limiting fluxes
are approximately and in the 0.5--2.0 keV and 2.0--8.0 keV
bands, respectively. Using simulations, we determine the catalog completeness
as a function of flux and assess uncertainties in the derived fluxes due to
incomplete spectral information. We present the differential and cumulative
flux distributions, which are in good agreement with the number counts from
previous deep X-ray surveys and with the predictions from an AGN population
synthesis model that can explain the X-ray background. In general, fainter
sources have harder X-ray spectra, consistent with the hypothesis that these
sources are mainly obscured AGN.Comment: Replaced original paper with the AJ-accepted version. New version
includes simulations on the uncertainties in the derived fluxes due to
incomplete spectral information and catalog completeness. Full catalog
available at http://www.astro.yale.edu/svirani/ecdfs
Detection of an X-Ray Hot Region in the Virgo Cluster of Galaxies with ASCA
Based on mapping observations with ASCA, an unusual hot region with a spatial
extent of 1 square degree was discovered between M87 and M49 at a center
coordinate of R. A. = 12h 27m 36s and Dec. = (J2000). The X-ray
emission from the region has a 2-10 keV flux of ergs
s cm and a temperature of keV, which is
significantly higher than that in the surrounding medium of keV. The
internal thermal energy in the hot region is estimated to be ergs with a gas density of cm. A power-law
spectrum with a photon index is also allowed by the data. The hot
region suggests there is an energy input due to a shock which is probably
caused by the motion of the gas associated with M49, infalling toward the M87
cluster with a velocity km s.Comment: 12 pages, 3 figures, accepted to ApJ
Gas temperature profiles in galaxy clusters with Swift XRT: observations and capabilities to map near R200
We investigate the possibility of using the X-ray telescope (XRT) on board
the Swift satellite to improve the current accuracy of the ICM temperature
measurements in the region close to the virial radius of nearby clusters. We
present the spectral analysis of the Swift XRT observations of 6 galaxy
clusters and their temperature profiles in the regions within 0.2-0.6 r200.
Four of them are nearby famous and very well studied objects (Coma, Abell 1795,
Abell 2029 and PKS0745-19). The remaining two, SWJ1557+35 and SWJ0847+13, at
redshift z=0.16 and z=0.36, were serendipitously observed by Swift-XRT. We
accurately quantify the temperature uncertainties, with particular focus on the
impact of the background scatter (both instrumental and cosmic). We extrapolate
these results and simulate a deep observation of the external region of Abell
1795 which is assumed here as a case study. In particular we calculate the
expected uncertainties in the temperature measurement as far as r200. We find
that, with a fairly deep observation (300 ks), the Swift XRT would be able to
measure the ICM temperature profiles in the external regions as far as the
virial radius, significantly improving the best accuracy among the previous
measurements. This can be achieved thanks to the unprecedented combination of
good PSF over the full field of view and very accurate control of the
instrumental background. Somehow unexpectedly we conclude that, among currently
operating telescope, the Swift-XRT is the only potentially able to improve the
current accuracy in plasma temperature measurement at the edges of the cluster
potential. This will be true until a newgeneration of low-background and large
field of view telescopes, aimed to the study of galaxy clusters, will operate.
These observations would be of great importance in developing the observing
strategy for suchmissions.Comment: Accepted for publication in A&A ;13 pages, 13 figure
From M-ary Query to Bit Query: a new strategy for efficient large-scale RFID identification
The tag collision avoidance has been viewed as one of the most important research problems in RFID communications and bit tracking technology has been widely embedded in query tree (QT) based algorithms to tackle such challenge. Existing solutions show further opportunity to greatly improve the reading performance because collision queries and empty queries are not fully explored. In this paper, a bit query (BQ) strategy based Mary query tree protocol (BQMT) is presented, which can not only eliminate idle queries but also separate collided tags into many small subsets and make full use of the collided bits. To further optimize the reading performance, a modified dual prefixes matching (MDPM) mechanism is presented to allow multiple tags to respond in the same slot and thus significantly reduce the number of queries. Theoretical analysis and simulations are supplemented to validate the effectiveness of the proposed BQMT and MDPM, which outperform the existing QT-based algorithms. Also, the BQMT and MDPM can be combined to BQMDPM to improve the reading performance in system efficiency, total identification time, communication complexity and average energy cost
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