66 research outputs found
A pulsing X-ray source in Circinus
Spectrum of pulsing X ray source observed in Circinu
Production of oriented nitrogen-vacancy color centers in synthetic diamond
The negatively charged nitrogen-vacancy (NV-) center in diamond is an
attractive candidate for applications that range from magnetometry to quantum
information processing. Here we show that only a fraction of the nitrogen
(typically < 0.5 %) incorporated during homoepitaxial diamond growth by
Chemical Vapor Deposition (CVD) is in the form of undecorated NV- centers.
Furthermore, studies on CVD diamond grown on (110) oriented substrates show a
near 100% preferential orientation of NV- centers along only the [111] and
[-1-11] directions, rather than the four possible orientations. The results
indicate that NV centers grow in as units, as the diamond is deposited, rather
than by migration and association of their components. The NV unit of the NVH-
is similarly preferentially oriented, but it is not possible to determine
whether this defect was formed by H capture at a preferentially aligned NV
center or as a complete unit. Reducing the number of NV orientations from 4
orientations to 2 orientations should lead to increased optically-detected
magnetic resonance contrast and thus improved magnetic sensitivity in
ensemble-based magnetometry.Comment: 13 Pages (inlcuding suplementary information), 4 figure
The X-ray Luminosity Function of Bright Clusters in the Local Universe
We present the X-ray luminosity function (XLF) for clusters of galaxies
derived from the RASS1 Bright Sample. The sample, selected from the ROSAT
All-Sky Survey in a region of 2.5 sr within the southern Galactic cap, contains
130 clusters with flux limits in the range ~ 3-4 x 10^-12 ergs/cm^2/s in the
0.5-2.0 keV band. A maximum-likelihood fit with a Schechter function of the XLF
over the entire range of luminosities (0.045 - 28. x 10^44 ergs/s), gives alpha
= 1.52 +/- 0.11, L_* = 3.80 +0.70 -0.55 x 10^44 ergs/s, and A = 5.07 +/- 0.45 x
10^-7 Mpc^-3 (10^44 ergs/s)^(\alpha-1). We investigate possible evolutionary
effects within the sample, out to our redshift limit (z ~ 0.3), finding no
evidence for evolution. Our results are in good agreement with other local
estimates of the XLF, implying that this statistic for the local universe is
now well determined. Comparison with XLFs for distant clusters (0.3 < z < 0.6),
shows that no evolution is present for L_X < 10^{44} ergs/s. However, we detect
differences at the 3 sigma level, between our local XLF and the distant one
estimated by Henry et al. for the EMSS sample. This difference is still present
when considering the EMSS sample revised by Nichol et al.Comment: 13 pages with 3 figures included, LaTex, aaspp4.sty and epsf.sty,
accepted for publication in ApJ Letters, only minor changes, added reference
Comparison of source detection procedures for XMM-Newton images
Procedures based on current methods to detect sources in X-ray images are
applied to simulated XMM images. All significant instrumental effects are taken
into account, and two kinds of sources are considered -- unresolved sources
represented by the telescope PSF and extended ones represented by a b-profile
model. Different sets of test cases with controlled and realistic input
configurations are constructed in order to analyze the influence of confusion
on the source analysis and also to choose the best methods and strategies to
resolve the difficulties.
In the general case of point-like and extended objects the mixed approach of
multiresolution (wavelet) filtering and subsequent detection by SExtractor
gives the best results. In ideal cases of isolated sources, flux errors are
within 15-20%. The maximum likelihood technique outperforms the others for
point-like sources when the PSF model used in the fit is the same as in the
images. However, the number of spurious detections is quite large.
The classification using the half-light radius and SExtractor stellarity
index is succesful in more than 98% of the cases. This suggests that average
luminosity clusters of galaxies (L_[2-10] ~ 3x10^{44} erg/s) can be detected at
redshifts greater than 1.5 for moderate exposure times in the energy band below
5 keV, provided that there is no confusion or blending by nearby sources.
We find also that with the best current available packages, confusion and
completeness problems start to appear at fluxes around 6x10^{-16} erg/s/cm^2 in
[0.5-2] keV band for XMM deep surveys.Comment: 20 pages, 16 figures. Accepted for publication in A&
The x-ray luminosity function of bright galaxy clusters in the local universe
We present the X-ray luminosity function (XLF) for clusters of galaxies derived from the RASS1 Bright Sample. The sample, selected from the ROSAT All-Sky Survey in a region of 2.5 sr within the southern Galactic cap, contains 130 clusters with flux limits in the range similar to 3-4 x 10(12) ergs cm(-2) s(-1) in the 0.5-2.0 keV band. A maximum likelihood fit with a Schechter function of the XLF over the entire range of luminosities (0.045-28.0 x 10(44) ergs s(-1)) gives alpha = 1.52(-0.11)(+0.11), L* = 3.80(-0.55)(+0.70) x 10(44) ergs s(-1), and A = 5.07 +/- 0.45 x 10(-7) Mpc(-3) (10(44) ergs s(-1))(alpha-1). We investigate possible evolutionary effects within the sample, out to our redshift limit (z similar to 0.3), finding no evidence for evolution. Our results are in good agreement with other local estimates of the XLF, implying that this statistic for the local universe is now well determined. Comparison with XLFs for distant clusters (0.3 < z < 0.6) shows that no evolution is present for L-X less than or similar to 10(44) ergs s(-1). However, we detect differences at the 3 sigma level between our local XLF and the distant one estimated by Henry et al. for the Einstein Extended Medium-Sensitivity Survey (EMSS) sample. This difference is still present when considering the EMSS sample revised by Nichol et al
A Flux-limited Sample of Bright Clusters of Galaxies from the Southern Part of the ROSAT All-Sky Survey: the Catalog and the LogN-LogS
We describe the selection of an X-ray flux-limited sample of bright clusters
of galaxies in the southern hemisphere, based on the first analysis of the
ROSAT All-Sky Survey data (RASS1). The sample is constructed starting from an
identification of candidate clusters in RASS1, and their X-ray fluxes are
remeasured using the Steepness Ratio Technique. This method is better suited
than the RASS1 standard algorithm for measuring flux from extended sources. The
final sample is count-rate-limited in the ROSAT hard band (0.5-2.0 keV), so
that due to the distribution of NH, its effective flux limit varies between
about 3-4 x 10**-12 ergs cm**-2 s**-1 over the selected area. This covers the
Decl<2.5 deg part of the south Galactic cap region (b<-20 deg) - with the
exclusion of patches of low RASS1 exposure time and of the Magellanic Clouds
area - for a total of 8235 deg**2. 130 candidate sources fulfill our selection
criteria for bonafide clusters of galaxies in this area. Of these, 101 are
Abell/ACO clusters, while 29 do not have a counterpart in these catalogs. Of
these clusters, 126 (97%) clusters have a redshift and for these we compute an
X-ray luminosity. 20% of the cluster redshifts come from new observations, as
part of the ESO Key Program REFLEX Cluster Survey that is under completion.
Considering the intrinsic biases and incompletenesses introduced by the flux
selection and source identification processes, we estimate the overall
completeness to be better than 90%. The observed number count distribution,
LogN-LogS, is well fitted by a power law with slope alpha = 1.34 +/- 0.15 and
normalization A = 11.87 +/- 1.04 sr**-1 (10**-11 ergs cm**-2 s**-1)**alpha, in
good agreement with other measurements.Comment: 27 pages, 8 figures and 3 tables included, LaTex, emulateapj.sty and
epsf.sty, accepted for publication in ApJ: scheduled for the March 20, 1999,
Vol.514. The cluster catalog is available at
http://www.merate.mi.astro.it/~degrand
The Chandra survey of the COSMOS field II: source detection and photometry
The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program,
that covers the central contiguous ~0.92 deg^2 of the COSMOS field. C-COSMOS is
the result of a complex tiling, with every position being observed in up to six
overlapping pointings (four overlapping pointings in most of the central ~0.45
deg^2 area with the best exposure, and two overlapping pointings in most of the
surrounding area, covering an additional ~0.47 deg^2). Therefore, the full
exploitation of the C-COSMOS data requires a dedicated and accurate analysis
focused on three main issues: 1) maximizing the sensitivity when the PSF
changes strongly among different observations of the same source (from ~1
arcsec up to ~10 arcsec half power radius); 2) resolving close pairs; and 3)
obtaining the best source localization and count rate. We present here our
treatment of four key analysis items: source detection, localization,
photometry, and survey sensitivity. Our final procedure consists of a two step
procedure: (1) a wavelet detection algorithm, to find source candidates, (2) a
maximum likelihood Point Spread Function fitting algorithm to evaluate the
source count rates and the probability that each source candidate is a
fluctuation of the background. We discuss the main characteristics of this
procedure, that was the result of detailed comparisons between different
detection algorithms and photometry tools, calibrated with extensive and
dedicated simulations.Comment: Accepted for publication in The Astrophysical Journal Supplement
Serie
The ROSAT All-Sky Survey: a Catalog of Clusters of Galaxies in a Region of 1 Ster around the South Galactic Pole
A field of 1.013 ster in the ROSAT all-sky survey (RASS), centered on the
south galactic pole (SGP), has been searched in a systematic, objective manner
for clusters of galaxies. The procedure relied on a correlation of the X-ray
positions and properties of ROSAT sources with the distribution of galaxies in
the COSMOS digitised data base, which was obtained by scanning the plates of
the UK Schmidt IIIa-J optical southern sky survey. The study used the second
ROSAT survey data base (RASS-2) and included several optical observing
campaigns to measure redshifts. The search, a precursor to the larger REFLEX
survey of the whole southern sky, reached the detection limits of the RASS and
the COSMOS data, and yielded a catalog of 186 clusters in which the lowest flux
is 1.5e-12 erg/cm2/s in the 0.1-2.4 keV band. Of these 157 have measured
redshifts. Using a limit of 3.0e-12 erg/cm2/s a complete subset of 112 clusters
was obtained, of which 110 have measured redshifts. The spatial distribution of
the X-ray clusters out to z = 0.15 shows an extension of the Local Supercluster
to the Pisces-Cetus supercluster (z<~0.07), and a more distant orthogonal
structure at 0.07<z<0.15.Comment: To be published in ApJ Supplements in February 2002: 53 pages: 18
figure
The DAFT/FADA survey. I.Photometric redshifts along lines of sight to clusters in the z=[0.4,0.9] interval
As a contribution to the understanding of the dark energy concept, the Dark
energy American French Team (DAFT, in French FADA) has started a large project
to characterize statistically high redshift galaxy clusters, infer cosmological
constraints from Weak Lensing Tomography, and understand biases relevant for
constraining dark energy and cluster physics in future cluster and cosmological
experiments. The purpose of this paper is to establish the basis of reference
for the photo-z determination used in all our subsequent papers, including weak
lensing tomography studies. This project is based on a sample of 91 high
redshift (z>0.4), massive clusters with existing HST imaging, for which we are
presently performing complementary multi-wavelength imaging. This allows us in
particular to estimate spectral types and determine accurate photometric
redshifts for galaxies along the lines of sight to the first ten clusters for
which all the required data are available down to a limit of I_AB=24/24.5 with
the LePhare software. The accuracy in redshift is of the order of 0.05 for the
range 0.2<z<1.5. We verified that the technique applied to obtain photometric
redshifts works well by comparing our results to with previous works. In
clusters, photoz accuracy is degraded for bright absolute magnitudes and for
the latest and earliest type galaxies. The photoz accuracy also only slightly
varies as a function of the spectral type for field galaxies. As a consequence,
we find evidence for an environmental dependence of the photoz accuracy,
interpreted as the standard used Spectral Energy Distributions being not very
well suited to cluster galaxies. Finally, we modeled the LCDCS 0504 mass with
the strong arcs detected along this line of sight.Comment: Accepted in A&
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