37,657 research outputs found
Chandra Publication Statistics
In this study we develop and propose publication metrics, based on an
analysis of data from the Chandra bibliographic database, that are more
meaningful and less sensitive to observatory-specific characteristics than the
traditional metrics. They fall in three main categories: speed of publication;
fraction of observing time published; and archival usage. Citation of results
is a fourth category, but lends itself less well to definite statements. For
Chandra, the median time from observation to publication is 2.36 years; after
about 7 years 90% of the observing time is published; after 10 years 70% of the
observing time is published more than twice; and the total annual publication
output of the mission is 60-70% of the cumulative observing time available,
assuming a two year lag between data retrieval and publication.Comment: 22 pages, 8 figures, 3 tables; revised manuscript submitted to PAS
The Luminosity Function of X-ray Selected Active Galactic Nuclei: Evolution of Supermassive Black Holes at High Redshift
We present a measure of the hard (2-8 keV) X-ray luminosity function (XLF) of
Active Galactic Nuclei up to z~5. At high redshifts, the wide area coverage of
the Chandra Multiwavength Project is crucial to detect rare and luminous (Lx >
10^44 erg s^-1) AGN. The inclusion of samples from deeper published surveys,
such as the Chandra Deep Fields, allows us to span the lower Lx range of the
XLF. Our sample is selected from both the hard (z 6.3x10^-16
erg cm^-2 s^-1) and soft (z > 3; f(0.5-2.0 keV) > 1.0x10^-16 erg cm^-2 s^-1)
energy band detections. Within our optical magnitude limits (r',i' < 24), we
achieve an adequate level of completeness (>50%) regarding X-ray source
identification (i.e., redshift). We find that the luminosity function is
similar to that found in previous X-ray surveys up to z~3 with an evolution
dependent upon both luminosity and redshift. At z > 3, there is a significant
decline in the numbers of AGN with an evolution rate similar to that found by
studies of optically-selected QSOs. Based on our XLF, we assess the resolved
fraction of the Cosmic X-ray Background, the cumulative mass density of
Supermassive Black Holes (SMBHs), and the comparison of the mean accretion rate
onto SMBHs and the star formation history of galaxies as a function of
redshift. A coevolution scenario up to z~2 is plausible though at higher
redshifts the accretion rate onto SMBHs drops more rapidly. Finally, we
highlight the need for better statistics of high redshift AGN at z > 3, which
is achievable with the upcoming Chandra surveys.Comment: Accepted for publication in ApJ; 25 pages, 18 figure
The Cluster Wind from Local Massive Star Clusters
Results of a study of the theoretically predicted and observed X-ray
properties of local massive star clusters are presented, with a focus on
understanding the mass and energy flow from these clusters into the ISM via a
cluster wind. A simple theoretical model, based on the work of Chevalier &
Clegg (1985), is used to predict the theoretical cluster properties, and these
are compared to those obtained from recent Chandra observations. The model
includes the effect of lower energy transfer efficiency and mass-loading. In
spite of limited statistics, some general trends are indicated; the observed
temperature of the diffuse X-ray emission is lower than that predicted from the
stellar mass and energy input rates, but the predicted scaling of X-ray
luminosity with cluster parameters is seen. The implications of these results
are discussed.Comment: 9 pages, 6 figues, accepted for publication in MNRA
Symmetry of matrix-valued stochastic processes and noncolliding diffusion particle systems
As an extension of the theory of Dyson's Brownian motion models for the
standard Gaussian random-matrix ensembles, we report a systematic study of
hermitian matrix-valued processes and their eigenvalue processes associated
with the chiral and nonstandard random-matrix ensembles. In addition to the
noncolliding Brownian motions, we introduce a one-parameter family of
temporally homogeneous noncolliding systems of the Bessel processes and a
two-parameter family of temporally inhomogeneous noncolliding systems of Yor's
generalized meanders and show that all of the ten classes of eigenvalue
statistics in the Altland-Zirnbauer classification are realized as particle
distributions in the special cases of these diffusion particle systems. As a
corollary of each equivalence in distribution of a temporally inhomogeneous
eigenvalue process and a noncolliding diffusion process, a stochastic-calculus
proof of a version of the Harish-Chandra (Itzykson-Zuber) formula of integral
over unitary group is established.Comment: LaTeX, 27 pages, 4 figures, v3: Minor corrections made for
publication in J. Math. Phy
An X-ray view of the active nucleus in NGC 4258
XMM-Newton observed the Seyfert 1.9 galaxy NGC 4258 in December 2000. At
energies above 2 keV a hard nuclear point source is resolved that can be fitted
by a highly absorbed power-law spectrum (NH = (8.0+-0.4)x10^22 cm^-2, photon
index 1.64+-0.08) with an unabsorbed luminosity of 7.5x10^40erg/s in the 2-10
keV band. No narrow iron Kalpha emission line is detected (90% upper limit of
equivalent width EW ~40 eV). The nuclear emission flux was observed to remain
constant over the observation. A short archival Chandra observation taken in
March 2000 further constrains the hard emission to a point source coincident
with the radio nucleus. A point source ~3" southwest of the nucleus does not
contribute significantly. Spectral results of the Chandra nuclear source are
comparable (within the limited statistics) to the XMM-Newton parameters. The
comparison of our iron line upper limit with reported detections indicates
variability of the line EW. These results can be explained by the relatively
low nuclear absorption of NGC 4258 (which is in the range expected for its
intermediate Seyfert type) and some variability of the absorbing material.
Reflection components as proposed to explain the large iron line EW of highly
absorbed Seyfert 2 galaxies and/or variations in the accretion disk are however
imposed by the time variability of the iron line flux.Comment: 6 pages, 5 figures, accepted for publication in A&
Deep XMM-Newton observations of the northern disc of M31. I. Source catalogue
We carried out new observations of two fields in the northern ring of M31
with XMM-Newton with two exposures of 100 ks each and obtained a complete list
of X-ray sources down to a sensitivity limit of ~7 x 10^34 erg s^-1 (0.5 - 2.0
keV). The major objective of the observing programme was the study of the hot
phase of the ISM in M31. The analysis of the diffuse emission and the study of
the ISM is presented in a separate paper. We analysed the spectral properties
of all detected sources using hardness ratios and spectra if the statistics
were high enough. We also checked for variability. We cross-correlated the
source list with the source catalogue of a new survey of the northern disc of
M31 carried out with Chandra and Hubble (Panchromatic Hubble Andromeda
Treasury, PHAT) as well as with other existing catalogues. We detected a total
of 389 sources, including 43 foreground stars and candidates and 50 background
sources. Based on the comparison to the Chandra/PHAT survey, we classify 24
hard X-ray sources as new candidates for X-ray binaries (XRBs). In total, we
identified 34 XRBs and candidates and 18 supernova remnants (SNRs) and
candidates. Three of the four brightest SNRs show emission mainly below 2 keV,
consistent with shocked ISM. The spectra of two of them also require an
additional component with a higher temperature. The SNR [SPH11] 1535 has a
harder spectrum and might suggest that there is a pulsar-wind nebula inside the
SNR. We find five new sources showing clear time variability. We also studied
the spectral properties of the transient source SWIFT J004420.1+413702, which
shows significant variation in flux over a period of seven months (June 2015 to
January 2016) and associated change in absorption. Based on the likely optical
counterpart detected in the Chandra/PHAT survey, the source is classified as a
low-mass X-ray binary.Comment: Accepted for publication in A&
New spectral classification technique for X-ray sources: quantile analysis
We present a new technique called "quantile analysis" to classify spectral
properties of X-ray sources with limited statistics. The quantile analysis is
superior to the conventional approaches such as X-ray hardness ratio or X-ray
color analysis to study relatively faint sources or to investigate a certain
phase or state of a source in detail, where poor statistics does not allow
spectral fitting using a model. Instead of working with predetermined energy
bands, we determine the energy values that divide the detected photons into
predetermined fractions of the total counts such as median (50%), tercile (33%
& 67%), and quartile (25% & 75%). We use these quantiles as an indicator of the
X-ray hardness or color of the source. We show that the median is an improved
substitute for the conventional X-ray hardness ratio. The median and other
quantiles form a phase space, similar to the conventional X-ray color-color
diagrams. The quantile-based phase space is more evenly sensitive over various
spectral shapes than the conventional color-color diagrams, and it is naturally
arranged to properly represent the statistical similarity of various spectral
shapes. We demonstrate the new technique in the 0.3-8 keV energy range using
Chandra ACIS-S detector response function and a typical aperture photometry
involving background subtraction. The technique can be applied in any energy
band, provided the energy distribution of photons can be obtained.Comment: 11 pages, 9 figures, accepted for publication in Ap
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