1,877 research outputs found
On the use of C-stat in testing models for X-ray spectra
It has been shown that for the analysis of X-ray spectra the C-statistic,
contrary to the chi^2-statistic, provides unbiased estimates of the model
parameters and their uncertainty ranges. However, it is often stated that the
C-statistic cannot be used to carry out statistical tests on the goodness of
fit of the model, and therefore several investigations are still based on
chi^2-statistics. Here we show that it is straightforward to calculate the
expected value and variance of the C-statistic so that it can be used in tests.
We provide formulae and simple numerical approximations to evaluate these
expected values and variances. We also give examples indicating that tests
based on only the expected value and variance of the C-statistic are reliable
for spectra even with only ~30 counts. Therefore the C-statistic can be used
for statistical tests such as assessing the goodness of fit of a spectral
model.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
The temperature structure in the core of Sersic 159-03
We present results from a new 120 ks XMM-Newton observation of the cluster of
galaxies Sersic 159-03. In this paper we focus on the high-resolution X-ray
spectra obtained with the Reflection Grating Spectrometer (RGS). The spectra
allow us to constrain the temperature structure in the core of the cluster and
determine the emission measure distribution as a function of temperature. We
also fit the line widths of mainly oxygen and iron lines.Comment: 7 pages and 4 figures. Contribution to the proceedings of the COSPAR
Scientific Assembly, session E1.2 "Clusters of Galaxies: New Insights from
XMM-Newton, Chandra and INTEGRAL", july 2004, Paris (France). Accepted for
publication in Advances in Space Researc
Optimal binning of X-ray spectra and response matrix design
A theoretical framework is developed to estimate the optimal binning of X-ray
spectra. We derived expressions for the optimal bin size for model spectra as
well as for observed data using different levels of sophistication. It is shown
that by taking into account both the number of photons in a given spectral
model bin and their average energy over the bin size, the number of model
energy bins and the size of the response matrix can be reduced by a factor of
. The response matrix should then contain the response at the bin
centre as well as its derivative with respect to the incoming photon energy. We
provide practical guidelines for how to construct optimal energy grids as well
as how to structure the response matrix. A few examples are presented to
illustrate the present methods.Comment: 16 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
The warm absorber and X-ray variability of the Seyfert 1 galaxy NGC 3516 as seen by the XMM-Newton RGS
We present a new analysis of the soft and medium energy X-ray spectrum of the
Seyfert 1 galaxy NGC 3516 taken with the Reflection Grating Spectrometer (RGS)
and European Photon Imaging Camera (EPIC) on board the XMM-Newton observatory.
We examine four observations made in October 2006. We investigate whether the
observed variability is due to absorption by the warm absorber and/or is
intrinsic to the source emission. We analyse in detail the EPIC-pn and RGS
spectra of each observation separately. The warm absorber in NGC 3516 is found
to consist of three phases of ionisation, two of which have outflow velocities
of more than 1000 km/s. The third phase (the least ionised one) is much slower
at 100 km/s. One of the high ionisation phases, with log xi of 2.4, is found to
have a partial covering fraction of about 60%. It has previously been suggested
that the passage of a cloud, part of a disk wind, in front of the source
(producing a change in the covering fraction) was the cause of a significant
dip in the lightcurve during one of the observations. From our modelling of the
EPIC-pn and RGS spectra, we find that variation in the covering fraction cannot
be solely responsible for this. We show that intrinsic change in the source
continuum plays a much more significant role in explaining the observed flux
and spectral variability than originally thought.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Simultaneous X-ray and Ultraviolet spectroscopy of the Seyfert galaxy NGC 5548. III. X-ray time variability
The Seyfert 1 galaxy NGC 5548 was observed for a week by Chandra using both
the HETGS and LETGS spectrometers. In this paper we study the time variability
of the continuum radiation. During our observation, the source showed a gradual
increase in flux over four days, followed by a rapid decrease and flattening of
the light curve afterwards. Superimposed upon these relatively slow variations
several short duration bursts or quasi-periodic oscillations occured with a
typical duration of several hours and separation between 0.6-0.9 days. The
bursts show a delay of the hard X-rays with respect to the soft X-rays of a few
hours. We interprete these bursts as due to a rotating, fluctuating hot spot at
approximately 10 gravitational radii; the time delay of the hard X-rays from
the bursts agree with the canonical picture of Inverse Compton scattering of
the soft accretion disk photons on a hot medium that is relatively close to the
central black hole.Comment: 6 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Contrasting the UV and X-ray O VI Column Density Inferred for the Outflow in NGC 5548
We compare X-ray and UV spectroscopic observations of NGC 5548. Both data
sets show O VI absorption troughs associated with the AGN outflow from this
galaxy. We find that the robust lower limit on the column density of the O VI
X-ray trough is seven times larger than the column density found in a study of
the O VI UV troughs. This discrepancy suggests that column densities inferred
for UV troughs of Seyfert outflows are often severely underestimated. We
identify the physical limitations of the UV Gaussian modeling as the probable
explanation of the O VI column density discrepancy. Specifically, Gaussian
modeling cannot account for a velocity dependent covering fraction, and it is a
poor representation for absorption associated with a dynamical outflow.
Analysis techniques that use a single covering fraction value for each
absorption component suffer from similar limitations. We conclude by suggesting
ways to improve the UV analysis.Comment: 16 pages, 1 figure, accepted for publication in Ap
X-ray emission from thin plasmas. Collisional ionization for atoms and ions of H to Zn
Every observation of astrophysical objects involving a spectrum requires
atomic data for the interpretation of line fluxes, line ratios and ionization
state of the emitting plasma. One of the processes which determines it is
collisional ionization. In this study an update of the direct ionization (DI)
and excitation-autoionization (EA) processes is discussed for the H to Zn-like
isoelectronic sequences. In the last years new laboratory measurements and
theoretical calculations of ionization cross sections have become available. We
provide an extension and update of previous published reviews in the
literature. We include the most recent experimental measurements and fit the
cross sections of all individual shells of all ions from H to Zn. These data
are described using an extension of Younger's and Mewe's formula, suitable for
integration over a Maxwellian velocity distribution to derive the subshell
ionization rate coefficients. These ionization rate coefficients are
incorporated in the high-resolution plasma code and spectral fitting tool SPEX
V3.0.Comment: Accepted for publication in A&A, 22 pages, 18 figure
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