154 research outputs found
On the Unusually High Temperature of the Cluster of Galaxies 1E 0657-56
A recent X-ray observation of the cluster 1E0657-56 (z=0.296) with ASCA
implied an unusually high temperature of ~17 keV. Such a high temperature would
make it the hottest known cluster and severely constrain cosmological since, in
a Universe with critical density Omega=1 the probability of observing such a
cluster is only 4e-5. Here we test the robustness of this observational result
since it has such important implications. We analysed the data using a variety
of different data analysis methods and spectral analysis assumptions and find a
temperature of ~11-12 keV in all cases, except for one class of spectral fits.
These are fits in which the absorbing column density is fixed at the Galactic
value. We show that a high temperature of ~17 keV is artificially obtained if
the true spectrum has a stronger low- energy cut-off than that for Galactic
absorption only. The extra absorption may be astrophysical in origin, or it may
be a problem with the low-energy CCD efficiency. Although significantly lower
than previous measurements, this temperature of kT ~11-12 keV is still
relatively high since only a few clusters have been found to have temperatures
higher than 10 keV and the data therefore still present some difficulty for an
Omega=1 Universe. Our results will also be useful to anyone who wants to
estimate the systematic errors involved in different methods of background
subtraction of ASCA data for sources with similar S/N to that of the 1E0657-56
data reported here.Comment: 14 pages plus 2 figures. Latex with separate postscript figure files.
AASTEX 4.0 macro. Accepted for the Astrophysical Journal Letter
NGC 4388: A Test Case for Relativistic Disk Reflection and Fe K Fluorescence Features
We present a new analysis of the Suzaku X-ray spectrum of the Compton-thin
Seyfert 2 galaxy NGC 4388. The spectrum above 2 keV can be described by a
remarkably simple and rather mundane model, consisting of a uniform, neutral
spherical distribution of matter, with a radial column density of cm, and an Fe abundance of
relative to solar. The model does not require any
phenomenological adjustments to self-consistently account for the low-energy
extinction, the Fe K and Fe K fluorescent emission lines, the Fe
K edge, and the Compton-scattered continuum from the obscuring material. The
spherical geometry is not a unique description, however, and the
self-consistent, solar abundance MYTORUS model, applied with toroidal and
non-toroidal geometries, gives equally good descriptions of the data. In all
cases, the key features of the spectrum are so tightly locked together that for
a wide range of parameters, a relativistic disk-reflection component
contributes no more than 2% to the net spectrum in the 2-20 keV band. We
show that the commonly invoked explanations for weak X-ray reflection features,
namely a truncated and/or very highly ionized disk, do not work for NGC 4388.
If relativistically-broadened Fe K lines and reflection are ubiquitous
in Seyfert 1 galaxies, they should also be ubiquitous in Compton-thin Seyfert 2
galaxies. The case of NGC 4388 shows the need for similar studies of more
Compton-thin AGN to ascertain whether this is true.Comment: MNRAS accepted. 21 pages, 7 figures, 4 tables; Appendix with
historical notes and 1 table. This version: Corrected minor typo and
affiliatio
Monte Carlo simulations of the Nickel K fluorescent emission line in a toroidal geometry
We present new results from Monte Carlo calculations of the flux and
equivalent width (EW) of the Ni Kalpha fluorescent emission line in the
toroidal X-ray reprocessor model of Murphy & Yaqoob (2009, MNRAS, 397, 1549).
In the Compton-thin regime, the EW of the Ni Kalpha line is a factor of ~22
less than that of the Fe Kalpha line but this factor can be as low as ~6 in the
Compton-thick regime. We show that the optically-thin limit for this ratio
depends only on the Fe to Ni abundance ratio, it being independent of the
geometry and covering factor of the reprocessor, and also independent of the
shape of the incident X-ray continuum. We give some useful analytic expressions
for the absolute flux and the EW of the Ni Kalpha line in the optically-thin
limit. When the reprocessor is Compton-thick and the incident continuum is a
power-law with a photon index of 1.9, the Ni Kalpha line EW has a maximum value
of ~3 eV and ~250 eV for non-intercepting and intercepting lines-of-sight
respectively. Larger EWs are obtained for flatter continua. We have also
studied the Compton shoulder of the Ni Ka line and find that the ratio of
scattered to unscattered flux in the line has a maximum value of 0.26, less
than the corresponding maximum for the Fe Kalpha line. However, we find that
the shape of the Compton shoulder profile for a given column density and
inclination angle of the torus is similar to the corresponding profile for the
Fe Ka line. Our results will be useful for interpreting X-ray spectra of active
galactic nuclei (AGNs) and X-ray binary systems in which the system parameters
are favorable for the Ni Kalpha line to be detected.Comment: Accepted for publication in MNRAS. 8 pages, 5 figure
On the Prospect of Constraining Black-Hole Spin Through X-ray Spectroscopy of Hotspots
Future X-ray instrumentation is expected to allow us to significantly improve
the constraints derivedfrom the Fe K lines in AGN, such as the black-hole
angular momentum (spin) and the inclination angle of the putative accretion
disk. We consider the possibility that measurements of the persistent,
time-averaged Fe K line emission from the disk could be supplemented by the
observation of a localized flare, or "hotspot", orbiting close to the black
hole. Although observationally challenging, such measurements would recover
some of the information loss that is inherent to the radially-integrated line
profiles. We present calculations for this scenario to assess the extent to
which, in principle, black-hole spin may be measured. We quantify the
feasibility of this approach using realistic assumptions about likely
measurement uncertainties.Comment: 7 pages, 7 figures. Accepted for publication in Ap
Monitoring the Violent Activity from the Inner Accretion Disk of the Seyfert 1.9 Galaxy NGC 2992 with RXTE
We present the results of a one year monitoring campaign of the Seyfert 1.9 galaxy NGC 2992 with RXTE. Historically, the source has been shown to vary dramatically in 2-10 keV flux over timescales of years and was thought to be slowly transitioning between periods of quiescence and active accretion. Our results show that in one year the source continuum flux covered almost the entire historical range, making it unlikely that the low-luminosity states correspond to the accretion mechanism switching off. During flaring episodes we found that a highly redshifted Fe K line appears, implying that the violent activity is occurring in the inner accretion disk, within 100 gravitational radii of the central black hole. We also found that the Compton y parameter for the X-ray continuum remained approximately constant during the large amplitude variability. These observations make NGC 2992 well-suited for future multi-waveband monitoring, as a test-bed for constraining accretion models
Parameter Estimation In X-ray Astronomy Revisited
The method of obtaining confidence intervals on a subset of the total number
of parameters (p) of a model used for fitting X-ray spectra is to perturb the
best-fitting model until, for each parameter, a range is found for which the
change in the fit statistic is equal to some critical value. This critical
value corresponds to the desired confidence level and is obtained from the
chi-square distribution for q degrees of freedom, where q is the number of
interesting parameters. With the advent of better energy-resolution detectors,
such as those onboard ASCA it has become more common to fit complex models with
narrow features, comparable to the instrumental energy resolution. To
investigate whether this leads to significant non-Gaussian deviations between
data and model, we use simulations based on ASCA data and we show that the
method is still valid in such cases. We also investigate the weak-source limit
as well as the case of obtaining upper limits on equivalents widths of weak
emission lines and find that for all practical purposes the method gives the
correct confidence ranges. However, upper limits on emission-line equivalent
widths may be over-estimated in the extreme Poisson limit.Comment: 21 pages, 6 Figures. Latex with separate postscript figure files.
Appears in ApJ. NOTE:replaces erroneous version sent out due to clerical
erro
The Cores of the Fe K Lines in Seyfert I Galaxies Observed by the Chandra High Energy Grating
We report on the results of 18 observations of the core, or peak, of the Fe K
emission line at keV in 15 Seyfert I galaxies using the {\it
Chandra} High Energy Grating (HEG). These data afford the highest precision
measurements of the peak energy of the Fe K line, and the highest spectral
resolution measurements of the width of the core of the line to date. We were
able to measure the peak energy in 17 data sets, and, excluding a very deep
observation of NGC 3783, we obtained a weighted mean of keV.
In all 15 sources the two-parameter, 99% confidence errors on the line peak
energy do not exclude fluorescent line emission from Fe {\sc i},
although two sources (Mkn 509 and 3C 120) stand out as very likely being
dominated by emission from Fe {\sc xvii} or so. We were able to
measure the line core width in 14 data sets and obtained a weighted mean of
2380 +/- 760 km/s FWHM (excluding the NGC 3783 deep exposure), a little larger
than the instrument resolution. However, there is evidence of underlying broad
line emission in at least 4 sources. In fact, the width of the peak varies
widely from source to source and it may in general have a contribution from the
outer parts of an accretion disk {\it and} more distant matter. For the disk
contribution to also peak at 6.4 keV requires greater line emissivity at
hundreds of gravitational radii than has been deduced from previous studies of
the Fe K line.Comment: Accepted for publication in the Astrophysical Journal. 15 pages, four
figures, two of them color. Abstract is slightly abridge
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