585 research outputs found
The X-ray absorption spectrum of 4U1700-37 and its implications for the stellar wind of the companion HD153919
The first high resolution non-dispersive 2-60 KeV X-ray spectra of 4U1700-37 is presented. The continuum is typical of that found from X-ray pulsars; that is a flat power law between 2 and 10 keV and, beyond 10 keV, an exponential decay of characteristic energy varying between 10 and 20 keV. No X-ray pulsations were detected between 160 ms and 6 min with an amplitude greater than approximately 2%. The absorption measured at binary phases approximately 0.72 is comparable to that expected from the stellar wind of the primary. The gravitational capture of material in the wind is found to be more than enough to power the X-ray source. The increase in the average absorption after phi o approximately 0.5 is confirmed. The minimum level of adsorption is a factor of 2 or 3 lower than that reported by previous observers, which may be related to a factor of approximately 10 decline in the average X-ray luminosity over the same interval. Short term approximately 50% variations in adsorption are seen for the first time which appear to be loosely correlated with approximately 10 min flickering activity in the X-ray flux. These most likely originate from inhomogeneities in the stellar wind of the primary
Modeling of Photoionized Plasmas
In this paper I review the motivation and current status of modeling of
plasmas exposed to strong radiation fields, as it applies to the study of
cosmic X-ray sources. This includes some of the astrophysical issues which can
be addressed, the ingredients for the models, the current computational tools,
the limitations imposed by currently available atomic data, and the validity of
some of the standard assumptions. I will also discuss ideas for the future:
challenges associated with future missions, opportunities presented by improved
computers, and goals for atomic data collection.Comment: 17 pages, 8 figures, to appear in the proceedings of Xray2010,
Utrecht, the Netherlands, March 15-17 201
Radiative and Auger decay data for modelling nickel K lines
Radiative and Auger decay data have been calculated for modelling the K lines
in ions of the nickel isonuclear sequence, from Ni up to Ni. Level
energies, transition wavelengths, radiative transition probabilities, and
radiative and Auger widths have been determined using Cowan's Hartree--Fock
with Relativistic corrections (HFR) method. Auger widths for the third-row ions
(Ni--Ni) have been computed using single-configuration average
(SCA) compact formulae. Results are compared with data sets computed with the
AUTOSTRUCTURE and MCDF atomic structure codes and with available experimental
and theoretical values, mainly in highly ionized ions and in the solid state.Comment: submitted to ApJS. 42 pages. 12 figure
Photoionization Modeling and the K Lines of Iron
We calculate the efficiency of iron K line emission and iron K absorption in
photoionized models using a new set of atomic data. These data are more
comprehensive than those previously applied to the modeling of iron K lines
from photoionized gases, and allow us to systematically examine the behavior of
the properties of line emission and absorption as a function of the ionization
parameter, density and column density of model constant density clouds. We show
that, for example, the net fluorescence yield for the highly charged ions is
sensitive to the level population distribution produced by photoionization, and
these yields are generally smaller than those predicted assuming the population
is according to statistical weight. We demonstrate that the effects of the many
strongly damped resonances below the K ionization thresholds conspire to smear
the edge, thereby potentially affecting the astrophysical interpretation of
absorption features in the 7-9 keV energy band. We show that the centroid of
the ensemble of K lines, the K energy, and the ratio of the
K to K components are all diagnostics of the ionization
parameter of our model slabsComment: 38 pages, submitted to Ap.J. Sup
Testing Hydrodynamic Models of LMC X-4 with UV and X-ray Spectra
We compare the predictions of hydrodynamic models of the LMC X-4 X-ray binary
system with observations of UV P Cygni lines with the GHRS and STIS
spectrographs on the Hubble Space Telescope. The hydrodynamic model determines
density and velocity fields of the stellar wind, wind-compressed disk,
accretion stream, Keplerian accretion disk, and accretion disk wind. We use a
Monte Carlo code to determine the UV P Cygni line profiles by simulating the
radiative transfer of UV photons that originate on the star and are scattered
in the wind. The qualitative orbital variation predicted is similar to that
observed, although the model fails to reproduce the strong orbital asymmetry
(the observed absorption is strongest for phi>0.5). The model predicts a
mid-eclipse X-ray spectrum, due almost entirely to Compton scattering, with a
factor 4 less flux than observed with ASCA. We discuss how the model may need
to be altered to explain the spectral variability of the system.Comment: 11 figures, accepted by Ap
Fe XXV and Fe XXVI Diagnostics of the Black Hole and Accretion Disk in Active Galaxies: Chandra Time-Resolved Spectroscopy of NGC 7314
We report the detection of Fe XXV and Fe XXVI emission lines from a
Chandra HETGS observation of the Seyfert~1 galaxy NGC 7314, made simultaneously
with RXTE. The lines are redshifted (cz ~ 1500 km/s) relative to the systemic
velocity and unresolved. We argue that the lines originate in a near face-on
(<7 degrees) disk having a radial line emissivity flatter than r^-2. Line
emission from ionization states of Fe in the range ~Fe I up to Fe XXVI is
observed. The ionization balance of Fe responds to continuum variations on
timescales less than 12.5 ks, supporting an origin of the lines close to the
X-ray source. We present additional, detailed diagnostics from this rich data
set. These results identify NGC 7314 as a key source to study in the future if
we are to pursue reverberation mapping of space-time near black-hole event
horizons. This is because it is first necessary to understand the ionization
structure of accretion disks and the relation between the X-ray continuum and
Fe K line emission. However, we also describe how our results are suggestive of
a means of measuring black-hole spin without a knowledge of the relation
between the continuum and line emission. Finally, these data emphasize that one
{\it can} study strong gravity with narrow (as opposed to very broad) disk
lines. In fact narrow lines offer higher precision, given sufficient energy
resolution.Comment: Accepted for publication in the Astrophysical Journal. 30 pages, six
figures, five of them color. Abstract is abridge
GEMS X-ray Polarimeter Performance Simulations
The Gravity and Extreme Magnetism Small explorer (GEMS) is an X-ray polarization telescope selected as a NASA small explorer satellite mission. The X-ray Polarimeter on GEMS uses a Time Projection Chamber gas proportional counter to measure the polarization of astrophysical X-rays in the 2-10 keV band by sensing the direction of the track of the primary photoelectron excited by the incident X-ray. We have simulated the expected sensitivity of the polarimeter to polarized X-rays. We use the simulation package Penelope to model the physics of the interaction of the initial photoelectron with the detector gas and to determine the distribution of charge deposited in the detector volume. We then model the charge diffusion in the detector,and produce simulated track images. Within the track reconstruction algorithm we apply cuts on the track shape and focus on the initial photoelectron direction in order to maximize the overall sensitivity of the instrument, using this technique we have predicted instrument modulation factors nu(sub 100) for 100% polarized X-rays ranging from 10% to over 60% across the 2-10 keV X-ray band. We also discuss the simulation program used to develop and model some of the algorithms used for triggering, and energy measurement of events in the polarimeter
Ubiquitous equatorial accretion disc winds in black hole soft states
High resolution spectra of Galactic Black Holes (GBH) reveal the presence of
highly ionised absorbers. In one GBH, accreting close to the Eddington limit
for more than a decade, a powerful accretion disc wind is observed to be
present in softer X-ray states and it has been suggested that it can carry away
enough mass and energy to quench the radio jet. Here we report that these
winds, which may have mass outflow rates of the order of the inner accretion
rate or higher, are an ubiquitous component of the jet-free soft states of all
GBH. We furthermore demonstrate that these winds have an equatorial geometry
with opening angles of few tens of degrees, and so are only observed in sources
in which the disc is inclined at a large angle to the line of sight. The
decrease in Fe XXV / Fe XXVI line ratio with Compton temperature, observed in
the soft state, suggests a link between higher wind ionisation and harder
spectral shapes. Although the physical interaction between the wind, accretion
flow and jet is still not fully understood, the mass flux and power of these
winds, and their presence ubiquitously during the soft X-ray states suggests
they are fundamental components of the accretion phenomenon.Comment: Accepted for publication in MNRAS Letter
Astrophysical Fluids of Novae: High Resolution Pre-decay X-ray spectrum of V4743 Sagittarii
Eight X-ray observations of V4743 Sgr (2002), observed with Chandra and
XMM-Newton are presented. The nova turned off some time between days 301.9 and
371, and the X-ray flux subsequently decreased from day 301.9 to 526 following
an exponential decline time scale of days. We use the absorption
lines present in the SSS spectrum for diagnostic purposes, and characterize the
physics and the dynamics of the expanding atmosphere during the explosion of
the nova. The information extracted from this first stage is then used as input
for computing full photoionization models of the ejecta in V4743 Sgr. The SSS
spectrum is modeled with a simple black-body and multiplicative Gaussian lines,
which provides us of a general kinematical picture of the system, before it
decays to its faint phase (Ness et al. 2003). In the grating spectra taken
between days 180.4 and 370, we can resolve the line profiles of absorption
lines arising from H-like and He-like C, N, and O, including transitions
involving higher principal quantum numbers. Except for a few interstellar
lines, all lines are significantly blue-shifted, yielding velocities between
1000 and 6000 km/s which implies an ongoing mass loss. It is shown that
significant expansion and mass loss occur during this phase of the explosion,
at a rate . Our measurements show that the efficiency of the amount of
energy used for the motion of the ejecta, defined as the ratio between the
kinetic luminosity and the radiated luminosity , is
of the order of one.Comment: 25 pages, 9 figures. Accepted in book: Recent Advances in Fluid
Dynamics with Environmental Applications, pp.365-39
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