15 research outputs found
Ion-by-Ion DEM Determination: I. Method
We describe a technique to derive constraints on the differential emission
measure (DEM) distribution, a measure of the temperature distribution, of
collisionally ionized hot plasmas from their X-ray emission line spectra. This
technique involves fitting spectra using a number of components, each of which
is the entire X-ray line emission spectrum for a single ion. It is applicable
to high-resolution X-ray spectra of any collisionally ionized plasma and
particularly useful for spectra in which the emission lines are broadened and
blended such as those of the winds of hot stars. This method does not require
that any explicit assumptions about the form of the DEM distribution be made
and is easily automated.Comment: This paper was split in two. This version is part I. Part II may be
found at astro-ph/050343
Constraints on the Velocity and Spatial Distribution of Helium-like Ions in the Wind of SMC X-1 from Observations with XMM-Newton/RGS
We present here X-ray spectra of the HMXB SMC X-1 obtained in an observation
with the XMM observatory beginning before eclipse and ending near the end of
eclipse. With the Reflection Grating Spectrometers (RGS) on board XMM, we
observe emission lines from hydrogen-like and helium-like ions of nitrogen,
oxygen, neon, magnesium, and silicon. Though the resolution of the RGS is
sufficient to resolve the helium-like n=2->1 emission into three line
components, only one of these components, the intercombination line, is
detected in our data. The lack of flux in the forbidden lines of the
helium-like triplets is explained by pumping by ultraviolet photons from the B0
star and, from this, we set an upper limit on the distance of the emitting ions
from the star. The lack of observable flux in the resonance lines of the
helium-like triplets indicate a lack of enhancement due to resonance line
scattering and, from this, we derive a new observational constraint on the
distribution of the wind in SMC X-1 in velocity and coordinate space. We find
that the solid angle subtended by the volume containing the helium-like ions at
the neutron star multiplied by the velocity dispersion of the helium-like ions
must be less than 4pi steradians km/s. This constraint will be satisfied if the
helium-like ions are located primarily in clumps distributed throughout the
wind or in a thin layer along the surface of the B0 star.Comment: 22 pages, 17 figures, ApJ accepted, discussion of relevant other work
adde
Differential Emission Measure Determination of Collisionally Ionized Plasma: II. Application to Hot Stars
In a previous paper we have described a technique to derive constraints on
the differential emission measure (DEM) distribution, a measure of the
temperature distribution, of collisionally ionized hot plasmas from their X-ray
emission line spectra. We apply this technique to the Chandra/HETG spectra of
all of the nine hot stars available to us at the time this project was
initiated. We find that DEM distributions of six of the seven O stars in our
sample are very similar but that theta Ori has an X-ray spectrum characterized
by higher temperatures. The DEM distributions of both of B stars in our sample
have lower magnitudes than those of the O stars and one, tau Sco, is
characterized by higher temperatures than the other, beta Cru. These results
confirm previous work in which high temperatures have been found for theta Ori
and tau Sco and taken as evidence for channeling of the wind in magnetic
fields, the existence of which are related to the stars' youth. Our results
demonstrate the utility of our method for deriving temperature information for
large samples of X-ray emission line spectra.Comment: The contents of this paper were formerly part of astro-ph/0403603
which was split into two paper
An X-Ray Spectroscopic Study of the SMC X-1/Sk 160 System
We have investigated the composition and distribution of the wind of Sk 160,
the supergiant companion of the X-ray star SMC X-1, by comparing an X-ray
spectrum of the source, obtained with the ASCA observatory, during an eclipse
with the computed spectra of reprocessed radiation from circumstellar matter
with various density distributions. We show that the metal abundance in the
wind of Sk 160 is no greater than a few tenths of solar, as has been determined
for other objects in the Magellanic Clouds. We also show that the observed
X-ray spectrum is not consistent with the density distributions of
circumstellar matter of the spherically symmetric form derived for line-driven
winds, nor with the density distribution derived from a hydrodynamic simulation
of the X-ray perturbed and line-driven wind by Blondin & Woo (1995).Comment: 35 pages including 16 figures, uses AASTeX v5.0.2, accepted to Ap
Resolving the Effects of Resonant X-ray Line Scattering in Cen X-3 with Chandra
The massive X-ray binary Cen X-3 was observed over approximately one quarter
of the system's 2.08 day orbit, beginning before eclipse and ending slightly
after eclipse center with the Chandra X-ray Observatory using its High-Energy
Transmission Grating Spectrometer. The spectra show K shell emission lines from
hydrogen- and helium-like ions of magnesium, silicon, sulfur, and iron as well
as a K-alpha fluorescence emission feature from near-neutral iron. The
helium-like n=2->1 triplet of silicon is fully resolved and the analogous
triplet of iron is partially resolved. The helium-like triplet component flux
ratios outside of eclipse are consistent with emission from recombination and
subsequent cascades (recombination radiation) from a photoionized plasma. In
eclipse, however, the w (resonance) lines of silicon and iron are stronger than
that expected for recombination radiation, and are consistent with emission
from a collisionally ionized plasma. The triplet line flux ratios at both
phases can be explained more naturally, however, as emission from a
photoionized plasma if the effects of resonant line scattering are included in
addition to recombination radiation. We show that the emissivity due to
resonant scattering depends sensitively on the line optical depth and, in the
case of winds in X-ray binaries, this allows constraints on the wind velocity
even when Doppler shifts cannot be resolved.Comment: 31 pages, 8 figures, ApJ accepted, abridged in accord with referee's
Comment
The X-Ray Photoionized Wind in Cen X-3/V779 Cen
We analyze the ASCA spectrum of the Cen X-3 X-ray binary system in eclipse
using atomic models appropriate to recombination-dominated level population
kinetics in an overionized plasma. In order to estimate the wind
characteristics, we first fit the eclipse spectrum to a single-zone
photoionized plasma model. We then fit spectra from a range of orbital phases
using global models of photoionized winds from the companion star and the
accretion disk that account for the continuous distribution of density and
ionization state. We find that the spectrum can be reproduced by a density
distribution of the form derived by Castor, Abbot, & Klein (1975) for
radiation-driven winds with with the value of the mass-loss rate divided by the
terminal velocity consistent with values for isolated stars of the same stellar
type. This is surprising because the neutron star is very luminous (~10^38
erg/s) and the X-rays from the neutron star should ionize the wind and destroy
the ions that provide the opacity for the radiation-driven wind. Using the same
functional form for the density profile, we also fit the spectrum to a
spherically symmetric wind centered on the neutron star, a configuration chosen
to represent a disk wind. We argue that the relatively modest orbital variation
of the discrete spectrum rules out a disk wind hypothesis.Comment: ApJ accepte
Quasi-Periodic Occultation by a Precessing Accretion Disk and Other Variabilities of SMC X-1
We have investigated the variability of the binary X-ray pulsar, SMC X-1, in
data from several X-ray observatories. We confirm the ~60-day cyclic variation
of the X-ray flux in the long-term monitoring data from the RXTE and CGRO
observatories. X-ray light curves and spectra from the ROSAT, Ginga, and ASCA
observatories show that the uneclipsed flux varies by as much as a factor of
twenty between a high-flux state when 0.71 second pulses are present and a
low-flux state when pulses are absent. In contrast, during eclipses when the
X-rays consist of radiation scattered from circumsource matter, the fluxes and
spectra in the high and low states are approximately the same. These
observations prove that the low state of SMC X-1 is not caused by a reduction
in the intrinsic luminosity of the source, or a spectral redistribution
thereof, but rather by a quasi-periodic blockage of the line of sight, most
likely by a precessing tilted accretion disk. In each of two observations in
the midst of low states a brief increase in the X-ray flux and reappearance of
0.71 second pulses occurred near orbital phase 0.2. These brief increases
result from an opening of the line of sight to the pulsar that may be caused by
wobble in the precessing accretion disk. The records of spin up of the neutron
star and decay of the binary orbit are extended during 1991-1996 by
pulse-timing analysis of ROSAT, ASCA, and RXTE PCA data. The pulse profiles in
various energy ranges from 0.1 to >21 keV are well represented as a combination
of a pencil beam and a fan beam. Finally, there is a marked difference between
the power spectra of random fluctuations in the high-state data from the RXTE
PCA below and above 3.4 keV. Deviation from the fitted power law around 0.06 Hz
may be QPO.Comment: Accepted to ApJ. 33 pages including 11 figure
Carina OB Stars: X-ray Signatures of Wind Shocks and Magnetic Fields
The Chandra Carina Complex contains 200 known O- and B type stars. The
Chandra survey detected 68 of the 70 O stars and 61 of 127 known B0-B3 stars.
We have assembled a publicly available optical/X-ray database to identify OB
stars that depart from the canonical Lx/Lbol relation, or whose average X-ray
temperatures exceed 1 keV. Among the single O stars with high kT we identify
two candidate magnetically confined wind shock sources: Tr16-22, O8.5 V, and LS
1865, O8.5 V((f)). The O4 III(fc) star HD 93250 exhibits strong, hard, variable
X-rays, suggesting it may be a massive binary with a period of >30 days. The
visual O2 If* binary HD 93129A shows soft 0.6 keV and hard 1.9 keV emission
components, suggesting embedded wind shocks close to the O2 If* Aa primary, and
colliding wind shocks between Aa and Ab. Of the 11 known O-type spectroscopic
binaries, the long orbital-period systems HD 93343, HD 93403 and QZ Car have
higher shock temperatures than short-period systems such as HD 93205 and FO 15.
Although the X-rays from most B stars may be produced in the coronae of unseen,
low-mass pre-main-sequence companions, a dozen B stars with high Lx cannot be
explained by a distribution of unseen companions. One of these, SS73 24 in the
Treasure Chest cluster, is a new candidate Herbig Be star.Comment: To be published in a special issue of the Astrophysical Journal
Supplement on the Chandra Carina Complex Projec
An X-ray spectroscopic study of the SMC X-1/Sk X-ray binary system
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999.Includes bibliographical references (p. 105-109).by Patrick Stephen Wojdowski.Ph.D