103 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
Outbursts, State Transitions, and Periodicities Observed with the RXTE All-Sky Monitor
Results from the All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer are
reviewed. A number of recurrent transient sources have been detected, while
only a few previously unreported sources have been discovered. The ASM light
curves show a wide variety of phenomena in general, and, in particular, those
of transient sources show a wide range of properties. Examples are used to
illustrate that the distinction between persistent and transient sources may be
very unclear. The results of searches for periodicities in the ASM light curves
are summarized, and other astrophysical investigations using ASM light curves
are suggested. The latter include investigations of the possible causes of
long-term quasiperiodic and chaotic variability, and comparative studies on the
basis of the observed variability.Comment: 8 pages, 4 figures, in proceedings of the conference "The Active
X-ray Sky: Results from BeppoSAX and Rossi-XTE, Rome, Italy, 21-24 October,
1997. Nuclear Physics B Proceedings Supplement
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
The Importance of XUV Radiation as a Solution to the P V Mass Loss Rate Discrepancy in O-Stars
A controversy has developed regarding the stellar wind mass loss rates in
O-stars. The current consensus is that these winds may be clumped which implies
that all previously derived mass loss rates using density-squared diagnostics
are overestimated by a factor of ~ 2. However, arguments based on FUSE
observations of the P V resonance line doublet suggest that these rates should
be smaller by another order of magnitude, provided that P V is the dominant
phosphorous ion among these stars. Although a large mass loss rate reduction
would have a range of undesirable consequences, it does provide a
straightforward explanation of the unexpected symmetric and un-shifted X-ray
emission line profiles observed in high energy resolution spectra. But
acceptance of such a large reduction then leads to a contradiction with an
important observed X-ray property: the correlation between He-like ion source
radii and their equivalent X-ray continuum optical depth unity radii. Here we
examine the phosphorous ionization balance since the P V fractional abundance,
q(P V), is fundamental to understanding the magnitude of this mass loss
reduction. We find that strong "XUV" emission lines in the He II Lyman
continuum can significantly reduce q(P V). Furthermore, owing to the unique
energy distribution of these XUV lines, there is a negligible impact on the S V
fractional abundance (a key component in the FUSE mass loss argument). We
conclude that large reductions in O-star mass loss rates are not required, and
the X-ray optical depth unity relation remains valid.Comment: Accepted for publication in ApJ Letters, 15 pages, 5 color figure
X-Rays From Massive OB Stars: Thermal Emission From Radiative Shocks
Chandra gratings spectra of a sample of 15 massive OB stars were analyzed
under the basic assumption that the X-ray emission is produced in an ensemble
of shocks formed in the winds driven by these objects. Shocks develop either as
a result of radiation-driven instabilities or due to confinement of the wind by
relatively strong magnetic field, and since they are radiative, a simple model
of their X-ray emission was developed that allows a direct comparison with
observations. According to our model, the shock structures (clumps, complete or
fractional shells) eventually become `cold' clouds in the X-ray sky of the
star. As a result, it is expected that for large covering factors of the hot
clumps, there is a high probability for X-ray absorption by the `cold' clouds,
resulting in blue-shifted spectral lines. Our analysis has revealed that such a
correlation indeed exists for the considered sample of OB stars. As to the
temperature characteristics of the X-ray emission plasma, the studied OB stars
fall in two groups: (i) one with plasma temperature limited to 0.1-0.4 keV;
(ii) the other wtih X-rays produced in plasmas at considerably higher
temperatures. We argue that the two groups correspond to different mechanisms
for the origin of X-rays: in radiative-driven instability shocks and in
magnetically-confined wind shocks, respectively.Comment: 11 pages, 4 figures, 2 tables; accepted for publication in MNRA
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
Chandra HETG Observations of the Colliding Stellar Wind System WR 147
We present an extended analysis of deep Chandra HETG observations of the
WR+OB binary system WR 147 that was resolved into a double X-ray source (Zhekov
& Park, 2010, ApJ, 709, L119). Our analysis of the profiles of strong emission
lines shows that their centroids are blue-shifted in the spectrum of the
northern X-ray source. We find no suppressed forbidden line in the He-like
triplets which indicates that the X-ray emitting region is not located near
enough to the stars in the binary system to be significantly affected by their
UV radiation. The most likely physical picture that emerges from the entire set
of HETG data suggests that the northern X-ray source can be associated with the
colliding stellar wind region in the wide WR+OB binary system, while the X-rays
of its southern counterpart, the WN8 star, are result from stellar wind
shocking onto a close companion (a hypothesized third star in the system).Comment: 22 pages, 6 figures, 2 Tables; accepted for publication in The
Astrophysical Journa
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