43 research outputs found
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 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
Resolving the Composite Fe K-alpha Emission Line in the Galactic Black Hole Cygnus X-1 with Chandra
We observed the Galactic black hole Cygnus X-1 with the Chandra High Energy
Transmission Grating Spectrometer for 30 kiloseconds on 4 January, 2001. The
source was in an intermediate state, with a flux that was approximately twice
that commonly observed in its persistent low/hard state. Our best-fit model for
the X-ray spectrum includes narrow Gaussian emission line (E = 6.415 +/- 0.007
keV, FWHM = 80 (+28, -19) eV, W = 16 (+3, -2) eV) and broad line (E = 5.82
(+0.06, -0.07) keV, FWHM = 1.9 (+0.5, -0.3) keV, W = 140 (+70, -40) eV)
components, and a smeared edge at 7.3 +/- 0.2 keV (tau ~ 1.0). The broad line
profile is not as strongly skewed as those observed in some Seyfert galaxies.
We interpret these features in terms of an accretion disk with irradiation of
the inner disk producing a broad Fe K-alpha emission line and edge, and
irradiation of the outer disk producing a narrow Fe K-alpha emission line. The
broad line is likely shaped predominantly by Doppler shifts and gravitational
effects, and to a lesser degree by Compton scattering due to reflection. We
discuss the underlying continuum X-ray spectrum and these line features in the
context of diagnosing the accretion flow geometry in Cygnus X-1 and other
Galactic black holes.Comment: Accepted for publication in Ap
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
Anomalous Low States and Long Term Variability in the Black Hole Binary LMC X-3
Rossi X-ray Timing Explorer observations of the black hole binary LMC X-3
reveal an extended very low X-ray state lasting from 2003 December 13 until
2004 March 18, unprecedented both in terms of its low luminosity (>15 times
fainter than ever before seen in this source) and long duration (~3 times
longer than a typical low/hard state excursion). During this event little to no
source variability is observed on timescales of ~hours-weeks, and the X-ray
spectrum implies an upper limit of 1.2x10^35 erg s^-1. Five years later another
extended low state occurs, lasting from 2008 December 11 until 2009 June 17.
This event lasts nearly twice as long as the first, and while significant
variability is observed, the source remains reliably in the low/hard spectral
state for the ~188 day duration. These episodes share some characteristics with
the "anomalous low states" in the neutron star binary Her X-1. The average
period and amplitude of the variability of LMC X-3 have different values
between these episodes. We characterize the long-term variability of LMC X-3
before and after the two events using conventional and nonlinear time series
analysis methods, and show that, as is the case in Her X-1, the characteristic
amplitude of the variability is related to its characteristic timescale.
Furthermore, the relation is in the same direction in both systems. This
suggests that a similar mechanism gives rise to the long-term variability,
which in the case of Her X-1 is reliably modeled with a tilted, warped
precessing accretion disk.Comment: 10 pages, 7 figures. Accepted for publication in Ap
XMM-Newton X-ray spectroscopy of the high-mass X-ray binary 4U1700-37 at low flux
We present results of a monitoring campaign of the high-mass X-ray binary
system 4U 1700-37/HD 153919, carried out with XMM-Newton in February 2001. The
system was observed at four orbital phase intervals, covering 37% of one
3.41-day orbit. The lightcurve includes strong flares, commonly observed in
this source. We focus on three epochs in which the data are not affected by
photon pile up: the eclipse, the eclipse egress and a low-flux interval in the
lightcurve around orbital phase phi ~0.25. The high-energy part of the
continuum is modelled as a direct plus a scattered component, each represented
by a power law with identical photon index (alpha ~1.4), but with different
absorption columns. We show that during the low-flux interval the continuum is
strongly reduced, probably due to a reduction of the accretion rate onto the
compact object. A soft excess is detected in all spectra, consistent with
either another continuum component originating in the outskirts of the system
or a blend of emission lines. Many fluorescence emission lines from
near-neutral species and discrete recombination lines from He- and H-like
species are detected during eclipse and egress. The detection of recombination
lines during eclipse indicates the presence of an extended ionised region
surrounding the compact object. The observed increase in strength of some
emission lines corresponding to higher values of the ionisation parameter xi
further substantiates this conclusion.Comment: 15 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Determination of the mass of the neutron star in SMC X-1, LMC X-4 and Cen X-3 with VLT/UVES
We present the results of a spectroscopic monitoring campaign of the OB-star
companions to the eclipsing X-ray pulsars SMC X-1, LMC X-4 and Cen X-3.
High-resolution optical spectra obtained with UVES on the ESO Very Large
Telescope are used to determine the radial-velocity orbit of the OB
(super)giants with high precision. The excellent quality of the spectra
provides the opportunity to measure the radial-velocity curve based on
individual lines, and to study the effect of possible distortions of the line
profiles due to e.g. X-ray heating on the derived radial-velocity amplitude.
Several spectral lines show intrinsic variations with orbital phase. The
magnitude of these variations depends on line strength, and thus provides a
criterion to select lines that do not suffer from distortions. The undistorted
lines show a larger radial-velocity amplitude than the distorted lines,
consistent with model predictions. Application of our line-selection criteria
results in a mean radial-velocity amplitude K(Opt) of 20.2 +/- 1.1, 35.1 +/-
1.5, and 27.5 +/- 2.3 km/s (1 sigma errors), for the OB companion to SMC X-1,
LMC X-4 and Cen X-3, respectively. Adding information on the projected
rotational velocity of the OB companion (derived from our spectra), the
duration of X-ray eclipse and orbital parameters of the X-ray pulsar (obtained
from literature), we arrive at a neutron star mass of 1.06^{+0.11}_{-0.10},
1.25^{+0.11}_{-0.10} and 1.34^{+0.16}_{-0.14} M{sun} for SMC X-1, LMC X-4 and
Cen X-3, respectively. The mass of SMC X-1 is near the minimum mass (~1 M{sun})
expected for a neutron star produced in a supernova. We discuss the
implications of the measured mass distribution on the neutron-star formation
mechanism, in relation to the evolutionary history of the massive binaries.Comment: Accepted for publication in Astronomy and Astrophysic
BeppoSAX spectroscopy of the luminous X-ray sources in M33
The nearby galaxy M33 was observed by the imaging X-ray instruments on-board
BeppoSAX. Two observations at different phases of the 105.9 day intensity cycle
of the luminous central source X-8 failed to reveal the expected modulation,
suggesting that it is probably transitory. Similar behavior has been observed
from several X-ray binary sources. This strengthens somewhat the idea that M33
X-8 is a black hole accreting from a binary companion. The 0.2-10 keV spectrum
of M33 X-8 can best be modeled by an absorbed power-law with a photon index of
1.89 +0.40 -0.79 and a disk-blackbody with a temperature, kT, of 1.10 +/0 0.05
keV and a projected inner-disk radius of 55.4 +6.0 -7.7 km. This spectral shape
is in good agreement with earlier ASCA results. The 2-10 keV spectra of M33
X-4, X-5, X-7, X-9 and X-10 are all consistent with power-law or bremsstrahlung
models, whilst that of X-6 appears to be significantly more complex and may be
reasonably well modeled with a disk-blackbody with kT = 1.7 +/- 0.2 keV and a
projected inner-disk radius of 7 +/- 2 km. The spectrum of M33 X-9 is rather
hard with a photon index of 1.3. Compared to earlier Einstein and ROSAT
observations, M33 X-7 and X-9 varied in intensity, M33 X-4, X-6, and X-10 may
have varied and M33 X-5 remained constant.Comment: 11 pages. To appear in A&