An X-ray Survey of Colliding Wind Binaries

Properties of Hot Luminous Stars, 1998

High-mass X-ray binaries (HMXBs) represent an important stage in the evolution of massive binary systems. The compact object (in most cases an X-ray pulsar) not only provides information on the orbital and stellar parameters, but also probes the stellar wind of the massive companion, an OB supergiant or Be star. The X-ray luminosity directly depends on the density and the velocity of the wind at the orbit of the X-ray source. Important constraints on the stellar-wind structure can be set by studying the orbital modulation of UV P-Cygni profiles. In this paper different aspects of the interactive wind-accretion process are highlighted, such as the highly variable X-ray luminosity, the influence of the X-rays on the radiative acceleration of the wind inside the ionization zone, and the large-scale structures that trail the X-ray source in its orbit.

Bulletin de la Société …, 2011

We report the preliminary results of the Suzaku observations of the W-R binary WR 140 (WC7+O5I). We executed the observations at four different epochs around periastron passage in Jan. 2009 to understand the W-R stellar wind as well as the wind-wind collision shocks. The total exposure was 210 ks. We detected hard X-ray excess in the HXD band (> 10 keV) for the first time from a W-R binary. Another notable discovery was a soft component which is less absorbed even by the dense wind. The spectra can be fitted by three different components; one is for the cool component with kT=0.1-0.6 keV, one for a dominant high-temperature component with kT ∼3 keV, and one for the hardest power-law component with the photon index of ∼2. As periastron approached, the column density of the high-temperature component increased, which can be explained as self-absorption by the W-R wind. The emission measure of the dominant, high-temperature component is not inversely proportional to the distance between the two stars.

Monthly Notices of the Royal Astronomical Society, 2003

We present theoretical X-ray line profiles from a range of model colliding wind systems. In particular, we investigate the effects of varying the stellar mass-loss rates, the wind speeds, and the viewing orientation. We find that a wide range of theoretical line profile shapes is possible, varying with orbital inclination and phase. At or near conjunction, the lines have approximately Gaussian profiles, with small widths (HWHM ∼ 0.1v ∞ ) and definite blue-or redshifts (depending on whether the star with the weaker wind is in front or behind). When the system is viewed at quadrature, the lines are generally much broader (HWHM ∼ v ∞ ), flat-topped and unshifted. Local absorption can have a major effect on the observed profiles -in systems with mass-loss rates of a few times 10 −6 M ⊙ yr −1 the lower energy lines (E < ∼ 1 keV) are particularly affected. This generally results in blueward-skewed profiles, especially when the system is viewed through the dense wind of the primary. The orbital variation of the line widths and shifts is reduced in a low inclination binary. The extreme case is a binary with i = 0 • , for which we would expect no line profile variation.

Monthly Notices of the Royal Astronomical Society, 2021

We report a detailed modelling of soft X-ray emission lines from two stellar wind-fed Galactic high-mass X-ray binary (HMXB) systems, Cyg X-3 and 4U 1538-522, and estimate physical parameters, e.g. hydrogen density, radiation field, chemical abundances, wind velocity, etc. The spectral synthesis code cloudy is utilized for this modelling. We model highly ionized X-ray spectral lines, such as Fe XXV (6.700 keV) and Fe XXVI (6.966 keV), and reproduce the observed line flux values. We find that for Cyg X–3 and 4U 1538-522, the inner radius of the ionized gas is at a distance of 1012.25 cm and 1010.43 cm, respectively, from the primary star, which is the main source of ionization. The densities of the ionized gas for Cyg X–3 and 4U 1538–522 are found to be ∼1011.35 cm−3 and 1011.99 cm−3, respectively. The corresponding wind velocities are 2000 km s−1 and 1500 km s−1. The respective predicted hydrogen column densities for Cyg X–3 and 4U 1538–522 are 1023.2 cm−2 and 1022.25 cm−2. In addit...

Astronomy & Astrophysics, 2015

Aims. We have developed a new code for the three-dimensional time-dependent raditation hydrodynamic simulation of the stellar wind in interacting binaries to improve models of accretion in high-mass X-ray binaries and to quantitatively clarify the observed variability of these objects. We used the code to test the influence of various parameters on the structure and properties of circumstellar matter. Methods. Our code takes into account acceleration of the wind due to the Roche effective potential, Coriolis force, gas pressure, and (CAK-) radiative pressure in the lines and continuum of the supergiant radiation field that is modulated by its gravity darkening and by the photo-ionization caused by X-ray radiation from the compact companion. The parameters of Cygnus X-1 were used to test the properties of our model. Results. Both two-and three-dimensional numerical simulations show that the Coriolis force substantially influences the mass loss and consequently the accretion rate onto the compact companion. The gravitational field of the compact companion focuses the stellar wind, which leads to the formation of a curved cone-like gaseous tail behind the companion. The changes of X-ray photo-ionization of the wind material during X-ray spectral-state transitions significantly influence the wind structure and offer an explanation of the variability of Cygnus X-1 in optical observations (the Hα emission).

AIP Conference Proceedings, 2010

The INTEGRAL satellite has revealed a major population of supergiant High Mass X-ray Binaries in our Galaxy, revolutionizing our understanding of binary systems and their evolution. This population, constituted of a compact object orbiting around a massive and luminous supergiant star, exhibits unusual properties, either being extremely absorbed, or showing very short and intense flares. An intensive set of multi-wavelength observations has led us to reveal their nature, and to show that these systems are wind-fed accretors, closely related to massive star-forming regions. In this paper I describe the characteristics of these sources, showing that this newly revealed population is closely linked to the evolution of active and massive OB stars with a compact companion. The last section emphasizes the formation and evolution of such High Mass X-ray Binaries hosting a supergiant star.

Monthly Notices of the …, 2010

Using data from observations made with XMM-Newton, we present an X-ray analysis of two Wolf-Rayet (WR) binaries: V444 Cyg and CD Cru. The X-ray light curves show the phase-locked variability in both binaries, where the flux increased by a factor of ∼2 for V444 Cyg and ∼1.5 for CD Cru, from minimum to maximum. The maximum luminosities in the 0.3-7.5 keV energy band were found to be 5.8 × 10 32 and 2.8 × 10 32 erg s −1 for V444 Cyg and CD Cru, respectively. The X-ray spectra of these stars confirmed large extinction and revealed hot plasma with prominent emission-line features of highly ionized Ne, Mg, Si, S, Ar, Ca and Fe; these are found to be consistent with a two-temperature plasma model. At a temperature of ∼0.6 keV, the cooler plasma was found to be constant at all phases for both binaries, which could be the result of a distribution of small-scale shocks in radiation-driven outflows. The hot components in these binaries were found to be phase-dependent. They varied from 1.85 to 9.61 keV for V444 Cyg and from 1.63 to 4.27 keV for CD Cru. The absorption of the hard component varied with the orbital phase and was found to be maximum during the primary eclipse of V444 Cyg. The high plasma temperature and variability with orbital phase suggest that the hard-component emission is caused by a colliding wind shock between the binary components.

The Astrophysical Journal, 2015

We present an overview of four deep phase-constrained Chandra HETGS X-ray observations of δ Ori A. Delta Ori A is actually a triple system which includes the nearest massive eclipsing spectroscopic binary, δ Ori Aa, the only such object that can be observed with little phasesmearing with the Chandra gratings. Since the fainter star, δ Ori Aa2, has a much lower X-ray luminosity than the brighter primary (δ Ori Aa1), δ Ori Aa provides a unique system with which to test the spatial distribution of the X-ray emitting gas around δ Ori Aa1 via occultation by the photosphere of, and wind cavity around, the X-ray dark secondary. Here we discuss the X-ray spectrum and X-ray line profiles for the combined observation, having an exposure time of nearly 500 ks and covering nearly the entire binary orbit. The companion papers discuss the X-ray variability seen in the Chandra spectra, present new space-based photometry and ground-based radial velocities obtained simultaneous with the X-ray data to better constrain the system parameters, and model the effects of X-rays on the optical and UV spectra. We find that the X-ray emission is dominated by embedded wind shock emission from star Aa1, with little contribution from the tertiary star Ab or the shocked gas produced by the collision of the wind of Aa1 against the surface of Aa2. We find a similar temperature distribution to previous X-ray spectrum analyses. We also show that the line half-widths are about 0.3 − 0.5 times the terminal velocity of the wind of star Aa1. We find a strong anti-correlation between line widths and the line excitation energy, which suggests that longer-wavelength, lower-temperature lines form farther out in the wind. Our analysis also indicates that the ratio of the intensities of the strong and weak lines of Fe XVII and Ne X are inconsistent with model predictions, which may be an effect of resonance scattering.

arXiv preprint arXiv: …, 2009

We present X-ray analysis of two Wolf-Rayet (WR) binaries: V444 Cyg and CD Cru using the data from observations with XMM-Newton. The X-ray light curves show the phase locked variability in both binaries, where the flux increased by a factor of ∼ 2 in the case of V444 Cyg and ∼ 1.5 in the case of CD Cru from minimum to maximum. The maximum luminosities in the 0.3-7.5 keV energy band were found to be 5.8 × 10 32 and 2.8 × 10 32 erg s −1 for V444 Cyg and CD Cru, respectively. X-ray spectra of these stars confirmed large extinction and revealed hot plasma with prominent emission line features of highly ionized Ne, Mg, Si, S, Ar, Ca and Fe, and are found to be consistent with a two-temperature plasma model. The cooler plasma at a temperature of ∼ 0.6 keV was found to be constant at all phases of both binaries, and could be due to a distribution of small-scale shocks in radiation-driven outflows. The hot components in these binaries were found to be phase dependent. They varied from 1.85 to 9.61 keV for V444 Cyg and from 1.63 to 4.27 keV for CD Cru. The absorption of the hard component varied with orbital phase and found to be maximum during primary eclipse of V444 Cyg. The high plasma temperature and variability with orbital phase suggest that the hard-component emission is caused by a colliding wind shock between the binary components.

Revista Mexicana De Astronomia Y Astrofisica, 2015

The centimeter spectra of Wolf-Rayet (WR) binaries often show a contribution from a wind-wind collision region (WCR) between the stars. In short period systems ( � &lt; 1 yr), such a component is expected to be absorbed by the unshocked winds, losing any effect from its binarity. Recent studies suggest that the WCR in these systems may also contribute to the emission at both centimeter and millimeter wavelengths. We analyzed and compared centimeter and millimeter observations of a total sample of 17 WR stars (including nine confirmed short-period systems) to detect any possible WCR contribution. More detailed observations are required in order to distinguish between different scenarios. We highlight the importance of analyzing the spectrum from quasi-simultaneous observations in a wide range of frequencies in order to properly characterize and distinguish any possible extra contribution.

Monthly Notices of the Royal Astronomical Society

We investigate the structure and X-ray emission from the colliding stellar winds in massive star binaries. We find that the opening angle of the contact discontinuity (CD) is overestimated by several formulae in the literature at very small values of the wind momentum ratio, η. We find also that the shocks in the primary (dominant) and secondary winds flare by ≈20 • compared to the CD, and that the entire secondary wind is shocked when η 0.02. Analytical expressions for the opening angles of the shocks, and the fraction of each wind that is shocked, are provided. We find that the X-ray luminosity L x ∝ η, and that the spectrum softens slightly as η decreases.

Proceedings of the International Astronomical Union

Strong winds from massive stars are a topic of interest to a wide range of astrophysical fields. In High-Mass X-ray Binaries the presence of an accreting compact object on the one side allows to infer wind parameters from studies of the varying properties of the emitted X-rays; but on the other side the accretor’s gravity and ionizing radiation can strongly influence the wind flow. Based on a collaborative effort of astronomers both from the stellar wind and the X-ray community, this presentation attempts to review our current state of knowledge and indicate avenues for future progress.

Astronomy and Astrophysics, 2004

We present XMM-Newton observations of γ 2 Velorum (WR 11, WC8+O7.5III, P = 78.53 d), a nearby Wolf-Ray binary system, at its X-ray high and low states. At high state, emission from a hot collisional plasma dominates from about 1 to 8 keV. At low state, photons between 1 and 4 keV are absorbed. The hot plasma is identified with the shock zone between the winds of the primary Wolf-Rayet star and the secondary O giant. The absorption at low state is interpreted as photoelectric absorption in the Wolf-Rayet wind. This absorption allows us to measure the absorbing column density and to derive a mass loss rate . M = 8×10 −6 M⊙yr −1 for the WC8 star. This mass loss rate, in conjunction with a previous Wolf-Rayet wind model, provides evidence for a clumped WR wind. A clumping factor of 16 is required. The X-ray spectra below 1 keV (12Å) show no absorption and are essentially similar in both states. There is a rather clear separation in that emission from a plasma hotter than 5 MK is heavily absorbed in low state while the cooler plasma is not. This cool plasma must come from a much more extended region than the hot material. The Neon abundance in the X-ray emitting material is 2.5 times the solar value. The unexpected detection of C v (25.3Å) and C vi (31.6Å) radiative recombination continua at both phases indicates the presence of a cool (∼ 40,000 K) recombination region located far out in the binary system.

WR 140 is the archetype long-period colliding wind binary (CWB) system, and is well known for dramatic variations in its synchrotron emission during its 7.9-yr, highly eccentric orbit. This emission is thought to arise from relativistic electrons accelerated at the global shocks bounding the wind-collision region (WCR). The presence of non-thermal electrons and ions should also give rise to X-ray and gamma-ray emission from several separate mechanisms, including inverse-Compton cooling, relativistic bremsstrahlung, and pion decay. We describe new calculations of this emission and make some preliminary predictions for the new generation of gamma-ray observatories. We determine that WR 140 will likely require several Megaseconds of observation before detection with INTEGRAL, but should be a reasonably strong source for GLAST.

Astronomy & Astrophysics, 2005

Massive X-ray binaries are usually classified depending on the properties of the donor star in classical, supergiant and Be X-ray binaries. The massive X-ray binary 4U 2206+54 does not fit in any of these groups, and deserves a detailed study to understand how the transfer of matter and the accretion on to the compact object take place. To this end we study an IUE spectrum of the donor and obtain a wind terminal velocity (v_inf) of ~350 km/s, which is abnormally slow for its spectral type. We also analyse here more than 9 years of available RXTE/ASM data. We study the long-term X-ray variability of the source and find it to be similar to that observed in the wind-fed supergiant system Vela X-1, reinforcing the idea that 4U 2206+54 is also a wind-fed system. We find a quasi-period decreasing from ~270 to ~130 d, noticed in previous works but never studied in detail. We discuss possible scenarios and conclude that long-term quasi-periodic variations in the mass-loss rate of the primary are probably driving such variability in the measured X-ray flux. We obtain an improved orbital period of 9.5591 d with maximum X-ray flux at MJD 51856.6. Our study of the orbital X-ray variability in the context of wind accretion suggests a moderate eccentricity around 0.15. Moreover, the low value of v_inf solves the long-standing problem of the relatively high X-ray luminosity for the unevolved nature of the donor, BD +53 2790, which is probably an O9.5 V star. We note that changes in v_inf and/or the mass-loss rate of the primary alone cannot explain the diferent patterns displayed by the orbital X-ray variability. We finally emphasize that 4U 2206+54, together with LS 5039, could be part of a new population of wind-fed HMXBs with main sequence donors, the natural progenitors of supergiant X-ray binaries. (Abridged)