Colliding Wind Shock Research Articles

Non-thermal X-rays from colliding wind shock acceleration in the massive binary Eta Carinae

Cosmic-ray acceleration has been a long-standing mystery and despite more than a century of study, we still do not have a complete census of acceleration mechanisms. The collision of strong stellar winds in massive binary systems creates powerful shocks, which have been expected to produce high-energy cosmic-rays through Fermi acceleration at the shock interface. The accelerated particles should collide with stellar photons or ambient material, producing non-thermal emission observable in X-rays and gamma-rays. The supermassive binary star eta Carinae drives the strongest colliding wind shock in the solar neighborhood. Observations with non-focusing high-energy observatories indicate a high energy source near eta Carinae, but have been unable to conclusively identify eta Carinae as the source because of their relatively poor angular resolution. Here we present the first direct focussing observations of the non-thermal source in the extremely hard X-ray band, which is found to be spatially coincident with the star within several arc-seconds. These observations show that the source of non-thermal X-rays varies with the orbital phase of the binary, and that the photon index of the emission is similar to that derived through analysis of the gamma-ray spectrum. This is conclusive evidence that the high-energy emission indeed originates from non-thermal particles accelerated at colliding wind shocks.

Modelling the colliding-wind spectra of the WC8d+O8-9IV binary CV Ser (WR 113)

Striking profile variations of the C iii λ5696 emission line are visible amongst the high signal-to-noise ratio, moderate resolution spectra of the 29.7 d WC8d+O8-9IV binary CV Ser (WR 113) presented here. Using a significantly revised code, we have modelled these variations assuming the emission originates from the undisturbed WR star wind and a colliding wind shock region that partially wraps around the O star. Changes to the modelling code are chiefly in the form of additional parameters, intended to refine the modelling and facilitate comparison with recent predictions arising from theoretical and hydrodynamical work. This modelling provides measurements of crucial parameters such as the orbital inclination (63.5° ± 2.5°) and thus, together with the RV orbits, the stellar masses (11.7 ± 0.9 M⊙ for the WR star and 33.3 ± 2.0 M⊙ for the O star). We find good agreement with expectations based on theoretical studies and hydrodynamical modelling of colliding wind systems. Moreover, it raises the exciting prospect of providing a reliable method to learn more about WR stellar masses and winds, and for studying the physics of colliding winds in massive stars.

ACHANDRAOBSERVATION OF THE ECLIPSING WOLF-RAYET BINARY CQ Cep

The short-period (1.64 d) near-contact eclipsing WN6+O9 binary system CQ Cep provides an ideal laboratory for testing the predictions of X-ray colliding wind shock theory at close separation where the winds may not have reached terminal speeds before colliding. We present results of a Chandra X-ray observation of CQ Cep spanning ~1 day during which a simultaneous Chandra optical light curve was acquired. Our primary objective was to compare the observed X-ray properties with colliding wind shock theory, which predicts that the hottest shock plasma (T > 20 MK) will form on or near the line-of-centers between the stars. The X-ray spectrum is strikingly similar to apparently single WN6 stars such as WR 134 and spectral lines reveal plasma over a broad range of temperatures T ~ 4 - 40 MK. A deep optical eclipse was seen as the O star passed in front of the Wolf-Rayet star and we determine an orbital period P = 1.6412400 d. Somewhat surprisingly, no significant X-ray variability was detected. This implies that the hottest X-ray plasma is not confined to the region between the stars, at odds with the colliding wind picture and suggesting that other X-ray production mechanisms may be at work. Hydrodynamic simulations that account for such effects as radiative cooling and orbital motion will be needed to determine if the new Chandra results can be reconciled with the colliding wind picture.

Measuring mass-loss rates and constraining shock physics using X-ray line profiles of O stars from the Chandra archive

We quantitatively investigate the extent of wind absorption signatures in the X-ray grating spectra of all non-magnetic, effectively single O stars in the Chandra archive via line profile fitting. Under the usual assumption of a spherically symmetric wind with embedded shocks, we confirm previous claims that some objects show little or no wind absorption. However, many other objects do show asymmetric and blueshifted line profiles, indicative of wind absorption. For these stars, we are able to derive wind mass-loss rates from the ensemble of line profiles, and find values lower by an average factor of 3 than those predicted by current theoretical models, and consistent with Hα if clumping factors of fcl ≈ 20 are assumed. The same profile fitting indicates an onset radius of X-rays typically at r ≈ 1.5R*, and terminal velocities for the X-ray emitting wind component that are consistent with that of the bulk wind. We explore the likelihood that the stars in the sample that do not show significant wind absorption signatures in their line profiles have at least some X-ray emission that arises from colliding wind shocks with a close binary companion. The one clear exception is ζ Oph, a weak-wind star that appears to simply have a very low mass-loss rate. We also reanalyse the results from the canonical O supergiant ζ Pup, using a solar-metallicity wind opacity model and find |$\dot{M}= 1.8 \times 10^{-6}$| |${\mathrm{M_{\odot }\,{\mathrm{y}r^{-1}}}}$|⁠, consistent with recent multiwavelength determinations.

A RADIO PULSAR SEARCH OF THE γ-RAY BINARIES LS I +61 303 AND LS 5039

LS I +61 303 and LS 5039 are exceptionally rare examples of HMXBs with MeV-TeV emission, making them two of only five known or proposed "gamma-ray binaries". There has been disagreement within the literature over whether these systems are microquasars, with stellar winds accreting onto a compact object to produce high energy emission and relativistic jets, or whether their emission properties might be better explained by a relativistic pulsar wind colliding with the stellar wind. Here we present an attempt to detect radio pulsars in both systems with the Green Bank Telescope. The upper limits of flux density are between 4.1-14.5 uJy, and we discuss the null results of the search. Our spherically symmetric model of the wind of LS 5039 demonstrates that any pulsar emission will be strongly absorbed by the dense wind unless there is an evacuated region formed by a relativistic colliding wind shock. LS I +61 303 contains a rapidly rotating Be star whose wind is concentrated near the stellar equator. As long as the pulsar is not eclipsed by the circumstellar disk or viewed through the densest wind regions, detecting pulsed emission may be possible during part of the orbit.

Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate

We present analysis of both the resolved X-ray emission line profiles and the broadband X-ray spectrum of the O2 If* star HD 93129A, measured with the Chandra HETGS. This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X-ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10% of the X-ray flux. We find results consistent with the predictions of numerical simulations of the line-driving instability, including line broadening indicating an onset radius of X-ray emission of several tenths Rstar. Helium-like forbidden-to-intercombination line ratios are consistent with this onset radius, and inconsistent with being formed in a wind-collision interface with the star's closest visual companion at a distance of ~100 AU. The broadband X-ray spectrum is fit with a dominant emission temperature of just kT = 0.6 keV along with significant wind absorption. The broadband wind absorption and the line profiles provide two independent measurements of the wind mass-loss rate: Mdot = 5.2_{-1.5}^{+1.8} \times 10^{-6} Msun/yr and Mdot = 6.8_{-2.2}^{+2.8} \times 10^{-6} Msun/yr, respectively. This is the first consistent modeling of the X-ray line profile shapes and broadband X-ray spectral energy distribution in a massive star, and represents a reduction of a factor of 3 to 4 compared to the standard H-alpha mass-loss rate that assumes a smooth wind.

X-ray emission characteristics of two Wolf-Rayet binaries: V444 Cyg and CD Cru

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 $\sim 2$ in the case of V444 Cyg and $\sim 1.5$ 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 $5.8\times10^{32}$ and $2.8\times10^{32}$ 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 $\sim$ 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.

High‐ResolutionChandraX‐Ray Imaging and Spectroscopy of the σ Orionis Cluster

We present results of a 90 ks Chandra X-ray observation of the young σ Orionis cluster (age ∼ 3 Myr ) obtained with the HETGS. We use the high-resolution grating spectrum and moderate-resolution CCD spectrum of the massive central star σ Ori AB (O 9.5 V + B 0.5 V ) to test wind shock theories of X-ray emission and also analyze the high spatial resolution zero-order ACIS-S image of the central cluster region. Chandra detected 42 X-ray sources on the primary CCD (ACIS-S3). All but five have near-IR or optical counterparts and about one-fourth are variable. Notable high-mass stellar detections are σ Ori AB, the magnetic B star σ Ori E, and the B5 V binary HD 37525. Most of the other detections have properties consistent with lower mass K- or M-type stars. We present the first X-ray spectrum of the unusual infrared source IRS 1, located ≈3'' north of σ Ori AB. Its X-ray properties and elongated mid-IR morphology suggest that it is an embedded low-mass T Tauri star whose disk/envelope is being photoevaporated by σ Ori AB. We focus on the radiative wind shock interpretation of the soft luminous X-ray emission from σ Ori AB, but also consider possible alternatives including magnetically confined wind shocks and colliding wind shocks. Its emission lines show no significant asymmetries or centroid shifts and are moderately broadened to HWHM ≈ 264 km s−1, or one-fourth the terminal wind speed. Forbidden lines in He-like ions are formally undetected, implying strong UV suppression. The Mg XI triplet forms in the wind acceleration zone within one stellar radius above the surface. These X-ray properties are consistent in several respects with the predictions of radiative wind shock theory for an optically thin wind, but explaining the narrow line widths presents a challenge to the theory.

XMM–Newton X-ray observations of the Wolf–Rayet binary system WR 147

We present results of an ≈20-ks X-ray observation of the Wolf–Rayet (WR) binary system WR 147 obtained with XMM–Newton. Previous studies have shown that this system consists of a nitrogen-type WN8 star plus an OB companion whose winds are interacting to produce a colliding wind shock. X-ray spectra from the pn and MOS detectors confirm the high extinction reported from infrared studies and reveal hot plasma including the first detection of the Fe Kα line complex at 6.67 keV. Spectral fits with a constant-temperature plane-parallel shock model give a shock temperature kTshock= 2.7 keV (Tshock≈ 31 MK), close to but slightly hotter than the maximum temperature predicted for a colliding wind shock. Optically thin plasma models suggest even higher temperatures, which are not yet ruled out. The X-ray spectra are harder than can be accounted for using 2D numerical colliding wind shock models based on nominal mass-loss parameters. Possible explanations include: (i) underestimates of the terminal wind speeds or wind abundances, (ii) overly simplistic colliding wind models or (iii) the presence of other X-ray emission mechanisms besides colliding wind shocks. Further improvement of the numerical models to include potentially important physics such as non-equilibrium ionization will be needed to rigorously test the colliding wind interpretation.

X-ray Observations of Binary and Single Wolf-Rayet Stars with XMM-Newton and Chandra

We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars with XMM-Newton and Chandra. These observations are aimed at determining the differences in X-ray properties between massive WR + OB binary systems and putatively single WR stars. A new XMM spectrum of the nearby WN8 + OB binary WR 147 shows hard absorbed X-ray emission (including the Fe Kα line complex), characteristic of colliding wind shock sources. In contrast, sensitive observations of four of the closest known single WC (carbon-rich) WR stars have yielded only non-detections. These results tentatively suggest that single WC stars are X-ray quiet. The presence of a companion may thus be an essential factor in elevating the X-ray emission of WC + OB stars to detectable levels.

Chandra X-ray observations of the young stellar cluster NGC 6193 in the Ara OB1 association

A 90-ks Chandra High Energy Transmission Grating observation of the young stellar cluster NGC 6193 in the southern Ara OB1 association detected 43 X-ray sources in a 2 × 2 arcmin2 core region centred on the massive O stars HD 150135 (O6.5 V) and HD 150136 (O3 + O6V). The cluster is dominated by exceptionally bright X-ray emission from the two O stars, which are separated by only 10 arcsec. The X-ray luminosity of HD 150136 is log LX= 33.39 (erg s−1), making it one of the most luminous O-star X-ray sources known. All of the fainter X-ray sources in the core region have near-infrared (near-IR) counterparts, but existing JHK photometry provides little evidence for near-IR excesses. These core sources have typical mean photon energies 〈E〉 ≈ 2 keV and about one-third are variable. It is likely that some are young low-mass stars in the cluster, but cluster membership remains to be determined. Grating spectra show that the X-ray properties of HD 150135 and HD 150136 are similar, but not identical. Both have moderately broadened unshifted emission lines and their emission is dominated by cool plasma at kT ≈ 0.3 keV, pointing to a wind-shock origin. However, the emission of HD 150136 is slightly hotter and four times more luminous than its optical twin HD 150135. We discuss the possibility that a radiative colliding wind shock contributes to the prodigious X-ray output of the short-period (2.66 d) spectroscopic binary HD 150136. A surprising result is that the X-ray emission of HD 150136 is slowly variable on a time-scale of <1 d. The origin of the variability is not yet known but the observed behaviour suggests that it is an occultation effect.

The high energy X-ray tail of $\mathsf{\eta}$ Car revealed by BeppoSAX

We report on the June 2000 long (100 ks) BeppoSAX exposure that unveiled a new very high energy component of the X-ray spectrum of η Car above 10 keV extending to at least 50 keV. We find that the 2-150 keV spectrum is best reproduced by a thermal + non-thermal model. The thermal component dominates the 2-10 keV spectral range with kTh = 5.5 ± 0.3 keV and log NHh = 22.68 ± 0.01. The spectrum displays a prominent iron emission line centred at 6.70 keV. Its equivalent width of 0.94 keV, if produced by the thermal source, gives a slightly sub-solar iron abundance ((Fe/H) = −0.15 ± 0.02). The high energy tail above 10 keV is best fitted by a power law with a photon index of 2.42 ± 0.04. The integrated 13-150 keV luminosity of ∼12 Lis comparable to that of the 2-10 keV thermal component (19 L� ). The present result can be explained, in the η Car binary star scenario, by Comptonisation of low frequency radiation by high energy electrons, probably generated in the colliding wind shock front, or in instabilities in the wind of the S Dor primary star. It is possible that the high energy tail had largely weakened near the minimum of the 5.53 yr cycle. Probably, the thermal component has a longer recovery time like that of the highest excitation optical emission lines. Both features can be associated with the large absorption measured by BeppoSAX at phase 0.05.

Nonthermal Emission from the Arches Cluster (G0.121+0.017) and the Origin of -ray Emission from 3EG J1746-2851

High-resolution Very Large Array observations of the Arches cluster near the Galactic center show evidence of continuum emission at λ = 3.6, 6, 20, and 90 cm. The continuum emission at λ = 90 cm is particularly striking because thermal sources generally become optically thick at longer wavelengths and fall off in brightness, whereas nonthermal sources increase in brightness. It is argued that the radio emission from this unique source has compact and diffuse components produced by thermal and nonthermal processes, respectively. Compact sources within the cluster arise from stellar winds of mass-losing stars, whereas diffuse emission is likely to be due to colliding wind shocks of the cluster flow that generate relativistic particles that are due to diffuse shock acceleration. We also discuss the possibility that γ-ray emission from 3EG J1746-2851, located within 33 of the Arches cluster, results from the inverse Compton scattering of the radiation field of the cluster.

High-resolution X-ray imaging of the colliding wind shock in WR 147 (WN8(h)+B0.5V)

We present the results of a high-resolution Chandra X-ray image of the Wolf-Rayet binary system WR 147, which consists of a WN8 star and an early-type companion located 0″.6 to its north. The image provides the first direct evidence for spatially extended X-ray emission in an early-type binary system, and shows that the emission peaks north of the WN8 star and close to the position of the radio bow shock. Simulated X-ray images of the wind-wind collision have a fwhm consistent with the data, but cannot account for the full spatial extension, unless emission from the stars is also included.

High-resolution X-ray imaging of the colliding wind shock in WR 147

We analyze new high-resolution Chandra X-ray images of the Wolf-Rayet binary system WR147. This system contains a WN8 star with an early-type companion located 0:6 00 to its north, and is the only known early- type binary with a separation on the sky large enough for the wind-wind collision between the stars to currently be resolved at X-ray energies. The 5 ksec Chandra HRC-I image provides the rst direct evidence for spatially extended X-ray emission in an early-type binary system. The X-ray emission peaks close to the position of the radio bow shock and north of the WN8 star. A deeper X-ray image is needed to accurately determine the degree of spatial extension, to exactly align the X-ray and optical/radio frames, and to determine whether part of the detected X-ray emission arises in the individual stellar winds. Simulated X-ray images of the wind-wind collision have a FWHM consistent with the data, and maximum likelihood ts suggest that a deeper observation may also constrain the inclination and wind momentum ratio of this system. However, as the WR wind dominates the colliding wind X-ray emission it appears unlikely that _ M OB and v1OB can be separately determined from X-ray observations. We also note an inconsistency between numerical and analytical estimates of the X-ray luminosity ratio of the stronger and weaker wind components, and conclude that the analytical results are in error.

X‐Ray Emission from Colliding Wind Shocks in the Wolf‐Rayet Binary WR 140

We analyze four ASCA X-ray observations of the Wolf-Rayet binary system WR 140 obtained between 1993 and 1997 by making use of hydrodynamic colliding wind (CW) shock models. The analysis shows that the CW shock models are able to accurately reproduce the X-ray spectra at different orbital phases using mass-loss and orbital parameters that are within the ranges allowed by the uncertainties. However, some adjustment in the currently accepted values of the semimajor axis and time of periastron passage may eventually be required. Models that allow for different electron and ion temperatures provide better fits to the data. Extra absorption is inferred from CW shock models above that expected from the winds and interstellar medium, the origin of which is not yet known. We also report the serendipitous discovery of hot plasma at temperatures in excess of ~2 keV and X-ray emission lines in spectra extracted from the diffuse Cygnus superbubble background in the vicinity of WR 140.

Simultaneous Radio and X‐Ray Observations of the Wolf‐Rayet Star WR 147

We present results of simultaneous ASCA/Very Large Array (VLA) observations of the Wolf-Rayet star WR 147 (AS 431). This WN 8 star is an optical double and may be a WR+OB colliding wind binary system. The new observations place tight constraints on the origin of its X-ray and radio emission. The X-ray emission is due to a multitemperature, optically thin thermal plasma, with the dominant contribution coming from plasma at kT ≈ 1 keV. The absorption column density derived from the X-ray spectrum is NH = 2 × 1022 cm-2, which agrees well with estimates based on the visual extinction but is too large to explain by wind absorption alone. The X-ray temperature structure is consistent with colliding wind shock emission, but the unabsorbed X-ray luminosity Lx = 1032.55 ergs s-1 (0.5-10 keV) is several times smaller than that predicted from colliding wind shock models. The VLA data provide the most complete picture ever obtained of the radio spectral energy distribution of a WR star and consist of near-simultaneous observations at five different wavelengths (1.3, 2, 3.6, 6, and 21 cm). The radio emission consists of a thermal free-free component from the WR wind and a nonthermal component. If the nonthermal emission is due to relativistic particles accelerated by the Fermi mechanism in wind shocks, then the flux is expected to decline at high frequencies according to Sν ∝ ν-0.5. However, the observed falloff is much steeper and cannot be reproduced by a simple power law or by synchrotron models that assume power-law electron energy distributions. A surprising result is that the nonthermal emission can be accurately modeled as synchrotron radiation from relativistic electrons that are nearly monoergetic.

X‐Ray Evidence for Wind‐Wind Collision in the Wolf‐Rayet Binary V444 Cygni

We report the results of three ASCA observations of the eclipsing Wolf-Rayet binary V444 Cyg (WN5+O6). These observations were obtained at orbital phases 0.0, 0.25, and 0.5, with the WN5 star in front at phase 0.0 and the O6 star in front at phase 0.5. Acceptable fits of the X-ray spectra using optically thin plasma models require at least two different temperature components, with a soft component at kT1≈0.6 keV and a harder component at kT2≈2 keV. The absorption of the hard component varies with orbital phase and is largest when the WN5 star is in front, whereas the X-ray luminosity of the hard component is at a minimum when the O6 star is in front. The high plasma temperature and variability with orbital phase suggest that the hard-component emission is caused by a colliding wind shock between the WN5 and O6 stars, with the shock most likely located near the surface of the O6 star. On the other hand, the soft-component emission at kT1≈0.6 keV has nearly constant absorption and X-ray luminosity. The soft-component luminosity is LX, 1=(6-11) × 1032 ergs s-1 (0.2-4 keV), implying LX, 1/Lbol~10−6 to 10-7. This luminosity ratio and the soft-component temperature are similar to those of single massive stars, leading us to attribute the soft emission to the individual O6 and WN5 components.

Hydrodynamic simulations of the colliding winds in Iota Orionis

Two-dimensional hydrodynamic simulations of the colliding winds in the eccentric binary Iota Orionis (HR 1889; HD 37043) have been conducted. With the inclusion of radiative driving, the realistic simulation of such a system becomes possible for the first time. The dynamics of the post-shock flow throughout the orbit are explored. Radiative inhibition and sudden radiative braking both occur, lowering the temperature of the post-shock gas. Instabilities in the collision region are ubiquitous, leading to a great deal of structure. Two separate models with different stellar mass-loss rates are examined. In both models the colliding wind shock collapses on to the photosphere of the secondary around periastron, owing to the imbalance between the wind momentum fluxes. However, the shock is able to detach from the surface of the secondary in the less extreme model as the secondary star heads towards apastron. A higher resolution simulation indicates that this result is currently resolution-dependent. The synthetic intrinsic X-ray emission is extremely dependent on the amount of cooling in the post-shock flow, and hence its nature changes substantially if the shock detaches. In such a case it is very soft at periastron, but much harder at apastron. During the former, the secondary star penetrates deep into the wind acceleration region of the primary, and the pre-shock velocity is reduced from ∼2000 to ∼1000 km s−1. The post-shock density also substantially increases, resulting in very strong cooling. In comparison, at apastron the post-shock density is low, and the pre-shock velocity is high, resulting in a very adiabatic wind collision. Synthetic X-ray light curves show a minimum in the 0.4–10.0 keV ASCA band centred on periastron with a duration of a couple of days. If the shock detaches, a reduction in the 0.1–0.5 keV ROSAT emission is also predicted. Such variation, if seen in ‘real’ data, may help in accurately determining the mass-loss rates of the stellar components.

Circumstellar shocks: colliding stellar winds

In this paper I will review some recent developments in the field of circumstellar shocks, particularly as they relate to colliding stellar winds. I shall review the basic physics of colliding winds and shocks, and discuss recent developments in hydrodynamic modelling of colliding winds. I shall also report on recent X-ray observations of shock emission in Wolf-Rayet binary systems where high resolution X-ray spectra of colliding wind shock emission is being seen. I will discuss the occurrence of colliding winds to such diverse systems as Wolf-Rayet binaries, pre-main sequence binaries, symbiotic stars as well as the Galactic center object IRS 7, where recent results on interacting winds are yielded insight into the structure of winds in general.