Svetozar A. Zhekov

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.

Modeling the Hubble Space Telescope ultraviolet and optical spectrum of spot 1 on the circumstellar ring of SN 1987A

We report and interpret Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) long-slit observations of the optical and ultraviolet (1150-10270 A) emission line spectra of the rapidly brightening spot 1 on the equatorial ring of SN 1987A between 1997 September and 1999 October (days 3869-4606 after outburst). The emission is caused by radiative shocks created where the supernova blast wave strikes dense gas protruding inward from the equatorial ring. We measure and tabulate line identifications, fluxes, and, in some cases, line widths and shifts. We compute flux correction factors to account for substantial interstellar line absorption of several emission lines. Nebular analysis shows that optical emission lines come from a region of cool (Te ≈ 104 K) and dense (ne ≈ 106 cm-3) gas in the compressed photoionized layer behind the radiative shock. The observed line widths indicate that only shocks with shock velocities Vs < 250 km s-1 have become radiative, while line ratios indicate that much of the emission must have come from yet slower (Vs 135 km s-1) shocks. Such slow shocks can be present only if the protrusion has atomic density n 3 × 104 cm-3, somewhat higher than that of the circumstellar ring. We are able to fit the UV fluxes with an idealized radiative shock model consisting of two shocks (Vs = 135 and 250 km s-1). The observed UV flux increase with time can be explained by the increase in shock surface areas as the blast wave overtakes more of the protrusion. The observed flux ratios of optical to highly ionized UV lines are greater by a factor of ~2-3 than predictions from the radiative shock models, and we discuss the possible causes. We also present models for the observed Hα line widths and profiles, which suggest that a chaotic flow exists in the photoionized regions of these shocks. We discuss what can be learned with future observations of all the spots present on the equatorial ring.

The X-Ray Spectrum of Supernova Remnant 1987A

We discuss the X-ray emission observed from supernova remnant 1987A with the Chandra X-Ray Observatory. We analyze a high-resolution spectrum obtained in 1999 October with the high-energy transmission grating (HETG). From this spectrum we measure the strengths and an average profile of the observed X-ray lines. We also analyze a high signal-to-noise ratio CCD spectrum obtained in 2000 December. The good statistics (?9250 counts) of this spectrum and the high spatial resolution provided by the telescope allow us to perform spectroscopic analyses of different regions of the remnant. We discuss the relevant shock physics that can explain the observed X-ray emission. The X-ray spectra are well fitted by plane-parallel shock models with postshock electron temperatures of ?2.6 keV and ionization ages of ?6 ? 1010 cm-3 s. The combined X-ray line profile has a FWHM of ?5000 km s-1, indicating a blast-wave speed of ?3500 km s-1. At this speed, plasma with a mean postshock temperature of ?17 keV is produced. This is direct evidence for incomplete electron-ion temperature equilibration behind the shock. Assuming this shock temperature, we constrain the amount of collisionless electron heating at the shock front at Te0/Ts = 0.11. We find that the plasma has low metallicity (abundances are ?0.1-0.4 solar) and is nitrogen enriched (N/O ? 0.8 by number), similar to abundances found for the equatorial ring. Analysis of the spectra from different regions of the remnant reveals slight differences in the parameters of the emitting plasma. The plasma is cooler near the optical spot 1 (at position angle ?30?) and in the eastern half of the remnant, where the bright optical spots are found, than in the western half, consistent with the presence of slower (?500 km s-1) shocks entering denser ring material. There is an overall flux asymmetry between the two halves, with the eastern half being 15%-50% brighter (depending on how the center of the remnant is defined). However, our spectroscopic analysis shows that less than 5% of the overall X-ray emission could come from a slow shock component. Therefore the flux asymmetry cannot fully be due to X-rays produced by the blast wave entering the ring, but rather indicates an asymmetry in the global interaction with the circumstellar material interior to the ring.

X-RAY EMISSION FROM NITROGEN-TYPE WOLF-RAYET STARS

We summarize new X-ray detections of four nitrogen-type Wolf-Rayet (WR) stars obtained in a limited survey aimed at establishing the X-ray properties of WN stars across their full range of spectral subtypes. None of the detected stars is so far known to be a close binary. We report Chandra detections of WR 2 (WN2), WR 18 (WN4), and WR 134 (WN6), and an XMM-Newton detection of WR79a (WN9ha). These observations clearly demonstrate that both WNE and WNL stars are X-ray sources. We also discuss Chandra archive detections of the WN6h stars WR 20b, WR 24, and WR 136 and ROSAT non-detections of WR 16 (WN8h) and WR 78 (WN7h). The X-ray spectra of all WN detections show prominent emission lines and an admixture of cool (kT 2 keV) plasma. The hotter plasma is not predicted by radiative wind shock models and other as yet unidentified mechanisms are at work. Most stars show X-ray absorption in excess of that expected from visual extinction (A V), likely due to their strong winds or cold circumstellar gas. Existing data suggest a falloff in X-ray luminosity toward later WN7-9 subtypes, which have higher L bol but slower, denser winds than WN2-6 stars. This provides a clue that wind properties may be a more crucial factor in determining emergent X-ray emission levels than bolometric luminosity.

XMM-Newton Detection of Hard X-Ray Emission in the Nitrogen-Type Wolf-Rayet Star WR 110

We have used the excellent sensitivity of XMM-Newton to obtain the first high-quality X-ray spectrum of a Wolf-Rayet (W-R) star that is not known to be a member of a binary system. Our target, the nitrogen-type star WR 110 (HD 165688), was also observed and detected with the Very Large Array at four different frequencies. The radio flux density increases with frequency according to a power law Sν ∝ ν+0.64±0.10, in very good agreement with the behavior expected for free-free wind emission. The radio data give an ionized mass-loss rate = 4.9 × 10-5 M☉ yr-1 for an assumed spherical constant-velocity wind. The undispersed CCD X-ray spectra reveal strong emission lines from He-like ions of Mg, Si, and S. The emission measure distribution shows a dominant contribution from cool plasma with a characteristic temperature kTcool ≈ 0.5 keV (≈6 MK). Little or no excess absorption of this cool component above the value expected from the visual extinction is present. We conclude that the bulk of the cool plasma detected by XMM-Newton lies at hundreds of stellar radii or more if the wind is approximately spherical and homogeneous, but it could lie closer to the star if the wind is clumped. If the cool plasma is due to instability-driven wind shocks, then typical shock velocities are vs ≈ 340-550 km s-1 and the average filling factor of X-ray-emitting gas in the wind is no larger than f ~ 10-6. A surprising result is the unambiguous detection of a hard X-ray component that is clearly seen in the hard-band images and the spectra. This hard component accounts for about half of the observed flux and can be acceptably fitted by a hot, optically thin thermal plasma or a power-law model. If the emission is thermal, then a temperature kThot ≥ 3 keV is derived. Such high temperatures are not predicted by current instability-driven wind shock models, and a different mechanism is thus required to explain the hard X-rays. We examine several possible mechanisms and show that the hard emission could be accounted for by the W-R wind shocking onto a close stellar companion that has so far escaped detection. However, until persuasive evidence for binarity is found, we are left with the intriguing possibility that the hard X-ray emission is produced entirely by the Wolf-Rayet star.

A Chandra view of the morphological and spectral evolution of supernova remnant 1987A

We present an update on the results of our monitoring observations of the X-ray remnant of supernova (SN) 1987A with Chandra. As of 2002 December, we have performed a total of seven observations of SN 1987A, which allows us to monitor the details of the earliest stage of the supernova remnant evolution in X-rays. The high angular resolution images from the latest data reveal developments of new X-ray-bright spots in the northwestern and the southwestern portions of the remnant, as well as changes on the eastern side. The observed soft X-ray flux is increasing more rapidly than ever, and the latest 0.5-2 keV band flux (fX ~ 6 × 10-13 ergs cm-2 s-1) is 4 times brighter than 3 yr earlier, when this monitoring began. The overall X-ray emission is primarily from the blast wave shock with kT ~ 2.4 keV. As the blast wave approaches the dense circumstellar material, the contribution from the decelerated slow shock (kT ~ 0.22 keV) to the observed X-ray emission is becoming significant. The increase of this slow shock contribution over the last 2 yr is particularly noticeable in the western half of the remnant. These results indicate that the shock front is now reaching the main body of the inner circumstellar ring and that SN 1987A will be a complete ring with dramatic brightening in coming years. Based on the best-fit two-shock spectral model, we derive approximate densities of the X-ray-emitting regions (ne ~ 235 cm-3 for the fast shock and ne ~ 7500 cm-3 for the slow shock). There is no direct observational evidence to date for a neutron star associated with supernova remnant 1987A. We obtain an upper limit on the observed X-ray luminosity of any embedded point source (LX ≤ 1.5 × 1034 ergs s-1) in the 2-10 keV band. The X-ray remnant continues to expand linearly at a rate of 4167 km s-1.

X-RAY EVOLUTION OF SNR 1987A: THE RADIAL EXPANSION

We present the evolution of the radial expansion of SNR 1987A as measured using Chandra X-ray observations taken over the last 10 years. To characterize the complex structure of the remnant and isolate the expansion measurement, we fit the images to several empirical models including: a simple circular torus, a torus with bilateral lobes, and a torus with four tangentially extended lobes. We discuss the results of this measure in the context of the overall evolution of the supernova remnant, for which we believe we have measured the end of the free expansion phase and its transition to the adiabatic phase (at least along the equatorial ring). The timing of this event is in agreement with early predictions of the remnant evolution.

XMM-Newton and Very Large Array Observations of the Variable Wolf-Rayet Star EZ Canis Majoris: Evidence for a Close Companion?

We present new X-ray and radio observations of the Wolf-Rayet star EZ CMa (HD 50896) obtained with XMM-Newton and the Very Large Array (VLA). This WN4 star exhibits optical and UV variability at a period of 3.765 days whose cause is unknown. Binarity may be responsible, but the existence of a companion has not been proven. The radio spectral energy distribution of EZ CMa determined from VLA observations at five frequencies is in excellent agreement with predictions for free-free wind emission, and the ionized mass-loss rate allowing for distance uncertainties is = 3.8 (±2.6) × 10-5 M☉ yr-1. The CCD X-ray spectra show prominent Si XIII and S XV emission lines and can be acceptably modeled as an absorbed multitemperature optically thin plasma, confirming earlier ASCA results. Nonsolar abundances are inferred with Fe notably deficient. The X-ray emission is dominated by cooler plasma at a temperature kTcool ≈ 0.6 keV, but a harder component is also detected, and the derived temperature is kThot ≈ 3.0-4.2 keV if the emission is thermal. This is too high to be explained by radiative wind shock models, and the X-ray luminosity of the hard component is 3 orders of magnitude lower than expected for accretion onto a neutron star companion. We show that the hard emission could be produced by the Wolf-Rayet wind shocking onto a normal (nondegenerate) stellar companion at close separation. Finally, using comparable data sets we demonstrate that the X-ray and radio properties of EZ CMa are strikingly similar to those of the WN5-6 star WR 110. This similarity points to common X-ray and radio emission processes in WN stars and discredits the idea that EZ CMa is anomalous within its class.

Colliding stellar wind models with non-equilibrium ionization: X-rays from WR 147

The effects of non-equilibrium ionization are explicitly taken into account in a numerical model which describes colliding stellar winds (CSW) in massive binary systems. This new model is used to analyse the most recent X-ray spectra of the WR+OB binary system WR 147. The basic result is that it can adequately reproduce the observed X-ray emission (spectral shape, observed flux) but some adjustment in the stellar wind parameters is required. Namely (i) the stellar wind velocities must be higher by a factor of 1.4-1.6 and (ii) the mass loss must be reduced by a factor of ∼2. The reduction factor for the mass loss is well within the uncertainties for this parameter in massive stars, but given the fact that the orbital parameters (e.g. inclination angle and eccentricity) are not well constrained for WR 147, even smaller corrections to the mass loss might be sufficient. Only CSW models with non-equilibrium ionization and equal (or nearly equal) electron and ion post-shock temperature are successful. Therefore, the analysis of the X-ray spectra of WR 147 provides evidence that the CSW shocks in this object must be collisionless.

Evolutionary Status of SNR 1987A at the Age of Eighteen

Approximately 18 yr after the supernova explosion, the blast wave of SNR 1987A is entering the main body of the equatorial circumstellar material, which is causing a dramatic brightening of the remnant. We recently reported observationalevidencefor thisevent fromourChandradata(theworkpublishedin2005NovemberbyParketal.).We present here the temporal evolution of the X-ray-emitting shock parameters and the detailed description of the spectral and image analysis of SNR 1987A, on which our 2005 November work was based. While the remnant becomes brighter, the softening of the overall X-ray spectrum continues and is enhanced around day 6200 (since the explosion). The two-component shock model indicates that the electron temperatures have been changing for the last � 6y r. The X-ray spectrum is now described by kT � 0.3 keVand 2.3 keV thermal plasmas that are believed to characteristically represent the shock-heated density gradient along the boundary between the H ii region and the dense inner ring. As the blast wave sweeps through the inner circumstellar ring shining in X-rays, we expect that the shock parameters will continue to change, revealing the density and abundance structure of the inner ring. Follow-up Chandra observationswillthusuncoverthepasthistoryoftheprogenitor’sstellarevolution.Theoriginoftherelativelyfaint hard X-ray emission (E > 3 keV) from SNR 1987A is still unclear (thermal vs. nonthermal). Considering the continuous brightening of the hard-band intensity, as well as the soft-band flux, follow-up monitoring observations will also be essential to reveal the origin of the hard X-ray emission of SNR 1987A.