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Register a new userObservations of Binary and Single Wolf-Rayet Stars with XMM-Newton and Chandra
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We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars with XMM-Newton and Chandra. A new XMM spectrum of the nearby WN8 + OB binary WR 147 shows hard absorbed X-ray emission, including the Fe K-alpha 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.
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The nature of the X-ray source RX J1914+24 has been the subject of much debate. It shows a prominent period of 569 sec in X-rays and the optical/infra-red: in most models this has been interpreted as the binary orbital period. We present our analysis of new XMM-Newton and Chandra data. We find a longer term trend in the XMM-Newton data and power at 556 and 585 sec in 5 sets of data. It is not clear if they are produced as a result of a beat between a longer intrinsic period and the 569 sec modulation or if they are due to secular variations. We obtain a good fit to the XMM-Newton spectrum with a low temperature thermal plasma model with an edge at 0.83keV. This model implies an unabsorbed bolometric X-ray luminosity of 1x10^{33} ergs/s (for a distance of 1kpc) - this is 2 orders of magnitude lower than our previous estimate (derived using a different model). If the distance is much less, as the absorption derived from the X-ray fits suggest, then it is even lower at ~3x10^{31} ergs/s.
Radioisotopes are natural clocks which can be used to estimate the age of the solar system. They also influence the shape of supernova light curves. In addition, the diffuse emission at 1.8 MeV from the decay of 26Al may provide a measure of the present day nucleosynthetic activity in the Galaxy. Therefore, even if radionuclides represent only a tiny fraction of the cosmic matter, they carry a unique piece of information. A large number of radioisotopes are produced by massive stars at the time of their supernova explosion. A more or less substantial fraction of them are also synthesized during the previous hydrostatic burning phases. These nuclides are then ejected either at the time of the supernova event, or through stellar winds during their hydrostatic burning phases. This paper focusses of the non explosive ejection of radionuclides by non-rotating or rotating Wolf-Rayet stars.
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.
The collapsar model for long gamma-ray bursts requires a rapidly rotating Wolf-Rayet star as progenitor. We test the idea of producing rapidly rotating Wolf-Rayet stars in massive close binaries through mass accretion and consecutive quasi-chemically homogeneous evolution; the latter had previously been shown to provide collapsars below a certain metallicity threshold for single stars. The binary channel presented here may provide a means for massive stars to obtain the high rotation rates required to evolve quasi-chemically homogeneous and fulfill the collapsar scenario. Moreover, it suggests that a possibly large fraction of long gamma-ray bursts occurs in runaway stars.
The archival XMM-Newton data of the central region of M31 were analyzed for diffuse X-ray emission. Point sources with the 0.5--10 keV luminosity exceeding $sim 4 times 10^{35}$ erg s$^{-1}$ were detected. Their summed spectra are well reproduced by a combination of a disk black-body component and a black-body component, implying that the emission mainly comes from an assembly of luminous low-mass X-ray binaries. After excluding these point sources, spectra were accumulated over a circular region of $6arcmin$ (1.2 kpc) centered on the nucleus. In the energy range above 2 keV, these residual spectra are understood mainly as contributions of unresolved faint sources and spill-over of photons from the excluded point sources. There is in addition a hint of a $sim 6.6$ keV line emission, which can be produced by a hot (temperature several keV) thin-thermal plasma. Below 2 keV, the spectra involve three additional softer components expressed by thin-thermal plasma emission models, of which the temperatures are $sim 0.6$, $sim 0.3$, and $sim 0.1$ keV. Their 0.5--10 keV luminosities within 6$arcmin$ are measured to be $sim 1.2 times 10^{38}$ erg s$^{-1}$, $sim 1.6 times 10^{38}$ erg s$^{-1}$, and $sim 4 times 10^{37}$ erg s$^{-1}$ in the order of decreasing temperature. The archival Chandra data of the central region of M31 yielded consistent results. By incorporating different annular regions, all the three softer thermal components were confirmed to be significantly extended. These results are compared with reports from previous studies. A discussion is presented on the origin of each thermal emission component.
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