Journal articles: 'X-rays: stars'

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Güdel, Manuel. "X–rays from stars." Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences 360, no. 1798 (July 30, 2002): 1935–49. http://dx.doi.org/10.1098/rsta.2002.1045.

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Petit, V., D. H. Cohen, Y. Nazé, M. Gagné, R. H. D. Townsend, M. A. Leutenegger, A. ud-Doula, S. P. Owocki, and G. A. Wade. "X-rays from magnetic massive OB stars." Proceedings of the International Astronomical Union 9, S302 (August 2013): 330–33. http://dx.doi.org/10.1017/s1743921314002427.

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AbstractThe magnetic activity of solar-type and low-mass stars is a well known source of coronal X-ray emission. At the other end of the main sequence, X-rays emission is instead associated with the powerful, radiatively driven winds of massive stars. Indeed, the intrinsically unstable line-driving mechanism of OB star winds gives rise to shock-heated, soft emission (~0.5 keV) distributed throughout the wind. Recently, the latest generation of spectropolarimetric instrumentation has uncovered a population of massive OB-stars hosting strong, organized magnetic fields. The magnetic characteristics of these stars are similar to the apparently fossil magnetic fields of the chemically peculiar ApBp stars. Magnetic channeling of these OB stars' strong winds leads to the formation of large-scale shock-heated magnetospheres, which can modify UV resonance lines, create complex distributions of cooled Halpha emitting material, and radiate hard (~2-5 keV) X-rays. This presentation summarizes our coordinated observational and modelling efforts to characterize the manifestation of these magnetospheres in the X-ray domain, providing an important contrast between the emission originating in shocks associated with the large-scale fossil fields of massive stars, and the X-rays associated with the activity of complex, dynamo-generated fields in lower-mass stars.

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Luna, G. J. M., J. L. Sokoloski, K. Mukai, and T. Nelson. "Symbiotic stars in X-rays." Astronomy & Astrophysics 559 (October 28, 2013): A6. http://dx.doi.org/10.1051/0004-6361/201220792.

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Ayres, Thomas R. "X‐Rays from Hybrid Stars." Astrophysical Journal 618, no. 1 (January 2005): 493–501. http://dx.doi.org/10.1086/425891.

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Cohen, David H. "X-ray Emission from O Stars." Proceedings of the International Astronomical Union 3, S250 (December 2007): 17–24. http://dx.doi.org/10.1017/s1743921308020309.

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AbstractYoung O stars are strong, hard, and variable X-ray sources; properties that strongly affect their circumstellar and galactic environments. After ≈ 1 Myr, these stars settle down to become steady sources of soft X-rays. I will use high-resolution X-ray spectroscopy and MHD modeling to show that young O stars like θ1 Ori C are well explained by the magnetically channeled wind shock scenario. After their magnetic fields dissipate, older O stars produce X-rays via shock heating in their unstable stellar winds. Here too I will use X-ray spectroscopy and numerical modeling to confirm this scenario. In addition to elucidating the nature and cause of the O star X-ray emission, modeling of the high-resolution X-ray spectra of O supergiants provides strong evidence that mass-loss rates of these O stars have been overestimated.

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Robrade, J. "eROSITA - Nearby Young Stars in X-rays." Proceedings of the International Astronomical Union 10, S314 (November 2015): 280–85. http://dx.doi.org/10.1017/s1743921315005943.

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AbstractX-ray surveys are well suited to detect, identify and study young stars based on their high levels of magnetic activity and thus X-ray brightness. The eROSITA instrument onboard the Spectrum-Roentgen-Gamma (SRG) satellite will perform an X-ray all-sky survey that surpasses existing data by a sensitivity increase of more than an order of magnitude. The 4 yr survey is expected to detect more than half a million stars and stellar systems in X-rays.

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Cassinelli, Joseph P., and David H. Cohen. "ROSAT Observations of B and Be Stars." Symposium - International Astronomical Union 162 (1994): 189–99. http://dx.doi.org/10.1017/s0074180900214824.

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We present results from a survey of X-ray emission properties of near main-sequence B stars, including several Be and β Cephei stars. The main conclusions of our survey are: 1) The X-rays are soft, probably because the shock velocity jumps are small since the terminal wind speeds are small. 2) A major fraction of the wind emission measure is hot, assuming wind theory estimates for the density distribution. A large fraction of the wind is not expected to be hot in current wind shock models. 3) A hard component is found to be present in τ Sco; possible causes are discussed. 4) For the Be stars, the X-rays emission is from a normal B-star wind that is coming from the poles as in the WCD model of Be stars. 5) None of the stars, including the β Cep stars, show noticeable variability in their X-rays. For the normal B stars we conclude from the lack of variability that the shocks are in the form of fragments in the wind instead of spherical shells. 6) Our observations suggest that all B stars are X-ray sources and that there is a basal amount of X-ray luminosity of about 10-8.5Lboi. The hot component in τ Sco and the high X-ray luminosity of B stars detected in the all-sky survey suggests that there is a source of X-ray emission in addition to wind shocks in some B stars.

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Nuñez, N. E., T. Nelson, K. Mukai, J. L. Sokoloski, and G. J. M. Luna. "SYMBIOTIC STARS IN X-RAYS. III.SUZAKUOBSERVATIONS." Astrophysical Journal 824, no. 1 (June 7, 2016): 23. http://dx.doi.org/10.3847/0004-637x/824/1/23.

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Hayakawa, Satio. "X-rays from accreting neutron stars." Physics Reports 121, no. 6 (May 1985): 317–406. http://dx.doi.org/10.1016/0370-1573(85)90053-5.

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Parker, Eugene N. "Why Do Stars Emit X Rays?" Physics Today 40, no. 7 (July 1987): 36–42. http://dx.doi.org/10.1063/1.881079.

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MacFarlane, Joseph J., Joseph P. Cassinelli, and D. H. Cohen. "X-Ray Evidence for Wind Instabilities." International Astronomical Union Colloquium 169 (1999): 201–2. http://dx.doi.org/10.1017/s0252921100071980.

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Hot stars are known to emit X-rays with LX/Lbol ~ 10−7 for O stars, falling to ~ 10−9 for B3 stars. These stars also lose mass at large rates through their high-speed winds. Over the years, several types of production mechanisms have been proposed to explain the X-ray emission from O stars, with source locations ranging from very near the stellar surface to very far from the star. A coronal X-ray source was originally proposed (Cassinelli and Olson 1979) to explain the presence of anomalously high ionization stages observed as P Cygni line profiles in the UV spectra of O stars. At the other extreme, Chlebowski (1989) suggested that the X-rays of O stars originate far from the star, and are produced by the interaction of the stellar wind with circumstellar matter. A model in which shocks forming due to instabilities in the line-driven winds of O stars was proposed by Lucy (1982), and studied in detail by Owocki et al. (1988), Cooper (1994), and Feldmeier (1996). In this case, the X-ray emission originates in a large number of shock-heated regions distributed throughout the wind. The shocked-wind model has also been shown to be consistent with the X-ray emission from early-B stars, such as τ Sco (MacFarlane and Cassinelli 1989). However, it appears difficult for shocked wind models to explain the X-ray emission from B3 and later stars because of their presumed low mass loss rates (Cohen et al.1997).

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Bard, Christopher, and Richard Townsend. "X-rays From Centrifugal Magnetospheres in Massive Stars." Proceedings of the International Astronomical Union 9, S307 (June 2014): 449–50. http://dx.doi.org/10.1017/s1743921314007352.

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AbstractIn the subset of massive OB stars with strong global magnetic fields, X-rays arise from magnetically confined wind shocks (Babel & Montmerle 1997). However, it is not yet clear what the effect of stellar rotation and mass-loss rate is on these wind shocks and resulting X-rays. Here, we present results from a grid of Arbitrary Rigid-Field Hydrodynamic simulations (ARFHD) of a B-star centrifugal magnetosphere with an eye towards quantifying the effect of stellar rotation and mass-loss rate on the level of X-ray emission. The results are also compared to a generalized XADM model for X-rays in dynamical magnetospheres (ud-Doula et al. 2014).

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Montmerle, Thierry. "Massive Stars and Giant HII Regions: The High-Energy Picture." Symposium - International Astronomical Union 143 (1991): 397–408. http://dx.doi.org/10.1017/s0074180900045502.

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Giant HII regions contain highly energetic objects: luminous, massive stars (including Wolf-Rayet stars) generating powerful winds, as well as, often, supernova remnants. These objects interact with the surrounding gas by creating shock waves. Part of the energy input is radiated away in the form of X-rays; also, protons and electrons may be accelerated in situ and generate γ-rays by collisions with the ionized gas. In addition, the stars themselves (including the accompanying low-mass PMS stars) are sources of X-rays, and W-R stars may emit continuum y-rays and are associated with nuclear γ-ray lines seen in the interstellar medium. Therefore, both through the stars they contain and through interactions within the gas, giant HII regions are, in addition to their more traditional properties and over nearly 7 decades in energy, important sources of high-energy radiation.

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Nazé, Yaël, Véronique Petit, Melanie Rinbrand, David Cohen, Stan Owocki, Asif ud-Doula, and Gregg Wade. "The X-ray properties of magnetic massive stars." Proceedings of the International Astronomical Union 9, S307 (June 2014): 437–42. http://dx.doi.org/10.1017/s1743921314007339.

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AbstractEarly-type stars are well-known to be sources of soft X-rays. However, this high-energy emission can be supplemented by bright and hard X-rays when magnetically confined winds are present. In an attempt to clarify the systematics of the observed X-ray properties of this phenomenon, a large series of Chandra and XMM observations was analyzed, over 100 exposures of 60% of the known magnetic massive stars listed recently by Petit et al. (2013). It is found that the X-ray luminosity is strongly correlated with mass-loss rate, in agreement with predictions of magnetically confined wind models, though the predictions of higher temperature are not always verified. We also investigated the behaviour of other X-ray properties (absorption, variability), yielding additional constraints on models. This work not only advances our knowledge of the X-ray emission of massive stars, but also suggests new observational and theoretical avenues to further explore magnetically confined winds.

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Nazé, Yael, Gregor Rauw, and Asif ud-Doula. "The surprising X-ray emission of Oe stars." Proceedings of the International Astronomical Union 6, S272 (July 2010): 624–25. http://dx.doi.org/10.1017/s1743921311011616.

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AbstractOe stars are thought to represent an extension of the Be phenomenon to higher temperatures. Dedicated XMM observations of HD 155806 revealed a surprising X-ray spectrum: soft character, absence of overluminosity, broad X-ray lines. These properties are fully compatible with the wind-shock model, which usually explains the X-rays from “normal”, single O-type stars. In contrast, some other Oe/Be stars display a completely different behaviour at high energies.

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Oskinova, L. M., R. Ignace, and D. P. Huenemoerder. "X-ray diagnostics of massive star winds." Proceedings of the International Astronomical Union 12, S329 (November 2016): 151–55. http://dx.doi.org/10.1017/s1743921317002952.

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AbstractObservations with powerful X-ray telescopes, such as XMM-Newton and Chandra, significantly advance our understanding of massive stars. Nearly all early-type stars are X-ray sources. Studies of their X-ray emission provide important diagnostics of stellar winds. High-resolution X-ray spectra of O-type stars are well explained when stellar wind clumping is taking into account, providing further support to a modern picture of stellar winds as non-stationary, inhomogeneous outflows. X-ray variability is detected from such winds, on time scales likely associated with stellar rotation. High-resolution X-ray spectroscopy indicates that the winds of late O-type stars are predominantly in a hot phase. Consequently, X-rays provide the best observational window to study these winds. X-ray spectroscopy of evolved, Wolf-Rayet type, stars allows to probe their powerful metal enhanced winds, while the mechanisms responsible for the X-ray emission of these stars are not yet understood.

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Iwazaki, Aiichi. "X rays from old neutron stars heated by axion stars." Physics Letters B 486, no. 1-2 (July 2000): 147–52. http://dx.doi.org/10.1016/s0370-2693(00)00745-0.

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Hudec, R., W. Wenzel, W. Goetz, B. Valníček, R. Peřestý, G. A. Richter, G. Hacke, H. Huth, A. Mrkos, and J. Tremko. "X-Ray and Optical Observations of the X-Ray Source EX0020528+1454.8." International Astronomical Union Colloquium 93 (1987): 697–700. http://dx.doi.org/10.1017/s0252921100105561.

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AbstractWe present preliminary results of the EXOSAT X-ray observations and quasisimultaneous and simultaneous optical photometry of the X-ray source EX0020528+1454.8 = 1E0205+149 found independently as an serendipitous source both with Einstein and EXOSAT satellites. The optical counterpart is a pair of dMe stars, Our results indicate that the object is variable both in X-rays and optical wavelenghts, and probably belongs to dMe flare stars.

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Montmerle, Thierry. "What can X-rays tell us about accretion, mass loss and magnetic fields in young stars?" Proceedings of the International Astronomical Union 3, S243 (May 2007): 23–30. http://dx.doi.org/10.1017/s1743921307009386.

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AbstractUntil recently, X-rays from low-mass young stars (105–106 yr) were thought to be a universal proxy for magnetic activity, enhanced by 3-4 orders of magnitude with respect to the Sun, but otherwise similar in nature to all low-mass, late-type convective stars (including the Sun itself). However, there is now evidence that other X-ray emission mechanisms are at work in young stars. The most frequently invoked mechanism is accretion shocks along magnetic field lines (“magnetic accretion”). In the case of the more massive A- and B-type stars, and their progenitors the Herbig AeBe stars, other, possibly more exotic mechanisms can operate: star-disk magnetic reconnection, magnetically channeled shocked winds, etc. In any case, magnetic fields, both on small scale (surface activity) and on large scale (dipolar magnetospheres), play a distinctive role in the emission of X-rays by young stars, probably throughout the IMF.

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Carlson, Eric D. "Pseudoscalar conversion and X-rays from stars." Physics Letters B 344, no. 1-4 (January 1995): 245–51. http://dx.doi.org/10.1016/0370-2693(94)01529-l.

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Güdel, Manuel, Kevin Briggs, Kaspar Arzner, Marc Audard, Jérôme Bouvier, Catherine Dougados, Eric Feigelson, et al. "Accretion and outflow-related X-rays in T Tauri stars." Proceedings of the International Astronomical Union 3, S243 (May 2007): 155–62. http://dx.doi.org/10.1017/s1743921307009519.

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AbstractWe report on accretion- and outflow-related X-rays from T Tauri stars, based on results from the “XMM-Newton Extended Survey of the Taurus Molecular Cloud.” X-rays potentially form in shocks of accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae may occur as well. We report on the discovery of a “soft excess” in accreting T Tauri stars supporting these scenarios. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal component and a very soft, weakly absorbed component probably related to shocks in microjets. The excessive coronal absorption points to dust-depletion in the accretion streams.

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Montmerle, Thierry. "The Quest for X-Rays from Protostars." Symposium - International Astronomical Union 188 (1998): 17–20. http://dx.doi.org/10.1017/s0074180900114330.

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The field of low-mass star formation and early evolution has made rapid progress in recent years, thanks in particular to observations in the IR and mm ranges. The current evolutionary scheme calls for two main stages, themselves divided into two substages (e.g., André & Montmerle 1994): (i) protostars, comprizing the newly discovered so-called “Class 0 sources”, detected mostly or only in the mm range, which are young protostars with estimated ages ~ 104 yrs, and “Class I sources”, visible in the near- to mid-IR, which are evolved protostars with estimated ages ~ 105 yrs; (ii) T Tauri stars, which are visible in the IR but also in the optical, the younger being the “classical” T Tauri stars (called “Class II” in the IR), and the “weak-line” T Tauri stars (“Class III” in the IR), with a large age spread of ~ 106 – 107 yrs. According to current models (e.g., Shu et al. 1987), protostars consist of a forming star surrounded by an extended envelope (up to ~ 10,000 AU in radius); the star forms via an accretion disk inside a cavity ~ several 100 AU in radius. The disk probably plays an important role in generating molecular outflows, running through the envelope. Classical T Tauri stars are only surrounded by a disk, which disappears at the weak-line T Tauri stage.

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Babel, J. "Diffusion, Winds and X-Rays from Magnetic Stars." Highlights of Astronomy 11, no. 2 (1998): 674–75. http://dx.doi.org/10.1017/s1539299600018426.

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AbstractWe propose a self-consistent approach of the CP star phenomenon and try to solve simultaneously the problems of abundance spots, wind and X-ray emission from these stars. We also discuss the periodic X-ray emission from the 07V star θ1 Orionis C and its link with Bp stars.

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Ruderman, M. "Neutron Star Powered Accelerators." Symposium - International Astronomical Union 195 (2000): 463–71. http://dx.doi.org/10.1017/s0074180900163508.

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Neutron stars can be the underlying source of energetic particle acceleration in several ways. The huge gravitational-collapse energy released in their birth, or the violent fusion at the end of the life of a neutron-star binary, is the energy source for an accelerator in the surrounding medium far from the star. This would be the case for: (a) cosmic rays from supernova explosions with neutron-star remnants; (b) energetic radiation from “plerions” around young neutron stars (e.g., the Crab Nebula, see Pacini 2000); and (c) “afterglow” and γ-rays of cosmic Gamma-Ray Burst (GRB) sources with possible neutron-star central engines. Particles can also be energetically accelerated if a neutron star's gravitational pull sustains an accretion disk fed by a companion. Examples are accretion-powered X-ray pulsars and low-mass X-ray binaries. A third family of “neutron-star powered” accelerators consists of those which do not depend on the surrounding environment. These are the accelerators which must exist in the magnetospheres of many solitary, spinning-down, magnetized neutron stars (“spinsters”) when they are observed as radio pulsars or γ-ray pulsars. (There are probably ~ 103 dead radio pulsars for each one in our Galaxy that is still active; the ratio for γ-ray pulsars may well exceed 105.)

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Meurs, E. J. A., and A. J. M. Piters. "OeBe-stars with possible compact companions observed at X-rays." Symposium - International Astronomical Union 163 (1995): 554–58. http://dx.doi.org/10.1017/s0074180900202672.

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A survey of bright O- and B-type stars has been conducted from the ROSAT All Sky Survey data, aiming at the detection of compact companions expected for Oe and Be stars. Concurrent Hα spectroscopy has been used to establish the level of activity of these stars around the time of the X-ray observations. The detection rate of OB and OeBe stars as well as their average X-ray over bolometric luminosities suggest little difference between these two groups of objects. Instead of accretion onto a compact object (NS, WD), the X-ray emission from OeBe stars may be produced by stellar winds as has been proposed for O- and early B-type stars in general. Flexibilities in the available models for X-ray emission from accreting WDs may allow the X-ray emission from such objects to remain below a detectable level. These investigations are now being extended to pointed observations in the ROSAT Archive.

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Nebot Gómez-Morán, A., and L. M. Oskinova. "The X-ray catalog of spectroscopically identified Galactic O stars." Astronomy & Astrophysics 620 (December 2018): A89. http://dx.doi.org/10.1051/0004-6361/201833453.

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The X-ray emission of O-type stars was first discovered in the early days of the Einstein satellite. Since then many different surveys have confirmed that the ratio of X-ray to bolometric luminosity in O-type stars is roughly constant, but there is a paucity of studies that account for detailed information on spectral and wind properties of O-stars. Recently a significant sample of O stars within our Galaxy was spectroscopically identified and presented in the Galactic O-Star Spectroscopic Survey (GOSS). At the same time, a large high-fidelity catalog of X-ray sources detected by the XMM-Newton X-ray telescope was released. Here we present the X-ray catalog of O stars with known spectral types and investigate the dependence of their X-ray properties on spectral type as well as stellar and wind parameters. We find that, among the GOSS sample, 127 O-stars have a unique XMM-Newton source counterpart and a Gaia data release 2 (DR2) association. Terminal velocities are known for a subsample of 35 of these stars. We confirm that the X-ray luminosities of dwarf and giant O stars correlate with their bolometric luminosity. For the subsample of O stars with measure terminal velocities we find that the X-ray luminosities of dwarf and giant O stars also correlate with wind parameters. However, we find that these correlations break down for supergiant stars. Moreover, we show that supergiant stars are systematically harder in X-rays compared to giant and dwarf O-type stars. We find that the X-ray luminosity depends on spectral type, but seems to be independent of whether the stars are single or in a binary system. Finally, we show that the distribution of log(LX/Lbol) in our sample stars is non-Gaussian, with the peak of the distribution at log(LX/Lbol) ≈ −6.6.

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Fletcher, C. L., V. Petit, Y. Nazé, G. A. Wade, R. H. Townsend, S. P. Owocki, D. H. Cohen, A. David-Uraz, and M. Shultz. "Investigating the Magnetospheres of Rapidly Rotating B-type Stars." Proceedings of the International Astronomical Union 12, S329 (November 2016): 369–72. http://dx.doi.org/10.1017/s1743921317002812.

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AbstractRecent spectropolarimetric surveys of bright, hot stars have found that ~10% of OB-type stars contain strong (mostly dipolar) surface magnetic fields (~kG). The prominent paradigm describing the interaction between the stellar winds and the surface magnetic field is the magnetically confined wind shock (MCWS) model. In this model, the stellar wind plasma is forced to move along the closed field loops of the magnetic field, colliding at the magnetic equator, and creating a shock. As the shocked material cools radiatively it will emit X-rays. Therefore, X-ray spectroscopy is a key tool in detecting and characterizing the hot wind material confined by the magnetic fields of these stars. Some B-type stars are found to have very short rotational periods. The effects of the rapid rotation on the X-ray production within the magnetosphere have yet to be explored in detail. The added centrifugal force due to rapid rotation is predicted to cause faster wind outflows along the field lines, leading to higher shock temperatures and harder X-rays. However, this is not observed in all rapidly rotating magnetic B-type stars. In order to address this from a theoretical point of view, we use the X-ray Analytical Dynamical Magnetosphere (XADM) model, originally developed for slow rotators, with an implementation of new rapid rotational physics. Using X-ray spectroscopy from ESA’s XMM-Newton space telescope, we observed 5 rapidly rotating B-types stars to add to the previous list of observations. Comparing the observed X-ray luminosity and hardness ratio to that predicted by the XADM allows us to determine the role the added centrifugal force plays in the magnetospheric X-ray emission of these stars.

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Grosso, Nicolas, Kenji Hamaguchi, David A. Principe, and Joel H. Kastner. "Evidence for magnetic activity at starbirth: a powerful X-ray flare from the Class 0 protostar HOPS 383." Astronomy & Astrophysics 638 (June 2020): L4. http://dx.doi.org/10.1051/0004-6361/202038185.

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Context. Class 0 protostars represent the earliest evolutionary stage of solar-type stars, during which the majority of the system mass resides in an infalling envelope of gas and dust and is not yet in the central, nascent star. Although X-rays are a key signature of magnetic activity in more evolved protostars and young stars, whether such magnetic activity is present at the Class 0 stage is still debated. Aims. We aim to detect a bona fide Class 0 protostar in X-rays. Methods. We observed HOPS 383 in 2017 December in X-rays with the Chandra X-ray Observatory (∼84 ks) and in near-infrared imaging with the Southern Astrophysical Research telescope. Results. HOPS 383 was detected in X-rays during a powerful flare. This hard (E > 2 keV) X-ray counterpart was spatially coincident with the northwest 4 cm component of HOPS 383, which would be the base of the radio thermal jet launched by HOPS 383. The flare duration was ∼3.3 h; at the peak, the X-ray luminosity reached ∼4 × 1031 erg s−1 in the 2−8 keV energy band, a level at least an order of magnitude larger than that of the undetected quiescent emission from HOPS 383. The X-ray flare spectrum is highly absorbed (NH ∼ 7 × 1023 cm−2), and it displays a 6.4 keV emission line with an equivalent width of ∼1.1 keV, arising from neutral or low-ionization iron. Conclusions. The detection of a powerful X-ray flare from HOPS 383 constitutes direct proof that magnetic activity can be present at the earliest formative stages of solar-type stars.

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Conway, Gail M., and You-Hua Chu. "X-ray emission from Planetary Nebulae." Symposium - International Astronomical Union 180 (1997): 214–15. http://dx.doi.org/10.1017/s0074180900130360.

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X-ray emission from planetary nebulae (PNe) may originate from two sources: central stars which are 100,000–200,000 K will emit soft X-rays, and shocked fast stellar winds reaching 106–107 K will emit harder X-rays. The former are point sources, while the shocked winds are expected to be extended sources emitting continuously out to the inner wall of the visible nebular shell (Weaver et al. 1977; Wrigge & Wendker 1996).

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Nazé, Yaël. "The X-ray stellar population of the LMC." Proceedings of the International Astronomical Union 4, S256 (July 2008): 20–29. http://dx.doi.org/10.1017/s1743921308028202.

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AbstractIn the study of stars, the high energy domain occupies a place of choice, since it is the only one able to directly probe the most violent phenomena: indeed, young pre-main sequence objects, hot massive stars, or X-ray binaries are best revealed in X-rays. However, previously available X-ray observatories often provided only crude information on individual objects in the Magellanic Clouds. The advent of the highly efficient X-ray facilities XMM-Newton and Chandra has now dramatically increased the sensitivity and the spatial resolution available to X-ray astronomers, thus enabling a fairly easy determination of the properties of individual sources in the LMC.

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Tanaka, S., S. Kitamoto, T. Suzuki, K. Torii, M. F. Corcoran, and W. Waldron. "Chemical Abundances of Early Type Stars." Symposium - International Astronomical Union 188 (1998): 224–25. http://dx.doi.org/10.1017/s007418090011486x.

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X-rays from early-type stars are emitted by the corona or the stellar wind. The materials in the surface layer of early-type stars are not contaminated by nuclear reactions in the stellar inside. Therefore, abundance study of the early-type stars provides us an information of the abundances of the original gas. However, the X-ray observations indicate low-metallicity, which is about 0.3 times of cosmic abundances. This fact raises the problem on the cosmic abundances.

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Leahy, D. A. "Binary Stucture of Accreting Neutron Stars." Symposium - International Astronomical Union 188 (1998): 105–6. http://dx.doi.org/10.1017/s0074180900114536.

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The study of X-ray binaries has made great progress with the advent in the past few years of a number of very capable X-ray astronomy missions. These are reviewed, for example, by Bradt et al 1992, and a set of recent relevant papers in Makino and Mitsuda, 1997. For example, ASCA has allowed a significant increase in sensitivity and spectral resolution in 0.5-10 keV X-rays (Tanaka et al 1994). Many recent Compton/GRO results on X-ray binaries are reviewed in the proceedings of the Second Compton Symposium (Fichtel et al 1994). Another source of recent results from analysis of data from several satellite missions is the proceedings of the Evolution of X-ray Binaries (Holt & Day, 1994). In this short paper, the emphasis is on guiding the reader to some relevant literature.

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Ryspaeva, Elizaveta B., and Alexander F. Kholtygin. "A possible nonthermal X-ray emission from γ Cas analogues stars." Open Astronomy 30, no. 1 (January 1, 2021): 132–43. http://dx.doi.org/10.1515/astro-2021-0018.

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Abstract We analyze the archival XMM-Newton X-ray observations of 15 γ Cas analogue stars and two candidates for such objects. The EPIC spectra of the considered stars in the range of 0.2-8 keV were extracted and fitted by different models. Our estimates show that assuming the X-ray emission from γ Cas analogues to be totally thermal, their model plasma temperatures can reach anomalously high values. However including an additional power components to the model spectra leads to significant decreasing of the plasma temperatures. The spectral index of the power component is about 1.5, and the fraction of this in the total model flux is rather large (50-90%). Moreover, it decreases with expanding temperature of the X-ray emitting plasma as compared to typical OB stars. We conclude that γ Cas analogues can produce nonthermal X-ray emission within the framework of the Chen & White (1991) model, while if the nonthermal X-rays from typical OB stars exists, they should be generated by different processes.

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Pavlov, G. G., and V. E. Zavlin. "Polarization of Thermal X‐Rays from Isolated Neutron Stars." Astrophysical Journal 529, no. 2 (February 2000): 1011–18. http://dx.doi.org/10.1086/308313.

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Chlebowski, Tomasz, and Catharine D. Garmany. "On winds and X-rays of O-type stars." Astrophysical Journal 368 (February 1991): 241. http://dx.doi.org/10.1086/169687.

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Robrade, Jan. "X-rays from magnetic intermediate mass Ap/Bp stars." Advances in Space Research 58, no. 5 (September 2016): 727–38. http://dx.doi.org/10.1016/j.asr.2015.12.045.

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Freund, S., J. Robrade, P. C. Schneider, and J. H. M. M. Schmitt. "Updated X-ray view of the Hyades cluster." Astronomy & Astrophysics 640 (August 2020): A66. http://dx.doi.org/10.1051/0004-6361/201937304.

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Aims. We revisit the X-ray properties of the main sequence Hyades members and the relation between X-ray emission and stellar rotation. Methods. As an input catalog for Hyades members, we combined three recent Hyades membership lists derived from Gaia DR2 data that include the Hyades core and its tidal tails. We searched for X-ray detections of the main sequence Hyades members in the ROSAT all-sky survey, and pointings from ROSAT, the Chandra X-Ray Observatory, and XMM-Newton. Furthermore, we adopted rotation periods derived from Kepler’s K2 mission and other resources. Results. We find an X-ray detection for 281 of 1066 bona fide main sequence Hyades members and provide statistical upper limits for the undetected sources. The majority of the X-ray detected stars are located in the Hyades core because of its generally smaller distance to the Sun. F- and G-type stars have the highest detection fraction (72%), while K- and M-type dwarfs have lower detection rates (22%). The X-ray luminosities of the detected members range from ∼2 × 1027 erg s−1 for late M-type dwarfs to ∼2 × 1030 erg s−1 for active binaries. The X-ray luminosity distribution functions formally differ for the members in the core and tidal tails, which is likely caused by a larger fraction of field stars in our Hyades tails sample. Compared to previous studies, our sample is slightly fainter in X-rays due to differences in the Hyades membership list used; furthermore, we extend the X-ray luminosity distribution to fainter luminosities. The X-ray activity of F- and G-type stars is well defined at FX/Fbol ≈ 10−5. The fractional X-ray luminosity and its spread increases to later spectral types reaching the saturation limit (FX/Fbol ≈ 10−3) for members later than spectral type M3. Confirming previous results, the X-ray flux varies by less than a factor of three between epochs for the 104 Hyades members with multiple epoch data, significantly less than expected from solar-like activity cycles. Rotation periods are found for 204 Hyades members, with about half of them being detected in X-rays. The activity-rotation relation derived for the coeval Hyades members has properties very similar to those obtained by other authors investigating stars of different ages.

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Park, Jongwon, Massimo Ricotti, and Kazuyuki Sugimura. "Population III star formation in an X-ray background – II. Protostellar discs, multiplicity, and mass function of the stars." Monthly Notices of the Royal Astronomical Society 508, no. 4 (October 20, 2021): 6193–208. http://dx.doi.org/10.1093/mnras/stab3000.

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ABSTRACT Disc fragmentation plays an important role in determining the number of primordial stars (Pop III stars), their masses, and hence the initial mass function. In this second paper of a series, we explore the effect of uniform far-ultraviolet H2-photodissociating and X-ray radiation backgrounds on the formation of Pop III stars using a grid of high-resolution zoom-in simulations. We find that, in an X-ray background, protostellar discs have lower surface density and higher Toomre Q parameter, so they are more stable. For this reason, X-ray irradiated discs undergo fewer fragmentations and typically produce either binary systems or low-multiplicity systems. In contrast, the cases with weak or no X-ray irradiation produce systems with a typical multiplicity of 6 ± 3. In addition, the most massive protostar in each system is smaller by roughly a factor of 2 when the disc is irradiated by X-rays, due to lower accretion rate. With these two effects combined, the initial mass function of fragments becomes more top-heavy in a strong X-ray background and is well described by a power law with slope 1.53 and high-mass cutoff of 61 M⊙. Without X-rays, we find a slope 0.49 and cutoff mass of 229 M⊙. Finally, protostars migrate outward after their formation likely due to the accretion of high-angular momentum gas from outside and the migration is more frequent and significant in absence of X-ray irradiation.

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Evans, Nancy Remage, Scott Engle, Ignazio Pillitteri, Edward Guinan, H. Moritz Günther, Scott Wolk, Hilding Neilson, et al. "X-Rays in Cepheids: Identifying Low-mass Companions of Intermediate-mass Stars*." Astrophysical Journal 938, no. 2 (October 1, 2022): 153. http://dx.doi.org/10.3847/1538-4357/ac6fdf.

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Abstract X-ray observations have been made of a sample of 20 classical Cepheids, including two new observations (Polaris and l Car) reported here. The occurrence of X-ray flux around the pulsation cycle is discussed. Three Cepheids are detected (δ Cep, β Dor, and Polaris). X-rays have also been detected from the low-mass F, G, and K companions of four Cepheids (V473 Lyr, R Cru, V659 Cen, and W Sgr) and one hot companion (S Mus). Upper limits on the X-ray flux of the remaining Cepheids provide an estimate that 28% have low-mass companions. This fraction of low-mass companions in intermediate-mass Cepheids is significantly lower than expected from random pairing with the field initial mass function (IMF). Combining the companion fraction from X-rays with that from ultraviolet observations results in a binary/multiple fraction of 57% ± 12% for Cepheids with ratios q > 0.1 and separations a >1 au. This is a lower limit since M stars are not included. X-ray observations detect less massive companions than other existing studies of intermediate-mass stars. Our measured occurrence rate of unresolved, low-mass companions to Cepheids suggests that intermediate-period binaries derive from a combination of disk and core fragmentation and accretion. This yields a hybrid mass ratio distribution that is skewed toward small values compared to a uniform distribution but is still top-heavy compared to random pairings drawn from the IMF.

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Güdel, M. "On radio emission and related X-rays in solar-like stellar coronae." Symposium - International Astronomical Union 176 (1996): 485–92. http://dx.doi.org/10.1017/s0074180900083509.

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Selected results relevant to coronal structuring in solar-like single stars based on combined radio and X-ray information are presented. Nonthermal radio and thermal soft X-ray emissions from coronal solar-like stars provide direct information on particle acceleration and coronal heating in the magnetically confined outer atmospheres. The structural relationship between the emission sources is mostly inferred from indirect arguments such as rotational modulation or gyroresonance emission. Direct VLBI provides evidence of resolved, extended coronae on solar-like stars.

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Cohen, David H., Emma E. Wollman, and Maurice A. Leutenegger. "X-ray spectral diagnostics of activity in massive stars." Proceedings of the International Astronomical Union 6, S272 (July 2010): 348–53. http://dx.doi.org/10.1017/s1743921311010763.

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AbstractX-rays give direct evidence of instabilities, time-variable structure, and shock heating in the winds of O stars. The observed broad X-ray emission lines provide information about the kinematics of shock-heated wind plasma, enabling us to test wind-shock models. And their shapes provide information about wind absorption, and thus about the wind mass-loss rates. Mass-loss rates determined from X-ray line profiles are not sensitive to density-squared clumping effects, and indicate mass-loss rate reductions of factors of 3 to 6 over traditional diagnostics that suffer from density-squared effects. Broad-band X-ray spectral energy distributions also provide mass-loss rate information via soft X-ray absorption signatures. In some cases, the degree of wind absorption is so high, that the hardening of the X-ray SED can be quite significant. We discuss these results as applied to the early O stars ζ Pup (O4 If), 9 Sgr (O4 V((f))), and HD 93129A (O2 If*).

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Hillier, D. John. "What Do We Really Know About the Winds of Massive Stars?" Proceedings of the International Astronomical Union 3, S250 (December 2007): 89–96. http://dx.doi.org/10.1017/s1743921308020371.

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AbstractThe standard theory of radiation driven winds has provided a useful framework to understand stellar winds arising from massive stars (O stars, Wolf-Rayet stars, and luminous blue variables). However, with new diagnostics, and advances in spectral modeling, deficiencies in our understanding of stellar winds have been thrust to the forefront of our research efforts. Spectroscopic observations and analyses have shown the importance of inhomogeneities in stellar winds, and revealed that there are fundamental discrepancies between predicted and theoretical mass-loss rates. For late O stars, spectroscopic analyses derive mass-loss rates significantly lower than predicted. For all O stars, observed X-ray fluxes are difficult to reproduce using standard shock theory, while observed X-ray profiles indicate lower mass-loss rates, the potential importance of porosity effects, and an origin surprisingly close to the stellar photosphere. In O stars with weak winds, X-rays play a crucial role in determining the ionization balance, and must be taken into account.

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Gagné, Marc, and Eduardo Janot Pacheco. "Discussion – Normal and active OB stars as extreme condition test beds." Proceedings of the International Astronomical Union 6, S272 (July 2010): 593–99. http://dx.doi.org/10.1017/s1743921311011483.

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Guillaume Dubus: As far as I can tell your model is exactly the same as standard models for high-mass x-ray binaries, the on difference is that you have a mechanism for generating γ rays. My question is, we know that there are dozens of other systems that are just LS 5039, so why are they not emitting γ rays?

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Farrell, Delaney, Pierre Baldi, Jordan Ott, Aishik Ghosh, Andrew W. Steiner, Atharva Kavitkar, Lee Lindblom, Daniel Whiteson, and Fridolin Weber. "Deducing neutron star equation of state parameters directly from telescope spectra with uncertainty-aware machine learning." Journal of Cosmology and Astroparticle Physics 2023, no. 02 (February 1, 2023): 016. http://dx.doi.org/10.1088/1475-7516/2023/02/016.

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Abstract Neutron stars provide a unique laboratory for studying matter at extreme pressures and densities. While there is no direct way to explore their interior structure, X-rays emitted from these stars can indirectly provide clues to the equation of state (EOS) of the superdense nuclear matter through the inference of the star's mass and radius. However, inference of EOS directly from a star's X-ray spectra is extremely challenging and is complicated by systematic uncertainties. The current state of the art is to use simulation-based likelihoods in a piece-wise method which relies on certain theoretical assumptions and simplifications about the uncertainties. It first infers the star's mass and radius to reduce the dimensionality of the problem, and from those quantities infer the EOS. We demonstrate a series of enhancements to the state of the art, in terms of realistic uncertainty quantification and a path towards circumventing the need for theoretical assumptions to infer physical properties with machine learning. We also demonstrate novel inference of the EOS directly from the high-dimensional spectra of observed stars, avoiding the intermediate mass-radius step. Our network is conditioned on the sources of uncertainty of each star, allowing for natural and complete propagation of uncertainties to the EOS.

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Günther, Hans Moritz, Carl Melis, J. Robrade, P. C. Schneider, Scott J. Wolk, and Rakesh K. Yadav. "Coronal and Chromospheric Emission in A-type Stars." Astronomical Journal 164, no. 1 (June 14, 2022): 8. http://dx.doi.org/10.3847/1538-3881/ac6ef6.

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Abstract Cool stars on the main sequence generate X-rays from coronal activity, powered by a convective dynamo. With increasing temperature, the convective envelope becomes smaller and X-ray emission fainter. We present Chandra/HRC-I observations of four single stars with early A spectral types. Only the coolest star of this sample, τ 3 Eri (T eff ≈ 8, 000 K), is detected with log ( L X / L bol ) = − 7.6 while the three hotter stars (T eff ≥ 8, 000 K), namely δ Leo, β Leo, and ι Cen, remain undetected with upper limits log ( L X / L bol ) < − 8.4 . The drop in X-ray emission thus occurs in a narrow range of effective temperatures around ∼8100 K and matches the drop of activity in the C iii and O vi transition region lines.

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PAREDES, J. M. "VHE GAMMA-RAYS FROM GALACTIC X-RAY BINARY SYSTEMS." International Journal of Modern Physics D 17, no. 10 (September 2008): 1849–58. http://dx.doi.org/10.1142/s0218271808013480.

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The detection of TeV gamma-rays from LS 5039 and the binary pulsar PSR B1259–63 by HESS, and from LS I +61 303 and the stellar-mass black hole Cygnus X-1 by MAGIC, provides clear evidence of very efficient acceleration of particles to multi-TeV energies in X-ray binaries. These observations demonstrate the richness of nonthermal phenomena in compact galactic objects containing relativistic outflows or winds produced near black holes and neutron stars. I review here some of the main observational results on very high energy (VHE) γ-ray emission from X-ray binaries, as well as some of the proposed scenarios to explain the production of VHE γ-rays. I put special emphasis on the flare TeV emission, suggesting that the flaring activity might be a common phenomena in X-ray binaries.

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Schmitt, J. H. M. M. "Rosat Observations of Stellar Flares." International Astronomical Union Colloquium 142 (1994): 735–42. http://dx.doi.org/10.1017/s0252921100078040.

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AbstractX-ray observations of stellar flares obtained during the ROSAT all-sky survey as well as in the ROSAT pointing program are discussed. The ROSAT all-sky survey allowed—for the first time—an unbiased search for stellar flares among all types of stars. A fundamentally new result obtained is that flares can occur on all types of late-type stars, thus supporting the view that the X-ray emission from these stars is controlled by magnetic processes. Long-duration flares can be studied with the all-sky survey data particularly well, and an especially well-observed long-duration flare event on the flare star EV Lacertae is presented and discussed in detail. Finally, the issue of time variability on the shortest detectable timescales and the question of microflaring is discussed using ROSAT data from a pointed observation of UV Ceti.Subject headings: stars: coronae — stars: flare — stars: late-type — X-rays: stars

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Langer, N., D. Baade, J. Bodensteiner, J. Greiner, Th Rivinius, Ch Martayan, and C. C. Borre. "γ Cas stars: Normal Be stars with discs impacted by the wind of a helium-star companion?" Astronomy & Astrophysics 633 (January 2020): A40. http://dx.doi.org/10.1051/0004-6361/201936736.

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γ Cas stars are a ∼1% minority among classical Be stars with hard (≥5−10 keV), but only moderately strong continuous thermal X-ray flux, and mostly very early-B spectral type. The X-ray flux has been suggested to originate from matter accelerated via magnetic disc-star interaction, by a rapidly rotating neutron star (NS) companion via the propeller effect, or by accretion onto a white dwarf (WD) companion. In view of the growing number of identified γ Cas stars and the only imperfect matches between these suggestions and the observations, alternative models should be pursued. Two of the three best-observed γ Cas stars, γ Cas itself and π Aqr, have a low-mass companion with low optical flux, whereas interferometry of BZ Cru is inconclusive. Binary-evolution models are examined for their ability to produce such systems. The OB+He-star stage of post-mass transfer binaries, which is otherwise observationally unaccounted, can potentially reproduce many observed properties of γ Cas stars. The interaction of the fast wind of helium stars with the circumstellar disc and/or with the wind of Be stars may give rise to the production of hard X-rays. While not modelling this process, it is shown that the energy budget is favourable, and that the wind velocities may lead to hard X-rays, as observed in γ Cas stars. Furthermore, the observed number of these objects appears to be consistent with the evolutionary models. Within the Be+He-star binary model, the Be stars in γ-Cas stars are conventional classical Be stars. They are encompassed by O-star+Wolf-Rayet systems towards higher mass, where no stable Be decretion discs exist, and by Be+sdO systems at lower mass, where the sdO winds may be too weak to cause the γ Cas phenomenon. In decreasing order of the helium-star mass, the descendants could be Be+black-hole, Be+NS, or Be+WD binaries. The interaction between the helium-star wind and the disc may provide new diagnostics of the outer disc.

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Krtička, Jiří, Jiří Kubát, and Iva Krtičková. "Wind inhibition by X-ray irradiation in high-mass X-ray binaries." Proceedings of the International Astronomical Union 12, S329 (November 2016): 417. http://dx.doi.org/10.1017/s1743921317002381.

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AbstractWinds of hot massive stars are driven radiatively by light absorption in the lines of heavier elements. Therefore, the radiative force depends on the wind ionization. That is the reason why the accretion powered X-ray emission of high-mass X-ray binaries influences the radiative force and may even lead to wind inhibition. We model the effect of X-ray irradiation on the stellar wind in high-mass X-ray binaries. The influence of X-rays is given by the X-ray luminosity, by the optical depth between a given point and the X-ray source, and by the distance to the X-ray source. The influence of X-rays is stronger for higher X-ray luminosities and in closer proximity of the X-ray source. There is a forbidden area in the diagrams of X-ray luminosity vs. the optical depth parameter. The observations agree with theoretical predictions, because all wind-powered high-mass X-ray binary primaries lie outside the forbidden area. The positions of real binaries in the diagram indicate that their X-ray luminosities are self-regulated.

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Nagase, Fumiaki. "Photoionized Plasmas in X-Ray Binary Pulsars: ASCA Observations." Symposium - International Astronomical Union 188 (1998): 101–4. http://dx.doi.org/10.1017/s0074180900114524.

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Massive X-ray binary pulsars have often evolved early-type companion stars which emanate strong stellar winds. X-rays emitted from the accreting neutron star irradiate and ionize the surrounding stellar wind, thus forming a photoionized sphere surrounding the neutron star. The photoionization structure of matter surrounding the neutron star was calculated by Hatchett and McCray (1977) and McCray et al. (1984), for Cen X-3 and Vela X-1 respectively.

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Journal articles on the topic 'X-rays: stars'

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