@article{LetoTrigilioOskinovaetal.2017, author = {Leto, Paolo and Trigilio, C. and Oskinova, Lida and Ignace, R. and Buemi, C. S. and Umana, G. and Ingallinera, A. and Todt, Helge Tobias and Leone, F.}, title = {The detection of variable radio emission from the fast rotating magnetic hot B-star HR 7355 and evidence for its X-ray aurorae}, series = {Monthly notices of the Royal Astronomical Society}, volume = {467}, journal = {Monthly notices of the Royal Astronomical Society}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stx267}, pages = {2820 -- 2833}, year = {2017}, abstract = {In this paper, we investigate the multiwavelength properties of the magnetic early B-type star HR 7355. We present its radio light curves at several frequencies, taken with the Jansky Very Large Array, and X-ray spectra, taken with the XMM-Newton X-ray telescope. Modelling of the radio light curves for the Stokes I and V provides a quantitative analysis of the HR 7355 magnetosphere. A comparison between HR 7355 and a similar analysis for the Ap star CU Vir allows us to study how the different physical parameters of the two stars affect the structure of the respective magnetospheres where the non-thermal electrons originate. Our model includes a cold thermal plasma component that accumulates at high magnetic latitudes that influences the radio regime, but does not give rise to X-ray emission. Instead, the thermal X-ray emission arises from shocks generated by wind stream collisions close to the magnetic equatorial plane. The analysis of the X-ray spectrum of HR 7355 also suggests the presence of a non-thermal radiation. Comparison between the spectral index of the power-law X-ray energy distribution with the non-thermal electron energy distribution indicates that the non-thermal X-ray component could be the auroral signature of the non-thermal electrons that impact the stellar surface, the same non-thermal electrons that are responsible for the observed radio emission. On the basis of our analysis, we suggest a novel model that simultaneously explains the X-ray and the radio features of HR 7355 and is likely relevant for magnetospheres of other magnetic early-type stars.}, language = {en} } @article{AbramowskiAharonianBenkhalietal.2014, author = {Abramowski, Attila and Aharonian, Felix A. and Benkhali, Faical Ait and Akhperjanian, A. G. and Ang{\"u}ner, Ekrem Oǧuzhan and Anton, Gisela and Balenderan, Shangkari and Balzer, Arnim and Barnacka, Anna and Becherini, Yvonne and Tjus, J. Becker and Bernl{\"o}hr, K. and Birsin, E. and Bissaldi, E. and Biteau, Jonathan and Boettcher, Markus and Boisson, Catherine and Bolmont, J. and Bordas, Pol and Brucker, J. and Brun, Francois and Brun, Pierre and Bulik, Tomasz and Carrigan, Svenja and Casanova, Sabrina and Cerruti, M. and Chadwick, Paula M. and Chalme-Calvet, R. and Chaves, Ryan C. G. and Cheesebrough, A. and Chretien, M. and Colafrancesco, Sergio and Cologna, Gabriele and Conrad, Jan and Couturier, C. and Dalton, M. and Daniel, M. K. and Davids, I. D. and Degrange, B. and Deil, C. and deWilt, P. and Dicldnson, H. J. and Djannati-Ata{\"i}, A. and Domainko, W. and Dubus, G. and Dutson, K. and Dyks, J. and Dyrda, M. and Edwards, T. and Egberts, Kathrin and Eger, P. and Espigat, P. and Farnier, C. and Fegan, S. and Feinstein, F. and Fernandes, M. V. and Fernandez, D. and Fiasson, A. and Fontaine, G. and Foerster, A. and Fuessling, Matthias and Gajdus, M. and Gallant, Y. A. and Garrigoux, T. and Giebels, B. and Glicenstein, J. F. and Grondin, M-H and Grudzinska, M. and Haeffner, S. and Hahn, J. and Harris, J. and Heinzelmann, G. and Henri, G. and Hermann, G. and Hervet, O. and Hillert, A. and Hinton, James Anthony and Hofmann, W. and Hofverberg, P. and Holler, Markus and Horns, D. and Jacholkowska, A. and Jahn, C. and Jamrozy, M. and Janiak, M. and Jankowsky, F. and Jung, I. and Kastendieck, M. A. and Katarzynski, K. and Katz, U. and Kaufmann, S. and Khelifi, B. and Kieffer, M. and Klepser, S. and Klochkov, D. and Kluzniak, W. and Kneiske, T. and Kolitzus, D. and Komin, Nu and Kosack, K. and Krakau, S. and Krayzel, F. and Krueger, P. P. and Laffon, H. and Lamanna, G. and Lefaucheur, J. and Lemiere, A. and Lemoine-Goumard, M. and Lenain, J-P and Lennarz, D. and Lohse, T. and Lopatin, A. and Lu, C-C and Marandon, V. and Marcowith, Alexandre and Marx, R. and Maurin, G. and Maxted, N. and Mayer, M. and McComb, T. J. L. and Mehault, J. and Menzler, U. and Meyer, M. and Moderski, R. and Mohamed, M. and Moulin, Emmanuel and Murach, T. and Naumann, C. L. and de Naurois, M. and Niemiec, J. and Nolan, S. J. and Oakes, L. and Ohm, S. and Wilhelmi, E. de Ona and Opitz, B. and Ostrowski, M. and Oya, I. and Panter, M. and Parsons, R. D. and Arribas, M. Paz and Pekeur, N. W. and Pelletier, G. and Perez, J. and Petrucci, P-O and Peyaud, B. and Pita, S. and Poon, H. and Puehlhofer, G. and Punch, M. and Quirrenbach, A. and Raab, S. and Raue, M. and Reimer, A. and Reimer, O. and Renaud, M. and de los Reyes, R. and Rieger, F. and Rob, L. and Romoli, C. and Rosier-Lees, S. and Rowell, G. and Rudak, B. and Rulten, C. B. and Sahakian, V. and Sanchez, David M. and Santangelo, A. and Schlickeiser, R. and Schuessler, F. and Schulz, A. and Schwanke, U. and Schwarzburg, S. and Schwemmer, S. and Sol, H. and Spengler, G. and Spies, F. and Stawarz, L. and Steenkamp, R. and Stegmann, Christian and Stinzing, F. and Stycz, K. and Sushch, Iurii and Szostek, A. and Tam, P. H. T. and Tavernet, J-P and Tavernier, T. and Taylor, A. M. and Terrier, R. and Tluczykont, M. and Trichard, C. and Valerius, K. and van Eldik, C. and Vasileiadis, G. and Venter, C. and Viana, A. and Vincent, P. and Voelk, H. J. and Volpe, F. and Vorster, M. and Wagner, S. J. and Wagner, P. and Ward, M. and Weidinger, M. and Weitzel, Q. and White, R. and Wierzcholska, A. and Willmann, P. and Woernlein, A. and Wouters, D. and Zacharias, M. and Zajczyk, A. and Zdziarski, A. A. and Zech, Alraune and Zechlin, H-S}, title = {Search for TeV Gamma-ray emission from GRB 100621A, an extremely bright GRB in X-rays, with HESS}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {565}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, organization = {HESS Collaboration}, issn = {0004-6361}, doi = {10.1051/0004-6361/201322984}, pages = {6}, year = {2014}, abstract = {The long gamma-ray burst (GRB) 100621A, at the time the brightest X-ray transient ever detected by Swift-XRT in the 0.3-10 keV range, has been observed with the H.E.S.S. imaging air Cherenkov telescope array, sensitive to gamma radiation in the very-high-energy (VHE, >100 GeV) regime. Due to its relatively small redshift of z similar to 0.5, the favourable position in the southern sky and the relatively short follow-up time (<700 s after the satellite trigger) of the H.E.S.S. observations, this GRB could be within the sensitivity reach of the HESS. instrument. The analysis of the HESS. data shows no indication of emission and yields an integral flux upper limit above similar to 380 GeV of 4.2 x 10(-12) cm(-2) s(-1) s (95\% confidence level), assuming a simple Band function extension model. A comparison to a spectral-temporal model, normalised to the prompt flux at sub-MeV energies, constraints the existence of a temporally extended and strong additional hard power law, as has been observed in the other bright X-ray GRB 130427A. A comparison between the HESS. upper limit and the contemporaneous energy output in X-rays constrains the ratio between the X-ray and VHE gamma-ray fluxes to be greater than 0.4. This value is an important quantity for modelling the afterglow and can constrain leptonic emission scenarios, where leptons are responsible for the X-ray emission and might produce VHE gamma rays.}, language = {en} } @article{RobradeOskinovaSchmittetal.2018, author = {Robrade, Jan and Oskinova, Lida and Schmitt, J. H. M. M. and Leto, Paolo and Trigilio, C.}, title = {Outstanding X-ray emission from the stellar radio pulsar CU Virginis}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {619}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201833492}, pages = {6}, year = {2018}, abstract = {Context. Among the intermediate-mass magnetic chemically peculiar (MCP) stars, CU Vir is one of the most intriguing objects. Its 100\% circularly polarized beams of radio emission sweep the Earth as the star rotates, thereby making this strongly magnetic star the prototype of a class of nondegenerate stellar radio pulsars. While CU Vir is well studied in radio, its high-energy properties are not known. Yet, X-ray emission is expected from stellar magnetospheres and confined stellar winds. Aims. Using X-ray data we aim to test CU Vir for intrinsic X-ray emission and investigate mechanisms responsible for its generation. Methods. We present X-ray observations performed with XMM-Newton and Chandra and study obtained X-ray images, light curves, and spectra. Basic X-ray properties are derived from spectral modelling and are compared with model predictions. In this context we investigate potential thermal and nonthermal X-ray emission scenarios. Results. We detect an X-ray source at the position of CU Vir. With LX approximate to 3 x 10(28) erg s(-1) it is moderately X-ray bright, but the spectrum is extremely hard compared to other Ap stars. Spectral modelling requires multi-component models with predominant hot plasma at temperatures of about T-X = 25MK or, alternatively, a nonthermal spectral component. Both types of model provide a virtually equivalent description of the X-ray spectra. The Chandra observation was performed six years later than those by XMM-Newton, yet the source has similar X-ray flux and spectrum, suggesting a steady and persistent X-ray emission. This is further confirmed by the X-ray light curves that show only mild X-ray variability. Conclusions. CU Vir is also an exceptional star at X-ray energies. To explain its full X-ray properties, a generating mechanism beyond standard explanations, like the presence of a low-mass companion or magnetically confined wind-shocks, is required. Magnetospheric activity might be present or, as proposed for fast-rotating strongly magnetic Bp stars, the X-ray emission of CU Vir is predominantly auroral in nature.}, language = {en} } @article{OskinovaTodtHuenemoerderetal.2015, author = {Oskinova, Lida and Todt, Helge Tobias and Huenemoerder, David P. and Hubrig, Swetlana and Ignace, Richard and Hamann, Wolf-Rainer and Balona, Luis}, title = {On X-ray pulsations in beta Cephei-type variables}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {577}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201525908}, pages = {5}, year = {2015}, abstract = {Context. beta Cep-type variables are early B-type stars that are characterized by oscillations observable in their optical light curves. At least one beta Cep-variable also shows periodic variability in X-rays. Aims. Here we study the X-ray light curves in a sample of beta Cep-variables to investigate how common X-ray pulsations are for this type of stars. Methods. We searched the Chandra and XMM-Newton X-ray archives and selected stars that were observed by these telescopes for at least three optical pulsational periods. We retrieved and analyzed the X-ray data for kappa Sco, beta Cru, and alpha Vir. The X-ray light curves of these objects were studied to test for their variability and periodicity. Results. While there is a weak indication for X-ray variability in beta Cru, we find no statistically significant evidence of X-ray pulsations in any of our sample stars. This might be due either to the insufficient data quality or to the physical lack of modulations. New, more sensitive observations should settle this question.}, language = {en} } @article{HuenemoerderOskinovaIgnaceetal.2012, author = {H{\"u}nem{\"o}rder, David P. and Oskinova, Lida and Ignace, Richard and Waldron, Wayne L. and Todt, Helge Tobias and Hamaguchi, Kenji and Kitamoto, Shunji}, title = {On the weak-wind problem in massive stars X-ray spectra reveal a massive hot wind in mu columbaea}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, volume = {756}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {2041-8205}, doi = {10.1088/2041-8205/756/2/L34}, pages = {5}, year = {2012}, abstract = {mu Columbae is a prototypical weak-wind O star for which we have obtained a high-resolution X-ray spectrum with the Chandra LETG/ACIS instrument and a low-resolution spectrum with Suzaku. This allows us, for the first time, to investigate the role of X-rays on the wind structure in a bona fide weak-wind system and to determine whether there actually is a massive hot wind. The X-ray emission measure indicates that the outflow is an order of magnitude greater than that derived from UV lines and is commensurate with the nominal wind-luminosity relationship for O stars. Therefore, the "weak-wind problem"-identified from cool wind UV/optical spectra-is largely resolved by accounting for the hot wind seen in X-rays. From X-ray line profiles, Doppler shifts, and relative strengths, we find that this weak-wind star is typical of other late O dwarfs. The X-ray spectra do not suggest a magnetically confined plasma-the spectrum is soft and lines are broadened; Suzaku spectra confirm the lack of emission above 2 keV. Nor do the relative line shifts and widths suggest any wind decoupling by ions. The He-like triplets indicate that the bulk of the X-ray emission is formed rather close to the star, within five stellar radii. Our results challenge the idea that some OB stars are "weak-wind" stars that deviate from the standard wind-luminosity relationship. The wind is not weak, but it is hot and its bulk is only detectable in X-rays.}, language = {en} } @article{OskinovaHuenemoerderHamannetal.2017, author = {Oskinova, Lida and Huenemoerder, D. P. and Hamann, Wolf-Rainer and Shenar, Tomer and Sander, Andreas Alexander Christoph and Ignace, R. and Todt, Helge Tobias and Hainich, Rainer}, title = {On the Binary Nature of Massive Blue Hypergiants: High-resolution X-Ray Spectroscopy Suggests That Cyg OB2 12 is a Colliding Wind Binary}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {845}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/aa7e79}, pages = {11}, year = {2017}, abstract = {The blue hypergiant Cyg OB2 12 (B3Ia(+)) is a representative member of the class of very massive stars in a poorly understood evolutionary stage. We obtained its high-resolution X-ray spectrum using the Chandra observatory. PoWR model atmospheres were calculated to provide realistic wind opacities and to establish the wind density structure. We find that collisional de-excitation is the dominant mechanism depopulating the metastable upper levels of the forbidden lines of the He-like ions Si XIV and Mg XII. Comparison between the model and observations reveals that X-ray emission is produced in a dense plasma, which could reside only at the photosphere or in a colliding wind zone between binary components. The observed X-ray spectra are well-fitted by thermal plasma models, with average temperatures in excess of 10 MK. The wind speed in Cyg OB2 12 is not high enough to power such high temperatures, but the collision of two winds in a binary system can be sufficient. We used archival data to investigate the X-ray properties of other blue hypergiants. In general, stars of this class are not detected as X-ray sources. We suggest that our new Chandra observations of Cyg OB2 12 can be best explained if Cyg OB2 12 is a colliding wind binary possessing a late O-type companion. This makes Cyg OB2 12 only the second binary system among the 16 known Galactic hypergiants. This low binary fraction indicates that the blue hypergiants are likely products of massive binary evolution during which they either accreted a significant amount of mass or already merged with their companions.}, language = {en} } @article{HubrigSchoellerKholtyginetal.2015, author = {Hubrig, Swetlana and Sch{\"o}ller, Markus and Kholtygin, Alexander F. and Tsumura, Hiroki and Hoshino, Akio and Kitamoto, Shunji and Oskinova, Lida and Ignace, Richard and Todt, Helge Tobias and Ilyin, Ilya}, title = {New multiwavelength observations of the Of?p star CPD-28 degrees 2561}, series = {Monthly notices of the Royal Astronomical Society}, volume = {447}, journal = {Monthly notices of the Royal Astronomical Society}, number = {2}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stu2516}, pages = {1885 -- 1894}, year = {2015}, abstract = {A rather strong mean longitudinal magnetic field of the order of a few hundred gauss was detected a few years ago in the Of?p star CPD -28 degrees 2561 using FORS2 (FOcal Reducer low dispersion Spectrograph 2) low-resolution spectropolarimetric observations. In this work, we present additional low-resolution spectropolarimetric observations obtained during several weeks in 2013 December using FORS 2 mounted at the 8-m Antu telescope of the Very Large Telescope (VLT). These observations cover a little less than half of the stellar rotation period of 73.41 d mentioned in the literature. The behaviour of the mean longitudinal magnetic field is consistent with the assumption of a single-wave variation during the stellar rotation cycle, indicating a dominant dipolar contribution to the magnetic field topology. The estimated polar strength of the surface dipole B-d is larger than 1.15 kG. Further, we compared the behaviour of the line profiles of various elements at different rotation phases associated with different magnetic field strengths. The strongest contribution of the emission component is observed at the phases when the magnetic field shows a negative or positive extremum. The comparison of the spectral behaviour of CPD -28 degrees 2561 with that of another Of?p star, HD 148937 of similar spectral type, reveals remarkable differences in the degree of variability between both stars. Finally, we present new X-ray observations obtained with the Suzaku X-ray Observatory. We report that the star is X-ray bright with log L-X/L-bol approximate to -5.7. The low-resolution X-ray spectra reveal the presence of a plasma heated up to 24 MK. We associate the 24 MK plasma in CPD -28 degrees 2561 with the presence of a kG strong magnetic field capable to confine stellar wind.}, language = {en} } @article{SandinSteffenSchoenberneretal.2016, author = {Sandin, C. and Steffen, M. and Schoenberner, D. and R{\"u}hling, Ute}, title = {Hot bubbles of planetary nebulae with hydrogen-deficient winds I. Heat conduction in a chemically stratified plasma}, series = {Frontiers in psychology}, volume = {586}, journal = {Frontiers in psychology}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201527357}, pages = {11}, year = {2016}, abstract = {Heat conduction has been found a plausible solution to explain discrepancies between expected and measured temperatures in hot bubbles of planetary nebulae (PNe). While the heat conduction process depends on the chemical composition, to date it has been exclusively studied for pure hydrogen plasmas in PNe. A smaller population of PNe show hydrogen-deficient and helium-and carbon-enriched surfaces surrounded by bubbles of the same composition; considerable differences are expected in physical properties of these objects in comparison to the pure hydrogen case. The aim of this study is to explore how a chemistry-dependent formulation of the heat conduction affects physical properties and how it affects the X-ray emission from PN bubbles of hydrogen-deficient stars. We extend the description of heat conduction in our radiation hydrodynamics code to work with any chemical composition. We then compare the bubble-formation process with a representative PN model using both the new and the old descriptions. We also compare differences in the resulting X-ray temperature and luminosity observables of the two descriptions. The improved equations show that the heat conduction in our representative model of a hydrogen-deficient PN is nearly as efficient with the chemistry-dependent description; a lower value on the diffusion coefficient is compensated by a slightly steeper temperature gradient. The bubble becomes somewhat hotter with the improved equations, but differences are otherwise minute. The observable properties of the bubble in terms of the X-ray temperature and luminosity are seemingly unaffected.}, language = {en} } @article{OskinovaGayleyHamannetal.2012, author = {Oskinova, Lida and Gayley, K. G. and Hamann, Wolf-Rainer and Huenemoerder, D. P. and Ignace, R. and Pollock, A. M. T.}, title = {HIGH-RESOLUTION X-RAY SPECTROSCOPY REVEALS THE SPECIAL NATURE OF WOLF-RAYET STAR WINDS}, series = {ASTROPHYSICAL JOURNAL LETTERS}, volume = {747}, journal = {ASTROPHYSICAL JOURNAL LETTERS}, number = {2}, publisher = {IOP PUBLISHING LTD}, address = {BRISTOL}, issn = {2041-8205}, doi = {10.1088/2041-8205/747/2/L25}, pages = {6}, year = {2012}, abstract = {We present the first high-resolutionX-ray spectrum of a putatively singleWolf-Rayet (WR) star. 400 ks observations of WR 6 by the XMM-Newton telescope resulted in a superb quality high-resolution X-ray spectrum. Spectral analysis reveals that the X-rays originate far out in the stellar wind, more than 30 stellar radii from the photosphere, and thus outside the wind acceleration zone where the line-driving instability (LDI) could create shocks. The X-ray emitting plasma reaches temperatures up to 50 MK and is embedded within the unshocked, "cool" stellar wind as revealed by characteristic spectral signatures. We detect a fluorescent Fe line at approximate to 6.4 keV. The presence of fluorescence is consistent with a two-component medium, where the cool wind is permeated with the hot X-ray emitting plasma. The wind must have a very porous structure to allow the observed amount of X-rays to escape. We find that neither the LDI nor any alternative binary scenario can explain the data. We suggest a scenario where X-rays are produced when the fast wind rams into slow "sticky clumps" that resist acceleration. Our new data show that the X-rays in single WR star are generated by some special mechanism different from the one operating in the O-star winds.}, language = {en} } @article{ShraderHamaguchiSturneretal.2015, author = {Shrader, C. R. and Hamaguchi, K. and Sturner, S. J. and Oskinova, Lida and Almeyda, T. and Petre, R.}, title = {Hifg-energy properties of the enigmatic be STAR gamma Cassiopeiae}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {799}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/799/1/84}, pages = {10}, year = {2015}, abstract = {We present the results of a broadband X-ray study of the enigmatic Be star Gamma Cassiopeiae (herein gamma Cas) based on observations made with both the Suzaku and INTEGRAL observatories.. Cas has long been recognized as the prototypical example of a small subclass of Be stars with moderately strong X-ray emission dominated by a hot thermal component in the 0.5-12 keV energy range (L-x approximate to 10(32)-10(33) erg s(-1)). This places them at the high end of the known luminosity distribution for stellar emission, but several orders of magnitude below typical accretion-powered Be X-ray binaries. The INTEGRAL observations spanned an eight-year baseline and represent the deepest measurement to date at energies above similar to 50 keV. We find that the INTEGRAL data are consistent within statistics to a constant intensity source above 20 keV, with emission extending up to similar to 100 keV, and that searches for all of the previously reported periodicities of the system at lower energies led to null results. We further find that our combined Suzaku and INTEGRAL spectrum, which we suggest is the most accurate broadband X-ray measurement of gamma Cas to date, is fitted extremely well with a thermal plasma emission model with a single absorption component. We found no compelling need for an additional non-thermal high-energy component. We discuss these results in the context of a currently favored models for gamma Cas and its analogs.}, language = {en} }