@article{ShenarRichardsonSablowskietal.2017, author = {Shenar, Tomer and Richardson, N. D. and Sablowski, Daniel P. and Hainich, Rainer and Sana, H. and Moffat, A. F. J. and Todt, Helge Tobias and Hamann, Wolf-Rainer and Oskinova, Lida and Sander, Andreas Alexander Christoph and Tramper, Frank and Langer, Norbert and Bonanos, Alceste Z. and de Mink, Selma E. and Gr{\"a}fener, G. and Crowther, Paul and Vink, J. S. and Almeida, Leonardo A. and de Koter, A. and Barb{\´a}, Rodolfo and Herrero, A. and Ulaczyk, Krzysztof}, title = {The tarantula massive binary monitoring}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {598}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201629621}, pages = {16}, year = {2017}, abstract = {We present the first SB2 orbital solution and disentanglement of the massive Wolf-Rayet binary R145 (P = 159 d) located in the Large Magellanic Cloud. The primary was claimed to have a stellar mass greater than 300 M-circle dot, making it a candidate for being the most massive star known to date. While the primary is a known late-type, H-rich Wolf-Rayet star (WN6h), the secondary has so far not been unambiguously detected. Using moderate-resolution spectra, we are able to derive accurate radial velocities for both components. By performing simultaneous orbital and polarimetric analyses, we derive the complete set of orbital parameters, including the inclination. The spectra are disentangled and spectroscopically analyzed, and an analysis of the wind-wind collision zone is conducted. The disentangled spectra and our models are consistent with a WN6h type for the primary and suggest that the secondary is an O3.5 If*/WN7 type star. We derive a high eccentricity of e = 0 : 78 and minimum masses of M-1 sin(3) i approximate to M-2 sin(3) i = 13 +/- 2 M-circle dot, with q = M-2/M-1 = 1.01 +/- 0.07. An analysis of emission excess stemming from a wind-wind collision yields an inclination similar to that obtained from polarimetry (i = 39 +/- 6 degrees). Our analysis thus implies M-1 = 53(-20)(+40) and M2 = 54(-20)(+40) M-circle dot, excluding M-1 > 300 M-circle dot. A detailed comparison with evolution tracks calculated for single and binary stars together with the high eccentricity suggests that the components of the system underwent quasi-homogeneous evolution and avoided mass-transfer. This scenario would suggest current masses of approximate to 80 M-circle dot and initial masses of M-i,M-1 approximate to 10(5) and M-i,M-2 approximate to 90 M-circle dot, consistent with the upper limits of our derived orbital masses, and would imply an age of approximate to 2.2 Myr.}, language = {en} } @article{PostnovOskinovaTorrejon2017, author = {Postnov, K. and Oskinova, Lida and Torrejon, J. M.}, title = {A propelling neutron star in the enigmatic Be-star gamma Cassiopeia}, series = {Monthly notices of the Royal Astronomical Society}, volume = {465}, journal = {Monthly notices of the Royal Astronomical Society}, number = {1}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnrasl/slw223}, pages = {L119 -- L123}, year = {2017}, abstract = {gamma Cassiopeia (gamma Cas), is known to be a binary system consisting of a Be-type star and a low-mass (M similar to 1M(circle dot)) companion of unknown nature orbiting in the Be-disc plane. Here, we apply the quasi-spherical accretion theory on to a compact magnetized star and show that if the low-mass companion of gamma Cas is a fast spinning neutron star, the key observational signatures of. Cas are remarkably well reproduced. Direct accretion on to this fast rotating neutron star is impeded by the propeller mechanism. In this case, around the neutron star magnetosphere a hot shell is formed which emits thermal X-rays in qualitative and quantitative agreement with observed properties of the X-ray emission from gamma Cas. We suggest that gamma Cas and its analogues constitute a new subclass of Be-type X-ray binaries hosting rapidly rotating neutron stars formed in supernova explosions with small kicks. The subsequent evolutionary stage of gamma Cas and its analogues should be the X Per-type binaries comprising low-luminosity slowly rotating X-ray pulsars. The model explains the enigmatic X-ray emission from gamma Cas, and also establishes evolutionary connections between various types of rotating magnetized neutron stars in Be-binaries.}, language = {en} } @article{SchoellerHubrigFossatietal.2017, author = {Sch{\"o}ller, Markus and Hubrig, Swetlana and Fossati, L. and Carroll, Thorsten Anthony and Briquet, Maryline and Oskinova, Lida and J{\"a}rvinen, S. and Ilyin, Ilya and Castro, N. and Morel, T. and Langer, N. and Przybilla, N. and Nieva, M. -F. and Kholtygin, A. F. and Sana, H. and Herrero, A. and Barba, R. H. and de Koter, A.}, title = {B fields in OB stars (BOB)}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {599}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, organization = {BOB Collaboration}, issn = {1432-0746}, doi = {10.1051/0004-6361/201628905}, pages = {11}, year = {2017}, abstract = {Aims. The B fields in OB stars (BOB) Collaboration is based on an ESO Large Programme to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. Methods. In the framework of this program, we carried out low-resolution spectropolarimetric observations of a large sample of massive stars using FORS2 installed at the ESO VLT 8m telescope. Results. We determined the magnetic field values with two completely independent reduction and analysis pipelines. Our in-depth study of the magnetic field measurements shows that differences between our two pipelines are usually well within 3 sigma errors. From the 32 observations of 28 OB stars, we were able to monitor the magnetic fields in CPD -57 degrees 3509 and HD164492C, confirm the magnetic field in HD54879, and detect a magnetic field in CPD -62 degrees 2124. We obtain a magnetic field detection rate of 6 +/- 3\% for the full sample of 69 OB stars observed with FORS 2 within the BOB program. For the preselected objects with a nu sin i below 60 km s(-1), we obtain a magnetic field detection rate of 5 +/- 5\%. We also discuss X-ray properties and multiplicity of the objects in our FORS2 sample with respect to the magnetic field detections.}, language = {en} } @article{RamachandranHainichHamannetal.2017, author = {Ramachandran, Varsha and Hainich, Rainer and Hamann, Wolf-Rainer and Oskinova, Lida and Shenar, T. and Sander, Andreas Alexander Christoph and Todt, Helge Tobias and Gallagher, John S.}, title = {Stellar population of the superbubble N206 in the LMC I. Analysis of the Of-type stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {609}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201731093}, pages = {26}, year = {2017}, abstract = {Context. Massive stars severely influence their environment by their strong ionizing radiation and by the momentum and kinetic energy input provided by their stellar winds and supernovae. Quantitative analyses of massive stars are required to understand how their feedback creates and shapes large scale structures of the interstellar medium. The giant H II region N206 in the Large Magellanic Cloud contains an OB association that powers a superbubble filled with hot X-ray emitting gas, serving as an ideal laboratory in this context. Aims. We aim to estimate stellar and wind parameters of all OB stars in N206 by means of quantitative spectroscopic analyses. In this first paper, we focus on the nine Of-type stars located in this region. We determine their ionizing flux and wind mechanical energy. The analysis of nitrogen abundances in our sample probes rotational mixing. Methods. We obtained optical spectra with the multi-object spectrograph FLAMES at the ESO-VLT. When possible, the optical spectroscopy was complemented by UV spectra from the HST, IUE, and FUSE archives. Detailed spectral classifications are presented for our sample Of-type stars. For the quantitative spectroscopic analysis we used the Potsdam Wolf-Rayet model atmosphere code. We determined the physical parameters and nitrogen abundances of our sample stars by fitting synthetic spectra to the observations. Results. The stellar and wind parameters of nine Of-type stars, which are largely derived from spectral analysis are used to construct wind momentum luminosity relationship. We find that our sample follows a relation close to the theoretical prediction, assuming clumped winds. The most massive star in the N206 association is an Of supergiant that has a very high mass-loss rate. Two objects in our sample reveal composite spectra, showing that the Of primaries have companions of late O subtype. All stars in our sample have an evolutionary age of less than 4 million yr, with the O2-type star being the youngest. All these stars show a systematic discrepancy between evolutionary and spectroscopic masses. All stars in our sample are nitrogen enriched. Nitrogen enrichment shows a clear correlation with increasing projected rotational velocities. Conclusions. The mechanical energy input from the Of stars alone is comparable to the energy stored in the N206 superbubble as measured from the observed X-ray and H alpha emission.}, language = {en} } @article{ShenarOskinovaJaervinenetal.2017, author = {Shenar, Tomer and Oskinova, Lida and Jaervinen, S. P. and Luckas, P. and Hainich, Rainer and Todt, Helge Tobias and Hubrig, Swetlana and Sander, Andreas Alexander Christoph and Ilyin, Ilya and Hamann, Wolf-Rainer}, title = {Constraining the weak-wind problem}, series = {Contributions Of The Astronomical Observatory Skalnate Pleso}, volume = {48}, journal = {Contributions Of The Astronomical Observatory Skalnate Pleso}, number = {1}, publisher = {Astronomick{\´y} {\´U}stav SAV}, address = {Tatransk{\´a} Lomnica}, issn = {1335-1842}, doi = {10.1051/0004-6361/201731291}, pages = {139 -- 143}, year = {2017}, abstract = {Mass-loss rates of massive, late type main sequence stars are much weaker than currently predicted, but their true values are very difficult to measure. We suggest that confined stellar winds of magnetic stars can be exploited to constrain the true mass-loss rates M of massive main sequence stars. We acquired UV, X-ray, and optical amateur data of HD 54879 (09.7 V), one of a few O-type stars with a detected atmospheric magnetic field (B-d greater than or similar to 2 kG). We analyze these data with the Potsdam Wolf-Rayet (PoWR) and XSPEC codes. We can roughly estimate the mass-loss rate the star would have in the absence of a magnetic field as log M-B=0 approximate to -9.0 M-circle dot yr(-1). Since the wind is partially trapped within the Alfven radius rA greater than or similar to 12 R-*,, the true mass-loss rate of HD 54879 is log M less than or similar to -10.2 M-circle dot yr(-1). Moreover, we find that the microturbulent, macroturbulent, and projected rotational velocities are lower than previously suggested (< 4 km s(-1)). An initial mass of 16 M-circle dot and an age of 5 Myr are inferred. We derive a mean X-ray emitting temperature of log T-x = 6.7 K and an X-ray luminosity of log L-x = 32 erg s(-1). The latter implies a significant X-ray excess (log L-x/L-Bol approximate to - 6.0), most likely stemming from collisions at the magnetic equator. A tentative period of P approximate to 5 yr is derived from variability of the Ha line. Our study confirms that strongly magnetized stars lose little or no mass, and supplies important constraints on the weak-wind problem of massive main sequence stars.}, language = {en} } @article{ToalaOskinovaIgnace2017, author = {Toala, Jes{\´u}s Alberto and Oskinova, Lida and Ignace, R.}, title = {On the Absence of Non-thermal X-Ray Emission around Runaway O Stars}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, volume = {838}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {2041-8205}, doi = {10.3847/2041-8213/aa667c}, pages = {1 -- 32}, year = {2017}, abstract = {Theoretical models predict that the compressed interstellar medium around runaway O stars can produce highenergy non-thermal diffuse emission, in particular, non-thermal X-ray and gamma-ray emission. So far, detection of nonthermal X-ray emission was claimed for only one runaway star, AE Aur. We present a search for non-thermal diffuse X-ray emission from bow shocks using archived XMM-Newton observations for a clean sample of six welldetermined runaway O stars. We find that none of these objects present diffuse X-ray emission associated with their bow shocks, similarly to previous X-ray studies toward. zeta ph and BD+ 43 degrees 3654. We carefully investigated multi-wavelength observations of AE Aur and could not confirm previous findings of non-thermal X-rays. We conclude that so far there is no clear evidence of non-thermal extended emission in bow shocks around runaway O stars.}, language = {en} } @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{PillitteriWolkRealeetal.2017, author = {Pillitteri, Ignazio and Wolk, Scott J. and Reale, Fabio and Oskinova, Lida}, title = {The early B-type star Rho Ophiuchi A is an X-ray lighthouse}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {602}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201630070}, pages = {10}, year = {2017}, abstract = {We present the results of a 140 ks XMM-Newton observation of the B2 star rho Oph A. The star has exhibited strong X-ray variability: a cusp-shaped increase of rate, similar to that which we partially observed in 2013, and a bright flare. These events are separated in time by about 104 ks, which likely correspond to the rotational period of the star (1.2 days). Time resolved spectroscopy of the X-ray spectra shows that the first event is caused by an increase of the plasma emission measure, while the second increase of rate is a major flare with temperatures in excess of 60 MK (kT similar to 5 keV). From the analysis of its rise, we infer a magnetic field of >= 300 G and a size of the flaring region of similar to 1.4-1.9 x 10(11) cm, which corresponds to similar to 25\%-30\% of the stellar radius. We speculate that either an intrinsic magnetism that produces a hot spot on its surface or an unknown low mass companion are the source of such X-rays and variability. A hot spot of magnetic origin should be a stable structure over a time span of >= 2.5 yr, and suggests an overall large scale dipolar magnetic field that produces an extended feature on the stellar surface. In the second scenario, a low mass unknown companion is the emitter of X-rays and it should orbit extremely close to the surface of the primary in a locked spin-orbit configuration, almost on the verge of collapsing onto the primary. As such, the X-ray activity of the secondary star would be enhanced by its young age, and the tight orbit as in RS Cvn systems. In both cases rho Oph would constitute an extreme system that is worthy of further investigation.}, 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{BozzoOskinovaLobeletal.2017, author = {Bozzo, Enrico and Oskinova, Lida and Lobel, A. and Hamann, Wolf-Rainer}, title = {The super-orbital modulation of supergiant high-mass X-ray binaries}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {606}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, organization = {LIGO Sci Collaboration \& Virgo;Fermi GBM;INTERGRAL;IceCube Collaboration;AstroSat Cadmium Zinc Telluride;IPN Collaboration;Insight-Hxmt Collaboration;ANTARES Collaboration;Swift Collaboration;AGILE Team;The 1M2H Team;Dark Energy Camera GW-EM;DLT40 Collaboration;GRAWITA GRAvitational Wave;Fermi Large Area Telescope;ATCA Australia Telescope;ASKAP Australian SKA Pathfinder;Las Cumbres Observatory Grp;OzGrav DWF Deeper Wider Faster;VINROUGE Collaboration;MASTER Collaboration;J-GEM;GROWTH JAGWAR CALTECH;Pan-STARRS;MAXI Team;TZAC Consortium;KU Collaboration;Nordic Optical Telescope;ePESSTO;GROUND;Texas Tech Univ;Salt Grp;Toros Transient Robotic Observat;BOOTES Collaboration;MWA Murchison Widefield Array;CALET Collaboration;IKI-GW Follow-up Collaboration;H E S S Collaboration;LOFAR Collaboration;LWA Long Wavelength Array;HAWC Collaboration;Pierre Auger Collaboration;ALMA Collaboration;Euro VLBI Team;Pi Sky Collaboration;Chandra Team McGill Univ;DFN Desert Fireball Network;ATLAS;High Time Resolution Universe;RIMAS RATIR;SKA South Africa MeerKAT}, issn = {1432-0746}, doi = {10.1051/0004-6361/201731930}, pages = {4}, year = {2017}, abstract = {The long-term X-ray light curves of classical supergiant X-ray binaries and supergiant fast X-ray transients show relatively similar super-orbital modulations, which are still lacking a sound interpretation. We propose that these modulations are related to the presence of corotating interaction regions (CIRs) known to thread the winds of OB supergiants. To test this hypothesis, we couple the outcomes of three-dimensional (3D) hydrodynamic models for the formation of CIRs in stellar winds with a simplified recipe for the accretion onto a neutron star. The results show that the synthetic X-ray light curves are indeed modulated by the presence of the CIRs. The exact period and amplitude of these modulations depend on a number of parameters governing the hydrodynamic wind models and on the binary orbital configuration. To compare our model predictions with the observations, we apply the 3D wind structure previously shown to well explain the appearance of discrete absorption components in the UV time series of a prototypical B0.5I-type supergiant. Using the orbital parameters of IGRJ 16493-4348, which has the same B0.5I donor spectral type, the period and modulations in the simulated X-ray light curve are similar to the observed ones, thus providing support to our scenario. We propose that the presence of CIRs in donor star winds should be considered in future theoretical and simulation efforts of wind-fed X-ray binaries.}, language = {en} } @article{SanjurjoFerrrinTorrejonPostnovetal.2017, author = {Sanjurjo-Ferrrin, G. and Torrejon, J. M. and Postnov, K. and Oskinova, Lida and Rodes-Roca, J. J. and Bernabeu, Guillermo}, title = {XMM-Newton spectroscopy of the accreting magnetar candidate 4U0114+65}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {606}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201630119}, pages = {4039 -- 4042}, year = {2017}, abstract = {Methods. We analysed the energy-resolved light curve and the time-resolved X-ray spectra provided by the EPIC cameras on board XMM-Newton. We also analysed the first high-resolution spectrum of this source provided by the Reflection Grating Spectrometer. Results. An X-ray pulse of 9350 +/- 160 s was measured. Comparison with previous measurements confirms the secular spin up of this source. We successfully fit the pulse-phase-resolved spectra with Comptonisation models. These models imply a very small (r similar to 3 km) and hot (kT similar to 2-3 keV) emitting region and therefore point to a hot spot over the neutron star (NS) surface as the most reliable explanation for the X-ray pulse. The long NS spin period, the spin-up rate, and persistent X-ray emission can be explained within the theory of quasi-spherical settling accretion, which may indicate that the magnetic field is in the magnetar range. Thus, 4U0114+65 could be a wind-accreting magnetar. We also observed two episodes of low luminosity. The first was only observed in the low-energy light curve and can be explained as an absorption by a large over-dense structure in the wind of the B1 supergiant donor. The second episode, which was deeper and affected all energies, may be due to temporal cessation of accretion onto one magnetic pole caused by non-spherical matter capture from the structured stellar wind. The light curve displays two types of dips that are clearly seen during the high-flux intervals. The short dips, with durations of tens of seconds, are produced through absorption by wind clumps. The long dips, in turn, seem to be associated with the rarefied interclump medium. From the analysis of the X-ray spectra, we found evidence of emission lines in the X-ray photoionised wind of the B1Ia donor. The Fe K alpha line was found to be highly variable and much weaker than in other X-ray binaries with supergiant donors. The degree of wind clumping, measured through the covering fraction, was found to be much lower than in supergiant donor stars with earlier spectral types. Conclusions. The XMM-Newton spectroscopy provided further support for the magnetar nature of the neutron star in 4U0114+65. The light curve presents dips that can be associated with clumps and the interclump medium in the stellar wind of the mass donor.}, language = {en} } @article{IgnaceHoleOskinovaetal.2017, author = {Ignace, R. and Hole, K. T. and Oskinova, Lida and Rotter, J. P.}, title = {An X-Ray Study of Two B plus B Binaries: AH Cep and CW Cep}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {850}, 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/aa93ea}, pages = {7}, year = {2017}, abstract = {AH Cep and CW Cep are both early B-type binaries with short orbital periods of 1.8. days and 2.7. days, respectively. All four components are B0.5V types. The binaries are also double-lined spectroscopic and eclipsing. Consequently, solutions for orbital and stellar parameters make the pair of binaries ideal targets for a study of the colliding winds between two B. stars. Chandra ACIS-I observations were obtained to determine X-ray luminosities. AH. Cep was detected with an unabsorbed X-ray luminosity at a 90\% confidence interval of (9-33) x 10(30) erg s(-1), or (0.5-1.7) x 10(-7) L-Bol , relative to the combined Bolometric luminosities of the two components. While formally consistent with expectations for embedded wind shocks, or binary wind collision, the near-twin system of CW Cep was a surprising nondetection. For CW Cep, an upper limit was determined with L-X/L-Bol < 10(-8), again for the combined components. One difference between these two systems is that AH Cep is part of a multiple system. The X-rays from AH. Cep may not arise from standard wind shocks nor wind collision, but perhaps instead from magnetism in any one of the four components of the system. The possibility could be tested by searching for cyclic X-ray variability in AH. Cep on the short orbital period of the inner B. stars.}, language = {en} } @article{BozzoBernardiniFerrignoetal.2017, author = {Bozzo, Enrico and Bernardini, F. and Ferrigno, Carlo and Falanga, M. and Romano, Patrizia and Oskinova, Lida}, title = {The accretion environment of supergiant fast X-ray transients probed with XMM-Newton}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {608}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201730398}, pages = {20}, year = {2017}, abstract = {Context. Supergiant fast X-ray (SFXT) transients are a peculiar class of supergiant X-ray binaries characterized by a remarkable variability in the X-ray domain, widely ascribed to accretion from a clumpy stellar wind. Aims. In this paper we performed a systematic and homogeneous analysis of the sufficiently bright X-ray flares observed with XMM-Newton from the supergiant fast X-ray transients to probe spectral variations on timescales as short as a few hundred seconds. Our ultimate goal is to investigate whether SFXT flares and outbursts are triggered by the presence of clumps, and to reveal whether strongly or mildly dense clumps are required. Methods. For all sources, we employ a technique developed by our group already exploited in a number of our previous papers, making use of an adaptive rebinned hardness ratio to optimally select the time intervals for the spectral extraction. A total of twelve observations performed in the direction of five SFXTs are reported, providing the largest sample of events available so far. Results. Using the original results reported here and those obtained with our technique from the analysis of two previously published XMM-Newton observations of IGR J17544-2619 and IGR J18410-0535, we show that both strongly and mildly dense clumps can trigger these events. In the former case, the local absorption column density may increase by a factor of >> 3, while in the latter case, the increase is only a factor of similar to 2-3 (or lower). An increase in the absorption column density is generally recorded during the rise of the flares/outbursts, while a drop follows when the source achieves peak flux. In a few cases, a re-increase of the absorption column density after the flare is also detected, and we discovered one absorption event related to the passage of an unaccreted clump in front of the compact object. Overall, there seems to be no obvious correlation between the dynamic ranges in the X-ray fluxes and absorption column densities in supergiant fast X-ray transients, with an indication that lower densities are recorded at the highest fluxes. Conclusions. The spectral variations measured in all sources are in agreement with the idea that the flares/outbursts are triggered by the presence of dense structures in the wind interacting with the X-rays from the compact object (leading to photoionization). The lack of correlation between the dynamic ranges in the X-ray fluxes and absorption column densities can be explained by the presence of accretion inhibition mechanism(s). Based on the knowledge acquired so far on the SFXTs, we propose a classification of the flares/outbursts from these sources in order to drive future observational investigations. We suggest that the difference between the classes of flares/outbursts is related to the fact that the mechanism(s) inhibiting accretion can be overcome more easily in some sources compared to others. We also investigate the possibility that different stellar wind structures, other than clumps, could provide the means to temporarily overcome the inhibition of accretion in supergiant fast X-ray transients.}, language = {en} }