References of "De Becker, Michaël"
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See detailL'émission non-thermique dans les binaires massives à collision de vents
De Becker, Michaël ULg

Scientific conference (2007, November 21)

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See detailNon-thermal emission processes in massive binaries
De Becker, Michaël ULg

in Astronomy and Astrophysics Review (2007), 14(3-4), 171-216

In this paper, I present a general discussion of several astrophysical processes likely to play a role in the production of non-thermal emission in massive stars, with emphasis on massive binaries. Even ... [more ▼]

In this paper, I present a general discussion of several astrophysical processes likely to play a role in the production of non-thermal emission in massive stars, with emphasis on massive binaries. Even though the discussion will start in the radio domain where the non-thermal emission was first detected, the census of physical processes involved in the non-thermal emission from massive stars shows that many spectral domains are concerned, from the radio to the very high energies. First, the theoretical aspects of the non-thermal emission from early-type stars will be addressed. The main topics that will be discussed are respectively the physics of individual stellar winds and their interaction in binary systems, the acceleration of relativistic electrons, the magnetic field of massive stars, and finally the non-thermal emission processes relevant to the case of massive stars. Second, this general qualitative discussion will be followed by a more quantitative one, devoted to the most probable scenario where non-thermal radio emitters are massive binaries. I will show how several stellar, wind and orbital parameters can be combined in order to make some semi-quantitative predictions on the high-energy counterpart to the non-thermal emission detected in the radio domain. These theoretical considerations will be followed by a census of results obtained so far, and related to this topic. These results concern the radio, the visible, the X-ray and the gamma-ray domains. Prospects for the very high energy gamma-ray emission from massive stars will also be addressed. Two particularly interesting examples-one O-type and one Wolf-Rayet binary-will be considered in details. Finally, strategies for future developments in this field will be discussed. [less ▲]

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See detailThe Struve-Sahade effect in the optical spectra of O-type binaries - I. Main-sequence systems
Linder, Natacha ULg; Rauw, Grégor ULg; Sana, H. et al

in Astronomy and Astrophysics (2007), 474(1), 193-204

Aims. We present a spectroscopic analysis of four massive binary systems that are known or are good candidates to display the Struve-Sahade effect (defined as the apparent strengthening of the secondary ... [more ▼]

Aims. We present a spectroscopic analysis of four massive binary systems that are known or are good candidates to display the Struve-Sahade effect (defined as the apparent strengthening of the secondary spectrum of the binary when the star is approaching, and the corresponding weakening of the lines when it is receding). Methods. We use high resolution optical spectra to determine new orbital solutions and spectral types of HD 165 052, HD 100 213, HD 159 176 and DH Cep. As good knowledge of the fundamental parameters of the considered systems is necessary to examine the Struve-Sahade effect. We then study equivalent width variations in the lines of both components of these binaries during their orbital cycle. Results. In the case of these four systems, variations appear in the equivalent widths of some lines during the orbital cycle, but the definition given above can any longer be valid, since it is now clear that the effect modifies the primary spectrum as much as the secondary spectrum. Furthermore, the lines affected, and the way in which they are affected, depend on the considered system. For at least two of them (HD 100 213 and HD 159 176) these variations probably reflect the ellipsoidal variable nature of the system. [less ▲]

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See detailINTEGRAL-ISGRI observations of the cygnus OB2 region - Searching for hard X-ray point sources in a region containing several non-thermal emitting massive stars
De Becker, Michaël ULg; Rauw, Grégor ULg; Pittard, J. M. et al

in Astronomy and Astrophysics (2007), 472(3), 905-910

Aims. We analyze INTEGRAL-ISGRI data in order to probe the hard X-ray emission ( above 20 keV) from point sources in the CygOB2 region and to investigate the putative non-thermal high-energy emission from ... [more ▼]

Aims. We analyze INTEGRAL-ISGRI data in order to probe the hard X-ray emission ( above 20 keV) from point sources in the CygOB2 region and to investigate the putative non-thermal high-energy emission from early-type stars (Wolf-Rayet and O-type stars). Among the targets located in the field of view, we focus on the still unidentified EGRET source 3EG2033+4118 that may be related to massive stars known to produce non-thermal emission in the radio domain, and on the wide colliding-wind binary WR 140. Methods. Using a large set of data obtained with the IBIS-ISGRI imager onboard INTEGRAL, we run the OSA software package in order to find point sources in the fully coded field of view of the instrument. Results. Our data do not allow the detection of a lower-energy counterpart of 3EGJ2033+4118 nor of any other new point sources in the field of view, and we derive upper limits on the high-energy flux for a few targets: 3EGJ2033+4118, TeVJ2032+4130, WR 140, WR 146 and WR 147. The results are discussed in the context of the multiwavelength investigation of these objects. Conclusions. The upper limits derived are valuable constraints for models aimed at understanding the acceleration of particles in non-thermal emitting massive stars, and of the still unidentified very-high gamma-ray source TeVJ2032+4130. [less ▲]

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See detailLong-term Spectroscopic Variability of Two Oe Stars
Rauw, Grégor ULg; Nazé, Yaël ULg; Marique, P. X. et al

in Information Bulletin on Variable Stars (2007), 5773

HD 45314 and HD 60848 are poorly studied Oe-type stars. Spectroscopic monitoring over 5 years reveals strong variations in the strength of the emission lines and for HD 45314 in the V/R ratio of the ... [more ▼]

HD 45314 and HD 60848 are poorly studied Oe-type stars. Spectroscopic monitoring over 5 years reveals strong variations in the strength of the emission lines and for HD 45314 in the V/R ratio of the double-peaked emissions. Part of these long-term variations could be recurrent. [less ▲]

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See detailEarly-type stars in the core of the young open cluster Westerlund 2
Rauw, Grégor ULg; Manfroid, Jean ULg; Gosset, Eric ULg et al

in Astronomy and Astrophysics (2007), 463(3), 981-991

Aims. The properties of the early-type stars in the core of the Westerlund 2 cluster are examined in order to establish a link between the cluster and the very massive Wolf-Rayet binary WR 20a as well as ... [more ▼]

Aims. The properties of the early-type stars in the core of the Westerlund 2 cluster are examined in order to establish a link between the cluster and the very massive Wolf-Rayet binary WR 20a as well as the H II complex RCW 49. Methods. Photometric monitoring as well as spectroscopic observations of Westerlund 2 are used to search for light variability and to establish the spectral types of the early-type stars in the cluster core. Results. The first light curves of the eclipsing binary WR 20a in B and V filters are analysed and a distance of 8 kpc is inferred. Three additional eclipsing binaries, which are probable late O or early B-type cluster members, are discovered, but none of the known early O-type stars in the cluster displays significant photometric variability above 1% at the 1-sigma level. The twelve brightest O-type stars are found to have spectral types between O3 and O6.5, significantly earlier than previously thought. Conclusions. The distance of the early-type stars in Westerlund 2 is established to be in excellent agreement with the distance of WR20a, indicating that WR20a actually belongs to the cluster. Our best estimate of the cluster distance thus amounts to 8.0 +/- 1.4 kpc. Despite the earlier spectral types, the currently known population of early-type stars in Westerlund 2 does not provide enough ionizing photons to account for the radio emission of the RCW49 complex. This suggests that there might still exist a number of embedded early O-stars in RCW49. [less ▲]

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See detailNon-thermal radio emission from O-type stars. II. HD167971
Blomme, Ronny; De Becker, Michaël ULg; Runacres, M. C. et al

in Astronomy and Astrophysics (2007), 464(2), 701-148

HD 167971 is a triple system consisting of a 3.3-day eclipsing binary (O5- 8 V + O5- 8 V) and an O8 supergiant. It is also a well known non-thermal radio emitter. We observed the radio emission of HD ... [more ▼]

HD 167971 is a triple system consisting of a 3.3-day eclipsing binary (O5- 8 V + O5- 8 V) and an O8 supergiant. It is also a well known non-thermal radio emitter. We observed the radio emission of HD 167971 with the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). By combining these data with VLA archive observations we constructed a radio lightcurve covering a 20-yr time-range. We searched for, but failed to find, the 3.3-day spectroscopic period of the binary in the radio data. This could be due to the absence of intrinsic synchrotron radiation in the colliding-wind region between the two components of the eclipsing binary, or due to the large amount of free-free absorption that blocks the synchrotron radiation. We are able to explain many of the observed characteristics of the radio data if the non-thermal emission is produced in a colliding-wind region between the supergiant and the combined winds of the binary. Furthermore, if the system is gravitationally bound, the orbital motion occurs over a period of similar to 20 years or longer, as suggested by the long-term variability in the radio data. We argue that the variability is due to the free-free absorption that changes with orbital phase or may also in part be due to changes in separation, should the orbit be eccentric. [less ▲]

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See detailNew Colliding-Wind Massive Binaries
De Becker, Michaël ULg; Rauw, Grégor ULg

in Massive Stars in Interactive Binaries (2007)

We report on the recent discovery of two new massive binary systems: WR20a and CygOB2#8A. For both systems, we briefly present the orbital solutions and we discuss the strong phase-locked line profile ... [more ▼]

We report on the recent discovery of two new massive binary systems: WR20a and CygOB2#8A. For both systems, we briefly present the orbital solutions and we discuss the strong phase-locked line profile variability of the Halpha and HeII lambda 4686 lines for WR20a, and the HeII &; 4686 line in the case of CygOB2#8A. In both cases, we show that the profile variability reveals the signature of a wind interaction. The prospects for future high-energy observations are also discussed. [less ▲]

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See detailXMM-Newton observations of the massive colliding wind binary and non-thermal radio emitter Cyg OB2#8A [O6If+O5.5III(f)]
De Becker, Michaël ULg; Rauw, Grégor ULg; Sana, H. et al

in Monthly Notices of the Royal Astronomical Society (2006), 371(3), 1280-1294

We report on the results of four XMM-Newton observations separated by about ten days from each other of Cyg OB2 #8A [O6If + O5.5III(f)]. This massive colliding wind binary is a very bright X-ray emitter ... [more ▼]

We report on the results of four XMM-Newton observations separated by about ten days from each other of Cyg OB2 #8A [O6If + O5.5III(f)]. This massive colliding wind binary is a very bright X-ray emitter-one of the first X-ray emitting O-stars discovered by the Einstein satellite-as well as a confirmed non-thermal radio emitter whose binarity was discovered quite recently. The X-ray spectrum between 0.5 and 10.0 keV is essentially thermal, and is best fitted with a three-component model with temperatures of about 3, 9 and 20 MK. The X-ray luminosity corrected for the interstellar absorption is rather large, i.e. about 1034 erg s(-1). Compared to the 'canonical' L-X/L-bol ratio of O-type stars, Cyg OB2 # 8A was a factor of 19-28 overluminous in X-rays during our observations. The EPIC spectra did not reveal any evidence for the presence of a non-thermal contribution in X-rays. This is not unexpected considering that the simultaneous detections of non-thermal radiation in the radio and soft X-ray (below 10.0 keV) domains is unlikely. Our data reveal a significant decrease in the X-ray flux from apastron to periastron with an amplitude of about 20 per cent. Combining our XMM Newton results with those from previous ROSAT-PSPC and ASCA-SIS observations, we obtain a light curve suggesting a phase-locked X-ray variability. The maximum emission level occurs around phase 0.75, and the minimum is probably seen shortly after the periastron passage. Using hydrodynamic simulations of the wind-wind collision, we find a high X-ray emission level close to phase 0.75, and a minimum at periastron as well. The high X-ray luminosity, the strong phase-locked variability and the spectral shape of the X-ray emission of Cyg OB2 # 8A revealed by our investigation point undoubtedly to X-ray emission dominated by colliding winds. [less ▲]

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See detailOn the nature of the hard X-ray source IGR J2018+4043
Bykov, Andrei Mikhailovich; Krassilchtchikov, Alexandre Mikhailovich; Uvarov, Yuri Alexandrovich et al

in Astrophysical Journal (2006), 649(1, Part 2), 21-24

We found a very likely counterpart to the recently discovered hard X-ray source IGR J2018 + 4043 in the multiwavelength observations of the source field. The source, originally discovered in the 20-40 keV ... [more ▼]

We found a very likely counterpart to the recently discovered hard X-ray source IGR J2018 + 4043 in the multiwavelength observations of the source field. The source, originally discovered in the 20-40 keV band, is now confidently detected also in the 40-80 keV band, with a flux of (1.4 +/- 0.4) x 10(-11) ergs cm(-2) s(-1). A 5 ks Swift observation of the IGR J2018 + 4043 field revealed a hard pointlike source with an observed 0.5-10 keV flux of 3.4(-0.8)(+0.7) x 10(-12) ergs cm(-2) s(-1) (90% confidence level) at alpha = 20(h)18(m)38(s).55, delta = +40 degrees 41'00.4" (with a 4".2 uncertainty). The combined Swift-INTEGRAL spectrum can be described by an absorbed power-law model with photon index Gamma = 1.3 +/- 0.2 and N-H = 6.1(-2.2)(+3.3) x 10(22) cm(-2). In archival optical and infrared data we found a slightly extended and highly absorbed object at the Swift source position. There is also an extended VLA 1.4 GHz source peaked at a beamwidth distance from the optical and X-ray positions. The observed morphology and multiwavelength spectra of IGR J2018 + 4043 are consistent with those expected for an obscured accreting object, i.e., an AGN or a Galactic X-ray binary. The identification suggests possible connection of IGR J2018 + 4043 to the bright gamma-ray source GEV J2020 + 4023 detected by COS B and CGRO EGRET. [less ▲]

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See detailEarly-type stars in the young open cluster IC 1805. II. The probably single stars HD 15570 and HD 15629, and the massive binary/triple system HD 15558
De Becker, Michaël ULg; Rauw, Grégor ULg; Manfroid, Jean ULg et al

in Astronomy and Astrophysics (2006), 456

Aims.We address the issue of the multiplicity of the three brightest early-type stars of the young open cluster IC 1805, namely HD 15570, HD 15629 and HD 15558. Methods: .For the three stars, we measured ... [more ▼]

Aims.We address the issue of the multiplicity of the three brightest early-type stars of the young open cluster IC 1805, namely HD 15570, HD 15629 and HD 15558. Methods: .For the three stars, we measured the radial velocity by fitting Gaussian curves to line profiles in the optical domain. In the case of the massive binary HD 15558, we also used a spectral disentangling method to separate the spectra of the primary and of the secondary in order to derive the radial velocities of the two components. These measurements were used to compute orbital solutions for HD 15558. Results: .For HD 15570 and HD 15629, the radial velocities do not present any significant trend attributable to a binary motion on time scales of a few days, nor from one year to the next. In the case of HD 15558 we obtained an improved SB1 orbital solution with a period of about 442 days, and we report for the first time on the detection of the spectral signature of its secondary star. We derive spectral types O5.5III(f) and O7V for the primary and the secondary of HD 15558. We tentatively compute a first SB2 orbital solution although the radial velocities from the secondary star should be considered with caution. The mass ratio is rather high, i.e. about 3, and leads to very extreme minimum masses, in particular for the primary object. Minimum masses of the order of 150 ± 50 and 50 ± 15 M_ȯ are found respectively for the primary and the secondary. Conclusions: .We propose that HD 15558 could be a triple system. This scenario could help to reconcile the very large minimum mass derived for the primary object with its spectral type. In addition, considering new and previously published results, we find that the binary frequency among O-stars in IC 1805 has a lower limit of 20%, and that previously published values (80%) are probably overestimated. [less ▲]

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See detailThe 4m international liquid mirror telescope (ILMT)
Surdej, Jean ULg; Absil, Olivier ULg; Bartczak, Przemyslaw ULg et al

in Stepp, Larry (Ed.) Ground-based and Airborne Telescopes (2006, July 01)

The entire funding has recently been obtained in Belgium for the construction of a 4m Liquid Mirror Telescope. Its prime focus will be equipped with a semi-conventional glass corrector allowing to correct ... [more ▼]

The entire funding has recently been obtained in Belgium for the construction of a 4m Liquid Mirror Telescope. Its prime focus will be equipped with a semi-conventional glass corrector allowing to correct for the TDI effect and a thinned, high quantum efficiency, 4K × 4K pixel equivalent CCD camera. It will be capable of subarcsecond imaging in the i'(760 nm) and possibly r', g' band(s) over a field of ~ 30' in diameter. This facility will be entirely dedicated to a deep photometric and astrometric variability survey over a period of ~ 5 years. In this paper, the working principle of liquid mirror telescopes is first recalled, along with the advantages and disadvantages of the latter over classical telescopes. Several science cases are described. For a good access to one of the galactic poles, the best image quality sites for the ILMT are located either in Northern Chile (latitude near -29°30') or in North-East India (Nainital Hills, latitude near +29°30'). At those geographic latitudes, a deep (i' = 22.5 mag.) survey will approximately cover 90 square degrees at high galactic latitude, which is very useful for gravitational lensing studies as well as for the identification of various classes of interesting galactic and extragalactic objects (cf. microlensed stars, supernovae, clusters, etc.). A description of the telescope, its instrumentation and the handling of the data is also presented. [less ▲]

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See detailNew light on the peculiar star HD108
Nazé, Yaël ULg; Barbieri, C.; Segafredo, A. et al

in Information Bulletin on Variable Stars (2006), 5693

HD 108 is a peculiar massive star of type Of?p, which present recurrent line profile variations. Archival data confirm the very long (50-60 yr) period of this star. Recent data show that the minimum state ... [more ▼]

HD 108 is a peculiar massive star of type Of?p, which present recurrent line profile variations. Archival data confirm the very long (50-60 yr) period of this star. Recent data show that the minimum state is not yet reached. [less ▲]

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See detailA mutliwavelength observational study of the non-thermal emission from O-type stars.
De Becker, Michaël ULg

Doctoral thesis (2005)

The present work is devoted to an observational multiwavelength study of the non-thermal emission of O-type stars. The non-thermal radio emission is believed to be synchrotron radiation, therefore ... [more ▼]

The present work is devoted to an observational multiwavelength study of the non-thermal emission of O-type stars. The non-thermal radio emission is believed to be synchrotron radiation, therefore requiring the existence of a magnetic field as well as of a population of relativistic electrons. The physical circumstances responsible for the acceleration of electrons up to relativistic velocities are not yet completely elucidated, but such an acceleration most probably occurs through the first order Fermi mechanism in the presence of hydrodynamic shocks, also called the Diffusive Shock Acceleration (DSA) mechanism. In the context of massive stars, the shocks responsible for this acceleration process may be intrinsic to their stellar winds, or arise from the collision between stellar winds in binary systems. In addition, the existence of non-thermal emission processes such as inverse Compton scattering suggests the possibility to detect a high-energy counterpart to the non-thermal emission observed in the radio domain. The objectives of this work are to (1) study the impact of the multiplicity on the non-thermal emission, (2) investigate the issue of the non-thermal emission of massive stars in the high-energy domain, and (3) discuss the possibility to constrain physical quantities such as the magnetic field strength through the study of these non-thermal emission processes. We investigate in detail several non-thermal emitting O-type stars, namely HD168112, HD167971, Cyg OB2 #8A, Cyg OB2 #9, and HD15558. For most of these targets, high quality XMM-Newton data are used in order to study their X-ray properties. In most case, the question of the multiplicity is also addressed through optical studies performed on the basis of data obtained with ground-based telescopes. The most intensive study is carried out on the Cyg OB2 region, for which we also obtained and analysed INTEGRAL data, in order to investigate the possible relation between the massive stars in Cyg OB2 and the unidentified EGRET source 3EG J2033+4118. In addition, high quality XMM-Newton data of the close massive binary HD159176 are also studied. Even though this latter target does not belong to the category of non-thermal radio emitters, its study unexpectedly turns out to be crucial in the context of this campaign as it might be the first O + O system likely to be classified as an n! on-thermal X-ray emitter. Considering our observational results, along with results from radio observations mainly coordinated by colleagues from the Royal Observatory of Belgium, we propose a general qualitative schematic view of the non-thermal emission from massive stars. First, it seems that the multiplicity plays a crucial role in the non-thermal emission processes. Second, we have found that the simultaneous detection of non-thermal radiation in the radio and X-ray (below 10.0 keV) domains is unlikely. Non-thermal radio emitters are indeed most probably binaries with periods longer than a few weeks, whilst only close binaries with periods of at most a few days may be non-thermal emitters in the soft X-ray domain. However, such a simultaneous detection is possible if the very hard X-rays or soft gamma-rays are considered, i.e. in a domain where the thermal emission from the colliding winds does not overwhelm the non-thermal emission. Moreover, it appears that the existence of non-thermal X-ray emitters without a non-thermal radio counterpart should be envisaged. Finally, the simultaneous detection of radio and high-energy non-thermal emissions is likely to lead to an indirect method to estimate the local magnetic field strength in the colliding-wind region of massive binaries. [less ▲]

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See detailA spectroscopic investigation of the young open cluster IC 1805
De Becker, Michaël ULg; Rauw, Grégor ULg

in Rauw, Grégor; Nazé, Yaël; Blomme, R. (Eds.) et al Massive Stars and High-Energy Emission in OB Associations (2005, November 01)

We present the main results of a spectroscopic campaign devoted to the O-type stars in the young open cluster IC 1805. This study aims at the investigation of the multiplicity and the line profile ... [more ▼]

We present the main results of a spectroscopic campaign devoted to the O-type stars in the young open cluster IC 1805. This study aims at the investigation of the multiplicity and the line profile variability of its brightest massive members. We present new results concerning the line profile variability of He II 4686 and H-beta for HD 15570 and HD 15629. For these two stars, our radial velocity analysis does not reveal any variations compatible with a binary motion. We also significantly improve the orbital parameters of the long period binary HD 15558. For this latter system, we present for the first time evidence for the detection of the secondary, leading to spectral types of O5.5 and O7 respectively for the two components. [less ▲]

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See detailOn the multiplicity of the non-thermal radio emitters 9 Sgr and HD 168112
Rauw, Grégor ULg; Sana, H.; Gosset, Eric ULg et al

in Massive Stars and High-Energy Emission in OB Associations (2005, November)

We discuss the first results of our ongoing optical spectroscopic monitoring campaign of the two O-type stars 9 Sgr and HD 168112. Both objects display a non-thermal radio emission and were considered as ... [more ▼]

We discuss the first results of our ongoing optical spectroscopic monitoring campaign of the two O-type stars 9 Sgr and HD 168112. Both objects display a non-thermal radio emission and were considered as single stars. Based on a large set of high-resolution spectra, we find that 9 Sgr is clearly an eccentric SB2 binary with an orbital period of several years. On the other hand, no evidence for radial velocity variations attributable to binary motion is found in our spectra of HD 168112. [less ▲]

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See detailXMM-Newton observations of the Cyg OB2 association
Rauw, Grégor ULg; De Becker, Michaël ULg; Linder, Natacha ULg

in Massive Stars and High-Energy Emission in OB Associations (2005, November)

We present the first results of an observing campaign of the Cyg OB2 association with the XMM-Newton observatory. The brightest OB-type stars exhibit rather hard spectra suggesting that at least part of ... [more ▼]

We present the first results of an observing campaign of the Cyg OB2 association with the XMM-Newton observatory. The brightest OB-type stars exhibit rather hard spectra suggesting that at least part of their X-ray emission arises in a wind-wind interaction. The EPIC images reveal a large number of fainter X-ray sources most of which are probably low-mass pre-main sequence stars belonging to Cyg OB2. [less ▲]

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See detailEvidence for phase-locked X-ray variations from the colliding-wind massive binary Cyg OB2 #8A
De Becker, Michaël ULg; Rauw, Grégor ULg

in Rauw, G.; Nazé, Y.; Blomme, R. (Eds.) et al Massive Stars and High-Energy Emission in OB Associations (2005, November)

We report on preliminary results of a multi-observatory investigation of the X-ray emission from the massive colliding wind binary Cyg OB2 #8A (O6If + O5.5III(f)). On the basis of our new XMM-Newton-EPIC ... [more ▼]

We report on preliminary results of a multi-observatory investigation of the X-ray emission from the massive colliding wind binary Cyg OB2 #8A (O6If + O5.5III(f)). On the basis of our new XMM-Newton-EPIC observations, along with archive ASCA-SIS and ROSAT-PSPC data, we show strong evidence for a significant phase-locked variability of the X-ray emission from Cyg OB2 #8A with the period of 21.9 days determined by De Becker et al. (2004). These results lend further support to the colliding wind scenario that was already suggested by optical data (De Becker & Rauw 2005). We briefly discuss the behaviour of the X-ray emission from this binary system as a function of phase in the context of the colliding wind scenario. [less ▲]

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