Radio emission models of colliding-wind binary systems

Dougherty, S. M.; Pittard, J. M.; Kasian, L.; Coker, R. F.; Williams, P. M.; Lloyd, Huw

doi:10.1051/0004-6361:20031048

Cited by 105 publications

(163 citation statements)

References 44 publications

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“…The radio emission calculations closely follow the methods outlined by Dougherty et al (2003), , and . This model includes thermal and nonthermal emission and absorption processes (free-free and synchrotron emission/absorption and the Razin effect).…”

Section: Radio Emissionmentioning

confidence: 99%

“…The main development in the present work has been modifying the radio calculations to deal with the 3D adaptivemeshes used in the hydrodynamic simulation. In essence, this involved taking the pre-existing AIR code that was originally written for X-ray emission calculations (Parkin 2011) and incorporating the radio emission calculations developed by − see also Dougherty et al (2003). In the tangled magnetic field limit the synchrotron power at frequency, ν for a single relativistic electron, is (Rybicki & Lightman 1979;)…”

Section: Radio Emissionmentioning

confidence: 99%

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The 2.35 year itch of Cygnus OB2 #9

Parkin

Pittard

Nazé

et al. 2014

A&A

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Context. The wind-wind collision in a massive star binary system leads to the generation of high temperature shocks that emit at X-ray wavelengths and, if particle acceleration is effective, may exhibit non-thermal radio emission. Cyg OB2#9 is one of a small number of massive star binary systems in this class. Aims. X-ray and radio data recently acquired as part of a project to study Cyg OB2#9 are used to constrain physical models of the binary system, providing in-depth knowledge about the wind-wind collision and the thermal, and non-thermal, emission arising from the shocks. Methods. We use a 3D, adaptive mesh refinement simulation (including wind acceleration, radiative cooling, and the orbital motion of the stars) to model the gas dynamics of the wind-wind collision. The simulation output is used as the basis for radiative transfer calculations considering the thermal X-ray emission and the thermal/non-thermal radio emission. Results. The flow dynamics in the simulation show that wind acceleration (between the stars) is inhibited at all orbital phases by the opposing star's radiation field, reducing pre-shock velocities below terminal velocities. To obtain good agreement with the X-ray observations, our initial mass-loss rate estimates require a down-shift by a factor of ∼7.7 to 6.5 × 10 −7 M yr −1 and 7.5 × 10 −7 M yr −1 for the primary and secondary star, respectively. Furthermore, the low gas densities and high shock velocities in Cyg OB2 #9 are suggestive of unequal electron and ion temperatures, and the X-ray analysis indicates that an immediately post-shock electron-ion temperature ratio of 0.1 is also required. The radio emission is dominated by non-thermal synchrotron emission. A parameter space exploration provides evidence against models assuming equipartition between magnetic and relativistic energy densities. However, fits of comparable quality can be attained with models having stark contrasts in the ratio of magnetic-to-relativistic energy densities. Both X-ray and radio lightcurves are largely insensitive to viewing angle. The variations in X-ray emission with orbital phase can be traced back to an inverse relation with binary separation and pre-shock velocity. The radio emission also scales with pre-shock velocity and binary separation, but to positive powers (i.e. not inversely). The radio models also reveal a subtle effect whereby inverse Compton cooling leads to an increase in emissivity as a result of the synchrotron characteristic frequency being significantly reduced. Finally, using the results of the radio analysis, we estimate the surface magnetic field strengths to be ≈0.3−52 G.

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Section: Radio Emissionmentioning

confidence: 99%

Section: Radio Emissionmentioning

confidence: 99%

The 2.35 year itch of Cygnus OB2 #9

Parkin

Pittard

Nazé

et al. 2014

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“…by the wind in front can create bow-shaped emission that is purely an absorption effect and which does not relate to the opening angle of the CWR (e.g., Dougherty et al 2003, their Fig. 11).…”

Section: Free-free Contribution Stellar Windsmentioning

confidence: 99%

The 2.35 year itch of Cygnus OB2 #9

Blomme

Nazé

Volpi

et al. 2013

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Context. Cyg OB2 #9 is one of a small set of non-thermal radio emitting massive O-star binaries. The non-thermal radiation is due to synchrotron emission in the colliding-wind region. Cyg OB2 #9 has only recently been discovered to be a binary system, and a multi-wavelength campaign was organized to study its 2011 periastron passage. Aims. We want to better determine the parameters of this system and model the wind-wind collision. This will lead to a better understanding of the Fermi mechanism that accelerates electrons up to relativistic speeds in shocks and its occurrence in collidingwind binaries. We report here on the results of the radio observations obtained in the monitoring campaign and present a simple model to interpret the data. Methods. We used the Expanded Very Large Array (EVLA) radio interferometer to obtain 6 cm and 20 cm continuum fluxes during the Cyg OB2 #9 periastron passage in 2011. We introduce a simple model to solve the radiative transfer in the stellar winds and the colliding-wind region, and thus determine the expected behaviour of the radio light curve. Results. The observed radio light curve shows a steep drop in flux sometime before periastron. The fluxes drop to a level that is comparable to the expected free-free emission from the stellar winds, suggesting that the non-thermal emitting region is completely hidden at that time. After periastron passage, the fluxes slowly increase. We use the asymmetry of the light curve to show that the primary has the stronger wind. This is somewhat unexpected if we use the astrophysical parameters based on theoretical calibrations. But it becomes entirely feasible if we take into account that a given spectral type-luminosity class combination covers a range of astrophysical parameters. The colliding-wind region also contributes to the free-free emission, which can help explain the high values of the spectral index seen after periastron passage. Combining our data with older Very Large Array (VLA) data allows us to derive a period P = 860.0 ± 3.7 days for this system. With this period, we update the orbital parameters that were derived in the first paper of this series. Conclusions. A simple model introduced to explain only the radio data already allows some constraints to be put on the parameters of this binary system. Future, more sophisticated, modelling that will also include optical, X-ray, and interferometric information will provide even better constraints.

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“…Both massive stars in the η Car system are expected to produce powerful stellar winds. Massloss rates and terminal wind velocities (as given in the table) are thought to be sufficiently high to form a wind-wind collision zone of shocked, hot gas, wherein particle acceleration (as illustrated by e.g., Eichler & Usov 1993;Dougherty et al 2003) in general and subsequent γ-ray emission in particular (as described by e.g., Reimer et al 2006;Bednarek & Pabich 2011) can occur. The conditions in the wind-wind collision zone depend on the orbital phase of the binary system.…”

Section: Introductionmentioning

confidence: 99%

Gamma-ray follow-up studies onηCarinae

Reitberger

Reimer

Werner

et al. 2012

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Aims. Observations of high-energy γ-rays recently revealed a persistent source in spatial coincidence with the binary system η Carinae. Since modulation of the observed γ-ray flux on orbital time scales has not been reported so far, an unambiguous identification was hitherto not possible. Particularly the observations made by the Fermi Large Area Telescope (LAT) posed additional questions regarding the actual emission scenario. Analyses show two energetically distinct components in the γ-ray spectrum, which are best described by an exponentially cutoff power-law function (CPL) at energies below 10 GeV and a power-law (PL) component dominant at higher energies. Methods. The increased exposure in conjunction with the improved instrumental response functions of the LAT now allow us to perform a more detailed investigation of location, spectral shape, and flux time history of the observed γ-ray emission. Results. We detect a weak but regular flux decrease over time. This can be understood and interpreted in a colliding-wind binary scenario for orbital modulation of the γ-ray emission. We find that the spectral shape of the γ-ray signal agrees with a single emitting particle population in combination with significant absorption by γ-γ pair production. Conclusions. We are able to report on the first unambiguous detection of GeV γ-ray emission from a colliding-wind massive star binary. Studying the correlation of the flux decrease with the orbital separation of the binary components allows us to predict the behaviour up to the next periastron passage in 2014.

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Radio emission models of colliding-wind binary systems

Cited by 105 publications

References 44 publications

The 2.35 year itch of Cygnus OB2 #9

The 2.35 year itch of Cygnus OB2 #9

The 2.35 year itch of Cygnus OB2 #9

Gamma-ray follow-up studies onηCarinae

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