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Winds in collision: high energy particles in massive binary systems

Winds in collision: high energy particles in massive binary systems. Sean M. Dougherty (NRC) In collaboration with: Julian M. Pittard (Leeds) Evan O’Connor (PEI) Nick Bolingbroke (Victoria) Perry M. Williams (IfA, Edinburgh) Tony Beasley (ALMA). Observations of Massive stars.

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Winds in collision: high energy particles in massive binary systems

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  1. Winds in collision: high energy particles in massive binary systems Sean M. Dougherty (NRC) In collaboration with: Julian M. Pittard (Leeds) Evan O’Connor (PEI) Nick Bolingbroke (Victoria) Perry M. Williams (IfA, Edinburgh) Tony Beasley (ALMA) 8th EVN Symposium, Torun

  2. Observations of Massive stars • Most massive stars (O or B-type) • positive spectra from IR to radio • brightness temperature ~ 104 K • Thermal emission – expected! • in a few systems • “flat” or negative spectra in the radio • brightness temperature ~ >106 K • Non-thermal radio emission – where from? • Also thermal/non-thermal X-rays, g-rays(?)

  3. The radio structure of a colliding-wind binary • two components - one thermal + one non-thermal • WR147 • High resolution observations - MERLIN @ 5GHz: • 50 mas = 77AU @ 650pc • IR obs resolve two stars • WR+ O/B type • Position of NT emission: consistent with position of wind-wind collision region Moran et al. 1989 Williams et al. 1997 8th EVN Symposium, Torun

  4. What is a wind-collision region? • Two massive stars with stellar winds • Contact discontinuity where ram pressures are equal • Standing shocks on either side of the CD • Thermal X-ray emission from shock-heated gas in collision region • Particle acceleration in wind-collision region • at the shocks • and/or through reconnection at the CD -> non-thermal emission • radio, X-ray etc. • Relativistic particles • Higher magnetic, particle & radiation densities than in SNR • Good particle acceleration laboratory D 8th EVN Symposium, Torun

  5. Modelling radio emission from CWB systems • Early models of CWB systems tended to be simple. • Point source non-thermal emission, radially symmetric winds – maintains analytic solutions • No consideration cooling mechanisms (e.g. Compton cooling – important - even for wide systems c.f. 146, 147) or other absorption. • Hydro-modelling of CWBs • radially symmetric, isothermal winds, collide at terminal velocity, axis-symmetric • constrained by radio spectrum and images • Radiative transfer • Assume cylindrical symmetry, ideal gas, adiabatic index=5/3 • Treatment of non-thermal emission • Urel = z Uthermal • tangled magnetic field • assumption of shock acceleration • power-law energy distribution at the shocks • p = 2 electron momentum spectrum accelerated at shocks • Electron energy spectrum evolves downstream due to IC cooling. • Thermal/non-thermal emission & absorption - determined from 2D hydro grid 8th EVN Symposium, Torun

  6. Hydro model of a CWB – spatial density distribution Wind-collision region O star – less dense wind Wind-collision regionhot plasma + NT particles WR star – dense stellar wind

  7. Typical radio spectra + IC cooling Intrinsic NT + Razin effect & ff absorption (max=1000) 8th EVN Symposium, Torun

  8. No IC cooling With IC cooling 1.6 GHz 22 GHz 8th EVN Symposium, Torun

  9. Modelling the radio spectrum of WR147 Total flux Thermal flux NT flux NT emission does not dominate – poor data constraints for modelling 8th EVN Symposium, Torun

  10. Spatial distribution in WR147 Simulated MERLIN 4.8 GHz 8th EVN Symposium, Torun

  11. WR146 • MERLIN obs - spatially resolved thermal and NT components (cf. WR147) • Brightest radio CWB – NT emission dominates • VLBI imaging • => Excellent data constraints WR146 8th EVN Symposium, Torun

  12. EVN 5 GHz 4.8-GHz EVN model (contours) 43-GHz model WR146 (2) (O’Connor et al. 2005, 2007) Greyscale - EVN 5 GHz Contours – VLA+PT 43 GHz Crosses denote stellar positions - HST 8th EVN Symposium, Torun

  13. NT emission in massive stars : binary required? • In spatially resolved WR-systems, NT emission is from a wind-collision region c.f. WR146, 147, 140 • Are all systems with NT emission “binary” systems? • 25 WR stars - mixture of both single and binary that have measured radio spectra • 11 systems have spectra identifying non-thermal emission • WR 11, 48, 98a, 104, 105, 112, 125, 137, 140, 146, 147 • 10 of these WR stars have OB-binary companions WR stars with NT radio emissionARE BINARY 8th EVN Symposium, Torun

  14. And O+O star systems?: • Cyg OB2 #5 • Eclipsing O6+O6 binary – 6-day period! • VLA 5GHz obs • thermal + non-thermal sources • Binary coincident with thermal emission • B-type star & NT emission (Contreras et al. 1999) • Wind-collision region? • Radio-detected O-stars • 60% exhibit non-thermal emission • “large” fraction are binary O stars with NT radio emissionARE BINARY? VLA 5 GHz 8th EVN Symposium, Torun

  15. State of Play: • Wind-collision regions are laboratories for investigating particle acceleration • Non-thermal emission in massive stars required a binary/companion • Certainly true for WR stars • Starting to look like the case for O stars • Successful models of the both radio spectrum and spatial distribution of emission

  16. WR 140 - the CWB laboratory • A major reason why non-thermal emission is clearly seen in WR147 + WR146 • the systems are very wide • free-free opacity along l.o.s. to the wind-collision zone is small • But --- “static” systems • families of satisfactory models • Ill-defined system parmeters = ill-constrained models • Shorter period, eccentric systems • possibility of well-specified orbit parameters • variable radiation density – IC cooling  variable high energy emission • variable ion density variable circumstellar ff opacity to WCR • WR 140 is the best studied WR+OB binary • WR + O in a 7.9 year, eccentric (e=0.88) orbit - orbit size ~ 15 AU • Radio-bright – dramatic variations in radio emission as orbit progresses • WCR resolved by VLBI -> good data constraints. • IC cooling important • Flow time ~ ROB/vWR ~ 100 hrs • IC Cooling tIC ~12 hrs @apastron @periastron ~250 times shorter! • considerably shorter than flow time • at all radio frequencies under consideration • High eccentricity + good data  excellent lab for studying wind-collision phenomena 8th EVN Symposium, Torun

  17. Cartoon of the colliding-wind region in WR140 Orbit parameters from Williams et al. 1990 - interaction region based on Eichler & Usov 1993 8th EVN Symposium, Torun

  18. The radio light curve of WR140 8 years of VLA observations (White & Becker 1995) + WSRT data (Williams p.c.) 2cm 6cm 21cm 8th EVN Symposium, Torun

  19. VLBA imaging of WR140 • 23 epochs @ 3.6 cm • phase~ 0.74 -> 0.93 (from Jan 1999 to Nov 2000) • Resolution ~ 2 mas • Linear res ~ 4 AU • Non-thermal emission (Tb~107 K) • Resolved – “curved” emission region => wind-collision region • Observe rotation & pm of emission region • Full orbit definition – particularly inclination • Distance independent of stellar parameters => Much needed modelling constraints 8th EVN Symposium, Torun

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  21. IOTA observation – Monnier et al. 2004 • “resolved” the binary components • 12.7 mas @ 151.7 degrees at phase 0.297 • Combined with other known orbit parms  families of solutions for a,W,i • Orbit definition could wait for more IOTA observation, but in the meanwhile….. 8th EVN Symposium, Torun

  22. Orbit inclination • VLBA obs • assume axis of symmetry along line-of-centres • Rotation of WCR as orbit progresses => O star moves from SE to E of WR star during observations => derive inclination. 8th EVN Symposium, Torun

  23. Orbit & distance of WR 140 • Orbit solution • a = 9.0+/-0.5 mas; W = 353+/- 3 degrees ; i=122+/-5 degrees • Distance – NOT from stellar parameters! • a sin i = 14.10 +/- 0.5 AU => a = 16.6 +/- 1.1 AU for i = 122 deg. • a = 9.0 +/- 0.5 mas • Distance = 1.85 +/- 0.16 kpc • O supergiant • All important system parms now defined!!! • Stellar types • Distance • All orbit parameters (including inclination) • ALL VERY IMPORTANT to modelling 8th EVN Symposium, Torun

  24. Modelling the spectra Using these orbit parms… Phase 0.837 A: =0.22, e=1.38x10-3, B=0.05 B: =0.02, e=5.36x10-3, B=0.05 • Constrain (& mass-loss)with thermal X-ray observations • independent of wind-clumping. • Successfully model individual orbit phases – good! • Most importantly, establish a value for B, the magnetic field strength 8th EVN Symposium, Torun

  25. Modelling 8 GHz VLBI observations of WR140 • Possible to constrain models with VLBI obs - demands good observations 8th EVN Symposium, Torun

  26. 22/43 15 8.3 5 1.6 Radiometry • New multi-frequency VLA observations • Repeat fluxes from previous orbit(s) • Suggests that emission arises from a “well-behaved” process • Similar behavior seen in O+O star binary systems 8th EVN Symposium, Torun

  27. First stab at modelling WR140 Looking good But… Relationship from one to another is UNCLEAR – bad Continues as a work in progress 8th EVN Symposium, Torun

  28. Modelling the spectra Models give all radio emission components Most important -- intrinsic Lsyn, the non-thermal radio power Lsyn Phase 0.837 Thermal stellar wind • Now have estimate of B and intrinsic Lsyn • And why are these so important? 8th EVN Symposium, Torun

  29. WR140 lies within the error box of 3EG J2022+4317 EGRET (100MeV – 20 GeV) Is WR140 a gamma-ray source? From Benaglia & Romero (2003) 8th EVN Symposium, Torun

  30. GLAST VERITAS INTEGRAL NT bremsstrahlung Inverse Compton Photon pair production opacity Emission from WR140 at phase 0.8 Radio ASCA Pion decay 8th EVN Symposium, Torun

  31. Looks like a duck, quacks like a duck – it’s a duck! • Cyg OB2 #9 • Not a spectroscopic binary • Apparently single! • Variable radio emission • 2.4-yr period • Radio obs => binary • Is there a WCR? • VLBA obs • looks like a WCR • Other evidence of companion? e.g. WCR rotate on plane-of-sky? 8th EVN Symposium, Torun

  32. Summary • Colliding winds in early-type binaries are useful laboratories for investigating particle acceleration • New insights into particle acceleration – at higher mass, B-field, and energy densities than in SNRs • Excellent data on a number of systems • Radiometry and imaging – WR140 and WR146 – more recently Cyg OB2 #9 • WR140 has well-constrained system parms from high-resolution imaging – very important for modelling • WR140 and Cyg OB2 #9 – similar flux orbit-to-orbit - emission arises from well-behaved process(es) • Hydro models of plasma distribution • Successful models of spectrum and spatial distribution of emission. • Some issues revealed in models of WR146 – better data constraints • high-frequency spectrum & spatial extent of emission • Models lead to intrinsic synchrotron radio emission and magnetic energy density • used to estimate the non-thermal X-ray and -ray emission • Insight into particle (ions & electrons) acceleration efficiencies, and the B-field • Exciting period with respect to new data from INTEGRAL, GLAST, HESS, VERITAS, etc. • Constrain models (e.g. pion decay signature of relativistic ion production). Dougherty, Beasley, Claussen, Zauderer, Bolingbroke, 2005, ApJ 623, 447 Pittard, Dougherty, Coker, O’Connor, Bolingbroke, 2006, A&A 446,1001 Pittard & Dougherty, 2006, MNRAS, in press O’Connor, Dougherty, Pittard, Williams, 2007, in prep 8th EVN Symposium, Torun

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