September 28 th , 2006 8 th EVN Symposium, Torun 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)
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September 28 th, 20068 th EVN Symposium, Torun Winds in collision: high energy particles in massive binary systems Sean M. Dougherty (NRC) In collaboration.
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September 28th, 20068th EVN Symposium, Torun
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• 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, -rays(?)
September 28th, 20068th EVN Symposium, Torun
Moran et al. 1989Williams et al. 1997
• WR147• High resolution observations
- MERLIN @ 5GHz: • 50 mas = 77AU @ 650pc
The radio structure of a colliding-wind binary
– two components - one thermal + one non-thermal
• IR obs resolve two stars– WR+ O/B type
• Position of NT emission: consistent with position of wind-wind collision region
September 28th, 20068th EVN Symposium, Torun
D
What is a wind-collision region?
• Two massive stars with stellar winds• Contact discontinuity where ram pressures are
equal
ratiomomentumwind
2/1
2/1
1D
rOB
• 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
O stars with NT radio emission ARE BINARY?ARE BINARY?
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
September 28th, 20068th EVN Symposium, Torun
• 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
WR 140 - the CWB laboratory
September 28th, 20068th EVN Symposium, Torun
Cartoon of the colliding-wind region in WR140
Orbit parameters from Williams et al. 1990 - interaction region based on Eichler & Usov 1993
September 28th, 20068th EVN Symposium, Torun
The radio light curve of WR140
8 years of VLA observations (White & Becker 1995) + WSRT data (Williams p.c.)
2cm
6cm
21cm
September 28th, 20068th EVN Symposium, Torun
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
September 28th, 20068th EVN Symposium, Torun
September 28th, 20068th EVN Symposium, Torun
• “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,• Orbit definition could wait for more IOTA observation, but in the meanwhile…..
IOTA observation – Monnier et al. 2004
September 28th, 20068th EVN Symposium, Torun
• 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
• 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)
• 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
Phase 0.837
September 28th, 20068th EVN Symposium, Torun
• Possible to constrain models with VLBI obs
Modelling 8 GHz VLBI observations of WR140
- demands good observations
September 28th, 20068th EVN Symposium, Torun
1.6
5
8.3
15
22/43
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
September 28th, 20068th EVN Symposium, Torun
First stab at modelling WR140
Looking good
But…Relationship from one to another is UNCLEAR –
badContinues as a work in progress
September 28th, 20068th EVN Symposium, Torun
Modelling the spectra
Models give all radio emission components Most important -- intrinsic Lsyn, the non-thermal radio power
• Now have estimate of B and intrinsic Lsyn
And why are these so important?
Phase 0.837
Thermal stellar wind
Lsyn
September 28th, 20068th EVN Symposium, Torun
WR140 lies within the error box of 3EG J2022+4317
EGRET (100MeV – 20 GeV)
From Benaglia & Romero (2003)
Is WR140 a gamma-ray source?
September 28th, 20068th EVN Symposium, Torun
NT bremsstrahlung
Pion decay
Emission from WR140 at phase 0.8
Photon pair production opacity Inverse Compton
Radio ASCA
INT
EG
RA
L
GL
AST
VE
RIT
AS
September 28th, 20068th EVN Symposium, Torun
Looks like a duck, quacks like a duck – it’s a duck!
• Cyg OB2 #9 • Not a spectroscopic binary
– Apparently single!
• VLBA obs– looks like a WCR
• Other evidence of companion?e.g. WCR rotate on plane-of-sky?
• Variable radio emission – 2.4-yr period
• Radio obs => binary• Is there a WCR?
September 28th, 20068th EVN Symposium, Torun
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).