GAMMA-RAYS FROM COLLIDING WINDS OF MASSIVE STARS Anita Reimer, Stanford University Olaf Reimer, Stanford University Martin Pohl, Iowa State University Courtesy: J. Pittard

Motivation Radio synchrotron radiation from collision region „proof“ for: existence of relativistic e - existence of magnetic field Gamma rays  non-thermal relativistic particle distribution Observational evidence of a colliding wind origin for the non-thermal radio emission from WR 147. inverse Compton (IC) scattering in photospheric radiation field & relativistic e - -bremsstrahlung are garantueed HE processes !  role of colliding winds from massive stars as  -ray emitter ?  >150 still unidentified EGRET-sources: - population studies imply correlation of some Unids with massive star populations (OB-associations, WR-, Of-stars, SNRs) [Montmerle 1979, Esposito et al. 1996, Kaul & Mitra 1997, Romero et al. 1999, …] - 15 TeV-Unids MERLIN 5-GHz map on top of an optical image from: Dougherty Anita Reimer, Stanford University -

Stagnation point (ram pressure balance): A schematic view on the colliding wind region from: Eichler & Usov 1993 Magnetic field: B

Radio band: free-free emission (S ~  for isothermal spherical wind) + synchrotron radiation („proof“ for existence of relativistic electrons!) Continuum Observations X-ray: thermal (shock-heated gas) + non-thermal ? often found: L x (binary) > L x (2 x single) phase-locked variations in binaries [from: Pittard et al. 2002] MERLIN 5-GHz map overlaid with contours from Chandra HRC-I-image WR 147 -Anita Reimer, Stanford University -

COS-B: WR 140 [Pollock 1987] EGRET: No individual binary system unambiguously identified, but intriguing spatial coincidences: 3EG J , 3EG J , 3EG J positional coincident with WR 137, WR 140, Cyg OB2#5 [Romero et al. 1999, Benaglia et al. 2001] SPI Observational „history“ at  -rays -Anita Reimer, Stanford University -

[e.g. White 1985, Chen & White 1991, White & Chen 1992,...: NT processes in single massive stars Usov 1992, Stevens et al. 1992,...: thermal X-ray production in massive binaries Eichler & Usov 1993, Benaglia & Romero 2003, Pittard et al. 2005,....: NT processes in m. binaries] expected mean  -ray (>100 MeV) luminosity ~ erg/s based on Thomson-limit appr. for IC emission process, NT bremsstrahlung,  0 -decay  s,... recently: (Reimer, Pohl & Reimer 2006, ApJ ) - Klein-Nishina (KN) & anisotropy effects in IC scattering process - propagation effects (t conv ~ t rad !) „Historical“ theory aspects -Anita Reimer, Stanford University -

uniform wind neglect interaction of stellar radiat. field on wind structure  restrict to wide binaries cylinder-like emission region (x >> r, emission from large r negligible) radiation field from WR-star negligible (D >> x) photon field of OB-comp. monochromatic: n(  ) ~  T ),  T  10 eV electron distribution isotropically convection velocity V = const. magnetic field B = const. throughout emission region The Model diffusion dominated convection dominated -Anita Reimer, Stanford University -

Basic Equations Spectral index depends on shock conditions & propagation parameters ! +1

Energy loss time scales Coulomb losses limit acceleration rate inverse Compton losses dominate radiation losses cutoff energy might be determined by synchrotron losses Thomson-formula deviates from KN-formula already at  <  TL =   -1 approximations for KN-losses to derive analytical solutions for e - spectra Bremsstrahlung-, Coulomb & sync. losses unimportant in convection zone -Anita Reimer, Stanford University -

Electron spectra D = 5 · 10 13, 10 14, 2 · 10 14, 5 · 10 14, cm  r = 10 11, 10 12, 10 13, cm deficit of high-energy particles in convection region ! -Anita Reimer, Stanford University -

 orbital variation of IC radiation expected from wide WR-binaries IC scattering in colliding winds of massive stars OB WR B=B= 90 o 0o0o 180 o i=45 o B=B= 0o0o 90 o, 270 o 180 o anisotropic IC scattering  emitted power increases with scattering angle ! propagation effect -Anita Reimer, Stanford University -

WR 140 (WC7+O4-5V) distance ~ 1.85 kpc period ~ 2899±10 days L O ~ erg/s T eff ~ K WC: V~2860 km/s, M~ M o /yr O: V~3100 km/s, M~ M o /yr e ~ 0.88±0.04, i ~ 122 o ±5 o,  ~47 o D ~ 0.3… cm 3EG J ? -Anita Reimer, Stanford University -

Phase=0.8 Phase=0.67 Phase=0.2 Phase=0.95 D~2.5AU WR 140 (WC7+O4-5V) -Anita Reimer, Stanford University -

[from: Niemela et al. 1998] WFPC2 WR 147 (WN8+B0.5V) distance ~ 650 pc L B ~ erg/s T eff ~ K WN: V~950 km/s, M~ M o /yr B: V~800 km/s, M~ M o /yr D/sin i ~ cm in vicinity of 3EG J Anita Reimer, Stanford University -

WR 147 (WN8+B0.5V) Phase=0 Phase=0.25 Phase=0.5 Phase=0.75 B0.5V WN l.o.s. -Anita Reimer, Stanford University -

Galactic WR-binaries and  -ray Unids positional coincidence ? physical relation ? Detectability issue /distance or source physics ? found for 9 WR-binaries [Romero et al. 1999, Benaglia et al. 2005] possible, but: - detection may be phase-dependent - large stellar separations preferred for IC dominated  -ray production process  physically similar (to WR 140,147) WR-binaries: (not complete!) WR 137, WR 138, WR 146 [spatial coincid. with Unids: Romero et al `99] WR 125, WR 112, WR 70: no convincing positional corr. to any 3EG Unid GLAST-LAT  -Anita Reimer, Stanford University -

KN-effects may influence spectral shape & cutoff energy of IC-spectrum propagation effects may lead to a deficit of high-energy photons in the convection region ( spectral softening of total spectrum) variation of  -ray flux expected due to - modulation of (target) radiation field density in eccentric orbits - changes in wind outflow - modulations of emitting region (size, geometry) - orbital variation of observed IC scattering angle (time scale of orbital period !)  massive binary systems are predicted to show (depend. on orbital system parameters more or less pronounced) orbital variability at  -ray energies WR 140 & WR 147 detectable with LAT if e - reach sufficient high energies establishing WR-binaries as  -ray emitters needs improved instrument performance GLAST-LAT Conclusions  -Anita Reimer, Stanford University -