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GAMMA-RAYS FROM COLLIDING WINDS OF MASSIVE STARS

GAMMA-RAYS FROM COLLIDING WINDS OF MASSIVE STARS. Anita Reimer, Stanford University Olaf Reimer , Stanford University Martin Pohl, Iowa State University. Courtesy: J. Pittard. -Anita Reimer, Stanford University -. Motivation. Gamma rays Þ non-thermal relativistic particle distribution.

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GAMMA-RAYS FROM COLLIDING WINDS OF MASSIVE STARS

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

  2. -Anita Reimer, Stanford University - Motivation Gamma rays Þnon-thermal relativistic particle distribution • Radiosynchrotron radiation from collision region • „proof“ for: existence of relativistic e- • existence of magnetic field Observational evidence of a colliding wind origin for the non-thermal radio emission from WR 147. Þ inverse Compton (IC) scattering in photospheric radiation field & relativistic e--bremsstrahlung are garantueed HE processes ! • >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 2002 Þrole of colliding winds from massive stars as g-ray emitter ?

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

  4. -Anita Reimer, Stanford University - Continuum Observations • Radio band: • free-free emission (S ~ n0.6 for isothermal spherical wind) + • synchrotron radiation („proof“ for existence of relativistic electrons!) • X-ray: • thermal (shock-heated gas) + non-thermal ? • often found: Lx(binary) > Lx(2 x single) • phase-locked variations in binaries WR 147 MERLIN 5-GHz map overlaid with contours from Chandra HRC-I-image [from: Pittard et al. 2002]

  5. EGRET: No individual binary system unambiguously identified, but intriguing spatial coincidences: 3EG J2016+3657, 3EG J2022+4317, 3EG J2033+4118 positional coincident withWR 137, WR 140, Cyg OB2#5 [Romero et al. 1999, Benaglia et al. 2001] SPI -Anita Reimer, Stanford University - Observational „history“ at g-rays COS-B:WR 140[Pollock 1987]

  6. -Anita Reimer, Stanford University - „Historical“ theory aspects [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 meang-ray (>100 MeV) luminosity ~1032-35 erg/s based on Thomson-limit appr. for IC emission process, NT bremsstrahlung, p0-decay gs,... recently: (Reimer, Pohl & Reimer 2006, ApJ ) - Klein-Nishina (KN)&anisotropy effects in IC scattering process - propagation effects(tconv ~ trad !)

  7. diffusion dominated convection dominated -Anita Reimer, Stanford University - The Model • uniform wind • neglect interaction of stellar radiat. field on wind structure Þ 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(e) ~ d(e-eT) , eT» 10 eV electron distribution isotropically • convection velocity V = const. • magnetic fieldB = const.throughout emission region

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

  9. -Anita Reimer, Stanford University - 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 g < gTL = eT-1 • approximations for KN-losses to derive analytical solutions for e- spectra • Bremsstrahlung-, Coulomb & sync. losses unimportant in convection zone

  10. deficit of high-energy particles in convection region ! -Anita Reimer, Stanford University - Electron spectra D = 5·1013, 1014, 2·1014, 5·1014, 1015 cm Dr = 1011, 1012, 1013, 5 1013 cm

  11. propagation effect jB= i=45o 0o 180o jB= 90o, 270o 0o OB 90o WR anisotropic IC scattering Þ emitted power increases with scattering angle ! 180o -Anita Reimer, Stanford University - IC scattering in colliding winds of massive stars Þorbital variation of IC radiation expected from wide WR-binaries

  12. -Anita Reimer, Stanford University - WR 140 (WC7+O4-5V) • distance ~ 1.85 kpc • period ~ 2899±10 days • LO ~ 6 1039 erg/s • Teff ~ 47400 K • WC: V~2860 km/s, M~4.3 10-5 Mo/yr • O: V~3100 km/s, M~8.7 10-6 Mo/yr • e ~ 0.88±0.04, i ~ 122o±5o, w~47o • D ~ 0.3…5 1014 cm • 3EG J2022+4317 ?

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

  14. WFPC2 [from: Niemela et al. 1998] -Anita Reimer, Stanford University - WR 147 (WN8+B0.5V) • distance ~ 650 pc • LB ~ 2 1038 erg/s • Teff ~ 28500 K • WN: V~950km/s, M~2.5 10-5Mo/yr • B: V~800km/s, M~4 10-7 Mo/yr • D/sin i ~ 6.2 1015 cm • in vicinity of 3EG J2033+4118

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

  16. 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 g-ray production process Þ 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 Þ GLAST-LAT -Anita Reimer, Stanford University - Galactic WR-binaries and g-ray Unids • positional coincidence ? • physical relation ? Detectability issue/distance or source physics ?

  17. -Anita Reimer, Stanford University - Conclusions • 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 g-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 !) • Þmassive binary systems are predicted to show (depend. on orbital system • parameters more or less pronounced) orbital variability at g-ray energies • WR 140 & WR 147 detectable with LATif e- reach sufficient high energies • establishing WR-binaries as g-ray emitters needs improved instrument performance • GLAST-LAT Þ

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