GAMMA-RAYS FROM COLLIDING WINDS OF MASSIVE STARS

Anita Reimer, Stanford University

Olaf Reimer , Stanford University

Martin Pohl, Iowa State UniversityCourtesy: 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 2002

-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: Lx (binary) > Lx (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 J2016+3657, 3EG J2022+4317, 3EG J2033+4118 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 ~1032-35 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 (tconv ~ trad !)

„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 Modeldiffusion

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 = 5·1013, 1014, 2·1014, 5·1014, 1015 cm

r = 1011, 1012, 1013, 5 1013 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=

90o

0o

180o

i=45o B=

0o

90o, 270o

180o

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

• 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, ~47o

• D ~ 0.3…5 1014 cm

• 3EG J2022+4317 ?

-Anita Reimer, Stanford University -

Phase=0.8

Phase=0.67Phase=0.2

Phase=0.95

D~2.5AU

WR 140 (WC7+O4-5V)

-Anita Reimer, Stanford University -

[from: Niemela et al. 1998]

WFPC2WR 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

-Anita Reimer, Stanford University -

WR 147 (WN8+B0.5V)

Phase=0 Phase=0.25

Phase=0.5Phase=0.75

B0.5V

WN80 0.5

0.25

0.75

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 -