The investigation of particle acceleration in colliding-wind massive binaries with SIMBOL-X
M. De Becker, G. Rauw, J. M. Pittard, R. Blomme, G. E. Romero, H. Sana, I. R. Stevens
aa r X i v : . [ a s t r o - ph ] S e p Mem. S.A.It. Vol. 75, 1 c (cid:13) SAIt 2004
The investigation of parti le a eleration in olliding-wind massive binaries with SIMBOL-X.
M. De Becker , G. Rauw , J.M. Pittard , R. Blomme , G.E. Romero , H. Sana , andI.R. Stevens Institut d’Astrophysique et de G´eophysique, Universit´e de Li`ege, FNRS, Belgiume-mail: [email protected] School of Physics and Astronomy, The University of Leeds, UK Royal Observatory of Belgium, Brussels, Belgium Facultad de Ciencias Astronom´ıcas y Geof´ısicas, Universidad Nacional de La Plata,Argentina European Southern Observatory, Chile School of Physics and Astronomy, University of Birmingham, UK
Abstract.
An increasing number of early-type (O and Wolf-Rayet) colliding wind binaries(CWBs) is known to accelerate particles up to relativistic energies. In this context, non-thermal emission processes such as inverse Compton (IC) scattering are expected to producea high energy spectrum, in addition to the strong thermal emission from the shock-heatedplasma. SIMBOL-X will be the ideal observatory to investigate the hard X-ray spectrum(above 10 keV) of these systems, i.e. where it is no longer dominated by the thermal emis-sion. Such observations are strongly needed to constrain the models aimed at understandingthe physics of particle acceleration in CWB. Such systems are important laboratories forinvestigating the underlying physics of particle acceleration at high Mach number shocks,and probe a di ff erent region of parameter space than studies of supernova remnants. Key words.
Stars: early-type – Radiation mechanisms: non-thermal – X-rays: stars –Acceleration of particles
1. Colliding-wind binaries as particleaccelerators
More than 30 early-type stars (most of themconfirmed or suspected binaries) exhibit syn-chrotron radiation in the radio domain (seeDe Becker 2007). This indicates that relativis-tic electrons are present, and therefore thata particle acceleration process is at work.The acceleration mechanism is most proba-
Send o ff print requests to : M. De Becker bly Di ff usive Shock Acceleration (DSA) in thepresence of strong hydrodynamic shocks (e.g.Pittard & Dougherty 2006). In massive CWBs,particle acceleration is expected to occur at theglobal shocks bounding the wind-wind colli-sion region, and perhaps also within this vol-ume (see Fig. 1). Such systems are further con-sidered to be candidates for the production ofcosmic-rays. In the context of CWBs, non-thermal X-rays are also expected to be pro-duced. Previous observations with XMM sug- De Becker et al.: Particle acceleration in colliding-wind binaries
Primary stellar wind Secondary stellar windSecondarystarPrimarystar Shocked gas Hydrodynamic shocks
Fig. 1.
Schematic view of a CWB system. gested hints for IC emission (De Becker 2007),although INTEGRAL data provided only up-per limits on the hard X-ray flux of CWBs(De Becker et al. 2007).
2. Cyg OB2
XMM-Newton observations of this O6If + O5.5III(f) binary (P ∼
22 d) revealed X-rayemission ( <
10 keV) dominated by the plasmaheated ( ∼
20 MK) by the colliding winds(De Becker et al. 2006). Any putative non-thermal emission component due to IC scat-tering is most probably overwhelmed by thethermal emission. The hard X-ray domain( >
10 keV) needs to be investigated in order tostudy the non-thermal emission from CWB.We simulated SIMBOL-X spectra on thebasis of a model combining the thermal emis-sion (see XMM results) and a power law (ICscattering) with a photon index equal to 2. Thislatter value is typical of relativistic electronsstrongly a ff ected by IC scattering that steep-ens the power law. We considered a ”bright”case with a flux a factor of 10 lower thatthe upper limits derived by INTEGRAL-ISGRIobservations (De Becker et al. 2007), and a”faint” case about a factor 10 lower again.The SIMBOL-X synthetic spectra obtained inboth cases with an exposure time of 100 ksare shown in Fig. 2. The quality of these spec-tra should allow us to derive the flux and thephoton index of the power law. Ideally, ob-servations at di ff erent orbtial phases would be
10 1001 N o r m a li ze d c oun t s / s ec / k e V E (keV) SIMBOL−X spectrum (100 ks)Cyg OB2 −5−4−3−2−1
10 1001 N o r m a li ze d c oun t s / s ec / k e V E (keV) −1 −2 −3 −4 −5 −6 SIMBOL−X spectrum (100 ks)MPD CZTCyg OB2
Fig. 2.
Synthetic SIMBOL-X MPD and CZT spec-tra of Cyg OB2 ff erent assumptionson the flux of the power law. taken in order to study the variation of the non-thermal emission along the eccentric orbit.
3. Conclusions
Our objective is to understand and quantify theparticle acceleration process in massive CWBand its potential relation with the productionof cosmic-rays, by putting constraints uponthe high-energy emission (photon index, flux,phase-locked variability) from such systems.
Acknowledgements.
MD is grateful to Giusi Micelafor her kind invitation to participate in this fruitfulworkshop.