Gamma-ray binaries are orbital modulated gamma-ray sources in the Galaxy detected both at GeV and TeV energies. The high-energy radiation may come from the interaction of energetic electrons injected by a young pulsar and photons from the massive companion star. We present a model for the production of high-energy radiation where emission, absorption and pair cascading are considered. New observations of LS I +61 303 by the Fermi Space Telescope revealed an exponential cut-off in the spectrum at a few GeV, inconsistent with gamma-gamma absorption. Electrons radiating at GeV and TeV have probably two different origins. We investigate whether the emission from the unshocked pulsar wind explains the GeV component in LS I +61 303.
Three binary systems have been discovered to produce non-thermal radiation up to very-high energies in the Galaxy (see also the contribution of J. Holder in these proceedings) namely 1 : LS I +61 o 303 [2,3], LS 5039 [4] and PSR B1259-63 [5]. HESS J0632+057, an unidentified gamma-ray source serendipitously discovered by HESS [6], may be the fourth system known [7]. These systems are composed of a Be or O companion star and a compact object in an eccentric orbit. Except for PSR B1259-63 where a 48 ms radio pulsar has been detected [8], the nature of the compact star remains unknown in the other two binaries.
These systems are known today as “gamma-ray binaries” as their non stellar luminosity peaks in gamma rays [9]. The main feature of these objects is probably the orbital modulation of the radiated gamma-ray flux. This property has been first uncovered at TeV energies by Cherenkov telescopes [10,11,12]. The Fermi Space Telescope has recently reported the first detections of the orbital modulation at GeV energies [13,14] (see Richard Dubois’ proceeding in this volume for more details).
In this proceeding, we report on our modeling investigations of the high and very-high energy radiation produced in gamma-ray binaries in the framework of the pulsar wind nebula scenario. In this model, the non-thermal radiation is emitted by a cooling plasma of relativistic electron-positron pairs injected and accelerated by a young pulsar (see Section 2). We present in Section 3 our modeling of the GeV and TeV modulation in LS 5039 where emission, absorption and pair cascading are considered. In Section 4, we discuss the new observations of LS I +61 o 303 by Fermi and investigate whether the emission from an 1 Note that the detection of Cyg X-3 by Fermi has been anounced at the Symposium (see S. Corbel’s proceeding). Note also that TeV emission from Cyg X-1 has been reported by MAGIC [1].
unshocked pulsar wind can explain the GeV component.
Maraschi & Treves [15] suggested that the radio to gamma-ray radiation could be powered by a young (∼ 10 4 -10 5 yr) fast-rotating pulsar in LS I +61 o 303 (scenario originally applied to Cyg X-3 [16]). The same idea has been recently revisited and proposed to be at work in the other gamma-ray binaries [9]. In this scenario (see the sketch in Fig. 1), the pulsar injects energetic electrons-positrons pairs in a relativistic wind. Particles propagate up to the termination shock where the pulsar wind momentum is balanced by the ram pressure of the massive star wind. The stellar wind can possibly confine the pulsar wind and a comet-like structure develops, spiraling around the system due to the orbital motion. At the shock, leptons are randomized, re-accelerated and produce nonthermal radiation.
In the ‘shocked’ wind, pairs emit synchrotron radiation and up-scatter the soft photons from the massive star to high energies by inverse Compton scattering. In the ‘unshocked’ wind region, pairs radiate only via inverse Compton scattering since upstream the termination shock, the magnetic field is frozen into the flow of particles. No synchrotron radiation is produced. The annihilation of gamma rays with the stellar photons can also be of major importance in some tight binaries such as LS 5039 (see next section). The production of gamma rays by inverse Compton scattering is not isotropic in gamma-ray binaries because the soft radiation field set by the massive star is anisotropic with respect to the location of electrons. Hence, the gamma-ray flux depends on the relative position of both stars and the observer (Fig. 1).
LS 5039 is composed of a O6.5V star in a compact 3.9 day orbit with its compact object. The orbital parameters are known with good accuracy (the latest solution can be found in [17]). The absence of a Be equatorial wind component makes this system ideal for modeling. TeV observations of this binary by HESS have shown a stable modulation of the gamma-ray flux with a maximum close to inferior conjunction and a minimum at superior conjunction [10]. The main behavior of this modulation can be accounted for by the effect of photon-photon annihilation γ + γ → e + + e - (see e.g. [18]). In the pulsar wind nebula scenario, the high-energy radiation has two distinct contributions.
The first component originates from the shocked wind where pairs are assumed to be isotropized and injected with a power-law energy distribution cooling down via synchrotron radiation and inverse Compton scattering. Combining emission and γγ-absorption of gamma rays, HESS observations can be reasonably well explained. Some physical quantities at the shock can be tightly constrained such as the magnetic field B = 0.8±0.2 G, the spectral index of the injected particle distribution p = 2 ± 0.3 and the pulsar spin down power L P = 10 36 erg.s -1 (see [19] for more details). This simple model can reproduce most of the spectral and temporal features at high energies but underestimates the flux observed by HESS at o
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