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Register a new userRelativistic Doppler-boosted emission in gamma-ray binaries
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Gamma-ray binaries could be compact pulsar wind nebulae formed when a young pulsar orbits a massive star. The pulsar wind is contained by the stellar wind of the O or Be companion, creating a relativistic comet-like structure accompanying the pulsar along its orbit. The X-ray and the very high energy (>100 GeV, VHE) gamma-ray emissions from the binary LS 5039 are modulated on the orbital period of the system. Maximum and minimum flux occur at the conjunctions of the orbit, suggesting that the explanation is linked to the orbital geometry. The VHE modulation has been proposed to be due to the combined effect of Compton scattering and pair production on stellar photons, both of which depend on orbital phase. The X-ray modulation could be due to relativistic Doppler boosting in the comet tail where both the X-ray and VHE photons would be emitted. Relativistic aberrations change the seed stellar photon flux in the comoving frame so Doppler boosting affects synchrotron and inverse Compton emission differently. The dependence with orbital phase of relativistic Doppler-boosted (isotropic) synchrotron and (anisotropic) inverse Compton emission is calculated, assuming that the flow is oriented radially away from the star (LS 5039) or tangentially to the orbit (LS I +61 303, PSR B1259-63). Doppler boosting of the synchrotron emission in LS 5039 produces a lightcurve whose shape corresponds to the X-ray modulation. The observations imply an outflow velocity of 0.15-0.33c consistent with the expected flow speed at the pulsar wind termination shock. In LS I +61 303, the calculated Doppler boosted emission peaks in phase with the observed VHE and X-ray maximum. Doppler boosting might provide an explanation for the puzzling phasing of the VHE peak in this system.
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More than a dozen binary systems are now established as sources of variable, high energy (HE, 0.1-100 GeV) gamma rays. Five are also established sources of very high energy (VHE, >100 GeV) gamma rays. The mechanisms behind gamma-ray emission in binaries are very diverse. My current understanding is that they divide up into four types of systems: gamma-ray binaries, powered by pulsar rotation; microquasars, powered by accretion onto a black hole or neutron star; novae, powered by thermonuclear runaway on a white dwarf; colliding wind binaries, powered by stellar winds from massive stars. Some of these types had long been suspected to emit gamma rays (microquasars), others have taken the community by surprise (novae). My purpose here is to provide a brief review of the current status of gamma-ray emission from binaries, in the context of related objects where similar mechanisms are at work (pulsar wind nebulae, active galactic nuclei, supernova remnants).
Detailed modeling of the high-energy emission from gamma-ray binaries has been propounded as a path to pulsar wind physics. Fulfilling this ambition requires a coherent model of the flow and its emission in the region where the pulsar wind interacts with the stellar wind of its companion. We developed a code that follows the evolution and emission of electrons in the shocked pulsar wind based on inputs from a relativistic hydrodynamical simulation. The code is used to model the well-documented spectral energy distribution and orbital modulations from LS 5039. The pulsar wind is fully confined by a bow shock and a back shock. The particles are distributed into a narrow Maxwellian, emitting mostly GeV photons, and a power law radiating very efficiently over a broad energy range from X-rays to TeV gamma rays. Most of the emission arises from the apex of the bow shock. Doppler boosting shapes the X-ray and VHE lightcurves, constraining the system inclination to $iapprox 35^{rm o}$. There is a tension between the hard VHE spectrum and the level of X-ray to MeV emission, which requires differing magnetic field intensities that are hard to achieve with a constant magnetisation $sigma$ and Lorentz factor $Gamma_{p}$ of the pulsar wind. Our best compromise implies $sigmaapprox 1$ and $Gamma_{p}approx 5times 10^{3}$, respectively higher and lower than the typical values in pulsar wind nebulae. The high value of $sigma$ derived here, where the wind is confined close to the pulsar, supports the classical picture that has pulsar winds highly magnetised at launch. However, such magnetisations will require further investigations to be based on relativistic MHD simulations.
Gamma-ray binaries (GBs) have been object of intense studies in the last decade. From an observational perspective, GBs are phenomenologically similar to most X-ray binary systems in terms of their broad-band emission across the entire electromagnetic spectrum, being segregated from this source population by showing a maximum of their spectral energy distribution in the gamma-ray band, either at high-energies (HE: 100 MeV - 100 GeV) or very-high energies (VHE: above 100 GeV). From a theoretical perspective, the broad-band emission from GBs is a unique case in which particle acceleration and emission/absorption mechanisms can be tested against periodically changing conditions of their immediate surroundings. In this proceedings we examine some of the key observational results of the multi-wavelength emission from GBs. We discuss the correlated/contemporaneous emission observed in several of these systems, from radio to gamma-rays, by considering a single underlying particle-emitting population and the properties of the nearby photon, matter and magnetic ambient fields.
Context. Gamma-ray binaries are systems that radiate the dominant part of their non-thermal emission in the gamma-ray band. In a wind-driven scenario, these binaries are thought to consist of a pulsar orbiting a massive star, accelerating particles in the shock arising in the wind collision. Aims. We develop a comprehensive, numerical model for the non-thermal emission of shock accelerated particles including the dynamical effects of fluid instabilities and orbital motion. We demonstrate the model on a generic binary system. Methods. The model is built on a dedicated three-dimensional particle transport simulation for the accelerated particles dynamically coupled to a simultaneous relativistic hydrodynamic simulation of the wind interaction. In a post-processing step, a leptonic emission model involving synchrotron and inverse Compton emission is evaluated based on resulting particle distributions and fluid solutions, consistently accounting for relativistic boosting and $gammagamma$-absorption in the stellar radiation field. The model is implemented as an extension to the Cronos code. Results. In the generic binary, the wind interaction leads to the formation of an extended, asymmetric wind-collision region distorted by the effects of orbital motion, mixing and turbulence giving rise to strong shocks terminating the pulsar wind and secondary shocks in the turbulent fluid flow. With the presented approach it is, for the first time, possible to consistently account for the dynamical shock structure in particle transport processes, yielding a complex distribution of accelerated particles. The predicted emission extends over a broad region of energy, with significant orbital modulation in all bands.
Gamma-ray binaries are a subclass of high-mass binary systems whose energy spectrum peaks at high energies (E$gtrsim$100 MeV) and extends to very high energies (E$gtrsim$100 GeV) $gamma$ rays. In this review we summarize properties of well-studied non-transient $gamma$-ray binaries as well as briefly discuss poorly known systems and transient systems hosting a microquasar. We discuss also theoretical models that have been used to describe spectral and timing characteristics of considered systems
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