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Register a new userRevealing Gamma-Ray Binaries
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I. F. Mirabel
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Recent ground based and space telescopes that detect high energy photons from a few up to hundreds of gigaelectron volts (GeV) have opened a new window on the universe. However, because of the relatively poor angular resolution of these telescopes, a large fraction of the thousands of sources of gamma-rays observed remains unknown. Compact astrophysical objects are among those high energy sources, and in the Milky Way there is a particular class called Gamma-Ray Binaries.
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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
After initial claims and a long hiatus, it is now established that several binary stars emit high (0.1-100 GeV) and very high energy (>100 GeV) gamma rays. A new class has emerged called gamma-ray binaries, since most of their radiated power is emitted beyond 1 MeV. Accreting X-ray binaries, novae and a colliding wind binary (eta Car) have also been detected - related systems that confirm the ubiquity of particle acceleration in astrophysical sources. Do these systems have anything in common ? What drives their high-energy emission ? How do the processes involved compare to those in other sources of gamma rays: pulsars, active galactic nuclei, supernova remnants ? I review the wealth of observational and theoretical work that have followed these detections, with an emphasis on gamma-ray binaries. I present the current evidence that gamma-ray binaries are driven by rotation-powered pulsars. Binaries are laboratories giving access to different vantage points or physical conditions on a regular timescale as the components revolve on their orbit. I explain the basic ingredients that models of gamma-ray binaries use, the challenges that they currently face, and how they can bring insights into the physics of pulsars. I discuss how gamma-ray emission from microquasars provides a window into the connection between accretion--ejection and acceleration, while eta Car and novae raise new questions on the physics of these objects - or on the theory of diffusive shock acceleration. Indeed, explaining the gamma-ray emission from binaries strains our theories of high-energy astrophysical processes, by testing them on scales and in environments that were generally not foreseen, and this is how these detections are most valuable.
We review the multiwavelength properties of the few known gamma-ray binaries, focusing on extended emission recently resolved with Chandra. We discuss the implications of these findings for the nature of compact objects and for physical processes operating in these systems.
Gamma-ray emitting binaries (GREBs) are complex systems. Its study became in the last years a major endeavour for the high-energy astrophysics community, both from an observational and a theoretical perspective. Whereas the accumulation of observation time for most Galactic gamma-ray sources is typically leading to highly accurate descriptions of their steady phenomenology, GREBs keep providing exceptions to the rule either through long-term monitoring of known systems or in the discovery of new sources of this class. Moreover, many GREBs have been identified as powerful radio, optical and X-ray emitters, and may significantly contribute as well to the Galactic cosmic-ray sea. Their understanding implies, therefore, solving a puzzle in a broad-band and multi-messenger context. In these proceedings we will summarise our current understanding of GREBs, emphasising the most relevant observational results and reviewing a number of controversial properties.
Gamma-ray binaries allow us to study physical processes such as particle acceleration up to very-high energies and gamma-ray emission and absorption with changing geometrical configurations on a periodic basis. These sources produce outflows of radio-emitting particles whose structure can be imaged with Very Long Baseline Interferometry (VLBI). We have studied the changing morphology of the gamma-ray binaries LS I +61 303 and LS 5039, and we have discovered the extended emission of PSR B1259-63 and HESS J0632+057. Based on these results, we have established the basic properties and behaviour of the radio emission of gamma-ray binaries on AU scales, and we have contributed to find characteristics that are common to all of them. Here we present the most relevant properties of each source and the general properties of gamma-ray binaries, and we describe the implications on the nature of these binary systems.
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