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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.
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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
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.
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.
This work is a summary of the X-ray spectral studies of 29 TeV $gamma$-ray emitting blazars observed with Swift/XRT, especially focusing on sources for which X-ray regime allows to study the low and the high energy ends of the particle distributions function. Variability studies require simultaneous coverage, ideally sampling different flux states of each source. This is achieved using X-ray observations by disentangling the high-energy end of the synchrotron emission and the low-energy end of the Compton emission, which are produced by the same electron population. We focused on a sample of 29 TeV gamma-ray emitting blazars with the best signal-to-noise X-ray observations collected with Swift/XRT in the energy range of 0.3-10 keV during 10 years of Swift/XRT operations. We investigate the X-ray spectral shapes and the effects of different corrections for neutral hydrogen absorption and decompose the synchrotron and inverse Compton components. In the case of 5 sources (3C 66A, S5 0716+714, W Comae, 4C +21.35 and BL Lacertae) a superposition of both components is observed in the X-ray band, permitting simultaneous, time resolved studies of both ends of the electron distribution. The analysis of multi-epoch observations revealed that the break energy of X-ray spectrum varies only by a small factor with flux changes. Flux variability is more pronounced in the synchrotron domain (high-energy end of the electron distribution) than in the Compton domain (low energy end of the electron distribution). The spectral shape of the Compton domain is stable, while the flux of the synchrotron domain is variable. These changes cannot be described by simple variations of the cut-off energy, suggesting that the high-energy end of the electron distribution is not generally well-described by cooling only.
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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