Microquasars in the GeV-TeV era

The discovery of non-thermal X-ray emission from the jets of some X-ray binaries, and especially the discovery of GeV-TeV gamma-rays in some of them, provide a clear evidence of very efficient acceleration of particles to multi-TeV energies in these systems. The observations demonstrate the richness of non-thermal phenomena in compact galactic objects containing relativistic outflows or winds produced near black holes and neutron stars. We review here some of the main observational results on the non-thermal emission from X-ray binaries as well as some of the proposed scenarios to explain the production of high-energy gamma-rays.

💡 Research Summary

The paper “Microquasars in the GeV‑TeV era” provides a comprehensive review of the recent discovery of non‑thermal X‑ray emission and GeV‑TeV gamma‑ray radiation from a handful of X‑ray binaries that host relativistic jets, often referred to as microquasars. The authors begin by outlining the historical context: early radio and X‑ray observations hinted at the presence of collimated outflows, but only with the advent of modern high‑energy observatories (Fermi‑LAT, H.E.S.S., MAGIC, VERITAS, and later CTA prospects) has it become possible to directly detect photons in the GeV‑TeV band from these compact systems.

The observational section focuses on the most compelling sources. LS 5039 was the first binary detected at TeV energies (H.E.S.S., 2005) and exhibits a strong orbital modulation of its gamma‑ray flux, indicating that the emission region is subject to varying photon‑photon absorption as the compact object moves through the intense stellar radiation field. LS I +61 303 shows a similar pattern, with both GeV (Fermi‑LAT) and TeV (MAGIC, VERITAS) components that peak at distinct orbital phases, again pointing to a geometry‑dependent opacity. Cygnus X‑1, a well‑known black‑hole binary, produced a short TeV flare contemporaneous with a hard X‑ray outburst, suggesting rapid particle acceleration near the jet base. Cygnus X‑3 displayed powerful GeV flares detected by Fermi‑LAT, correlated with radio outbursts, which supports a scenario where the jet interacts with the dense wind of its Wolf‑Rayet companion.

In the theoretical part the authors compare two broad families of models. The “microquasar scenario” attributes the high‑energy emission to internal shocks or magnetic reconnection within the relativistic jet. Electrons accelerated to multi‑TeV energies radiate via synchrotron (producing the hard X‑ray tail) and inverse‑Compton scattering of either their own synchrotron photons (SSC) or the stellar photon field (external Compton). Protons may also be accelerated, leading to hadronic channels such as p‑γ or p‑p collisions that generate neutral pions and, consequently, gamma‑rays and neutrinos. The alternative “pulsar‑wind scenario” assumes that the compact object is a rotation‑powered neutron star; its relativistic wind collides with the stellar wind, forming a shock that accelerates particles. This model naturally explains orbital modulation through the changing geometry of the wind‑wind interaction region.

Both frameworks can reproduce the observed spectral energy distributions (SEDs) and variability, but they diverge in key predictions. Hadronic models anticipate a detectable neutrino flux and a relatively hard gamma‑ray spectrum extending beyond several TeV, whereas leptonic models predict stronger dependence on the stellar photon field and more pronounced orbital absorption features. The paper emphasizes that current data are insufficient to decisively favor one over the other; multi‑messenger observations (simultaneous radio, X‑ray, gamma‑ray, and neutrino monitoring) are required.

The authors also discuss the role of photon‑photon absorption (γ‑γ pair production) in shaping the observed TeV light curves. The optical depth depends sensitively on the binary separation, eccentricity, and inclination, which explains why LS 5039’s TeV emission peaks near inferior conjunction while LS I +61 303’s peaks near apastron.

Looking ahead, the authors highlight the upcoming Cherenkov Telescope Array (CTA) as a game‑changer. CTA’s order‑of‑magnitude improvement in sensitivity and its ability to resolve sub‑minute variability will allow precise mapping of the emission region, discrimination between leptonic and hadronic processes, and detection of spectral cut‑offs that pinpoint the maximum particle energy. Complementary neutrino detectors such as IceCube‑Gen2 and KM3NeT will test the hadronic hypothesis by searching for temporally correlated neutrino events.

In summary, the paper argues that the GeV‑TeV era has opened a new window onto particle acceleration in microquasars, revealing that these Galactic systems can operate as efficient “mini‑blazars.” Continued coordinated, multi‑wavelength, and multi‑messenger campaigns, together with next‑generation gamma‑ray facilities, are essential to unravel the detailed physics of jet formation, particle acceleration, and high‑energy radiation in these compact binaries.