Cherenkov Telescope results on gamma-ray binaries

In the past ten years of regular operations, a new generation of Cherenkov telescopes have established binary systems as a new class of Very High Energy gamma-ray (VHE) emitters. Particle acceleration in these systems may occur either in an accretion-powered jet (microquasar) or in the shock between a pulsar wind and a stellar wind (wind-wind). This paper describes the phenomenology of the three VHE binaries PSR~B1259-63, LS 5039 and LS I +61 303. Two other objects may belong to this new class: HESS J0632+057 is a point-like variable VHE source whose multiwavelength behaviour resembles that of the other binaries, whereas Cyg X-1 is a well-known accreting system which may have been detected in VHE during a flaring episode. The paper concludes with a review of the latest searches for other binaries with Cherenkov telescopes, with special emphasis on Cyg X-3.

💡 Research Summary

The paper reviews a decade of very‑high‑energy (VHE, >100 GeV) observations of gamma‑ray binaries performed with the current generation of imaging atmospheric Cherenkov telescopes (IACTs), namely H.E.S.S., MAGIC and VERITAS. It begins with a brief description of the VHE band and the capabilities of ground‑based IACTs, then focuses on the three systems that have been firmly established as VHE emitters: PSR B1259‑63/LS 2883, LS 5039, and LS I +61 303.

PSR B1259‑63 is a 48 ms pulsar orbiting a Be star on a highly eccentric (e ≈ 0.87) 3.4‑year orbit. The VHE emission is interpreted as inverse‑Compton scattering of shock‑accelerated leptons at the interface between the pulsar wind and the dense equatorial disc of the Be star. H.E.S.S. observed two periastron passages (2004 and 2007). In both cases the source reached ≈10 % of the Crab Nebula flux, but the detailed light‑curve shapes differed, suggesting that the geometry and density of the disc, as well as radiative losses, vary from one orbit to the next. The spectrum is a stable power law with photon index Γ≈2.7, corresponding to a VHE luminosity of ~8 × 10³² erg s⁻¹ (≈0.1 % of the pulsar spin‑down power).

LS 5039 consists of an O6.5 V star and an unknown compact object (neutron star or low‑mass black hole) on a short (P≈3.906 d), mildly eccentric (e≈0.337) orbit. H.E.S.S. measured a strong orbital modulation of the VHE flux (5–15 % Crab) and a clear variation of the photon index with orbital phase. At inferior conjunction the spectrum hardens dramatically above ~0.3 TeV, while at superior conjunction it softens, a behavior naturally explained by anisotropic inverse‑Compton scattering and γ‑γ absorption in the intense stellar photon field. The lack of detected pulsations leaves the nature of the compact object ambiguous, and VLBA imaging shows a complex, orbit‑dependent morphology that can be interpreted either as a pulsar‑wind bow shock or as a micro‑quasar jet.

LS I +61 303 pairs a B0 V star with an unknown compact object (e≈0.54, P≈26.5 d). MAGIC first detected VHE emission in 2006, finding a highly variable flux that peaks at orbital phases 0.6–0.7 (≈15 % Crab) and is essentially absent near periastron and inferior conjunction. The VHE spectrum is Crab‑like (Γ≈2.6). Simultaneous X‑ray observations with XMM‑Newton and Swift in 2007 revealed a strong correlation (r≈0.81) between the X‑ray and VHE fluxes, indicating a common electron population responsible for both synchrotron X‑rays and inverse‑Compton VHE photons. However, VERITAS observations in 2008–2010 failed to detect the source, setting upper limits of ~2 % Crab at the previously bright phases. This disappearance may be linked to the long‑term (≈1667 d) super‑orbital modulation of the Be star’s circumstellar disc, which can alter the density of target photons and the efficiency of particle acceleration.

The paper also discusses two candidate VHE binaries that have not been firmly confirmed. Cyg X‑1 is a high‑mass X‑ray binary containing a ≳13 M⊙ black hole and an O9.7 I companion. MAGIC observed a brief (≈80 min) VHE excess on 2006 September 24 with a post‑trial significance of 4.1σ and a soft spectrum (Γ≈3.2, flux ≈10 % Crab at 1 TeV). The excess coincided with enhanced hard X‑ray emission measured by Swift/BAT and RXTE/ASM, but subsequent observations did not reproduce the signal, leaving its VHE nature uncertain.

HESS J0632+057 is a point‑like, variable VHE source whose multi‑wavelength behaviour (X‑ray, radio) resembles that of known binaries, but a definitive binary counterpart has not yet been identified.

Finally, the authors review systematic searches for VHE emission from other X‑ray binaries, especially Cyg X‑3, which remain undetected despite deep observations. They emphasize that the next generation of Cherenkov facilities (e.g., CTA) will provide an order‑of‑magnitude improvement in sensitivity and temporal resolution, enabling detailed phase‑resolved spectroscopy, the detection of weaker systems, and a decisive test of the competing “micro‑quasar jet” versus “pulsar wind‑stellar wind” acceleration scenarios. The paper concludes that VHE gamma‑ray binaries constitute a unique laboratory for studying relativistic particle acceleration under extreme radiation and magnetic field conditions, and that continued multi‑wavelength campaigns are essential for unraveling their complex phenomenology.