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Abstract. Among the X-ray/Be systems, A 0535 + 26/HDE 245770 has been noted, since its discovery, for its peculiar features in several respects, in a wide energy range. For this reason and for a series of concomitant favorable causes, this system has been one of the most studied among the massive X-ray binary systems. The most remarkable incident was that its optical identification with an early-type-emission-line star (O9.7IIie) has led to a deep studies on Be stars and their interactions with neutron stars, which have allowed to discover, without unbiguity, the presence of optical indicators of consequent X-ray flares, as well as that Be stars in X-ray/Be systems behave just as 'normal' Be stars. Overmore, thanks to the multifrequency coordinated observations of this system, the X-ray emissions from binary companion of the Be stars are best explained by assuming the presence of a thick equatorial disk with low expansion velocity and a thin polar region with high expansion velocity. This picture reconciled the strong discrepancy in mass loss rate evaluations coming from IR and from UV measurements, assuming that the observed regions are enterely distinct from each other, one being a high-density, low-velocity region, and the other being a low-density, very hot, rapidly-expanding disk-like zone.Since, this picture seems to be the best up-to-date frame to cuckold all the experimental panorama available on X-ray/Be systems, we would like to paint in this paper the multifrequency behaviour of A 0535 + 26/HDE 245770, which is the best studied among such systems, in order to stimulate future coordinated experimental-theoretical works on this very interesting class of objects.

TeV γ-ray emission has recently been discovered by Cherenkov telescopes from two microquasars, LS 5039 and LS I +61 o 303. This emission is likely to be produced inside the binary system since in both cases variability with the orbital period of the binary has been discovered. In fact, such emission features have recently been predicted by the inverse Compton (IC) e ± pair cascade model. In this model, electrons accelerated in the jet develop a cascade in the anisotropic radiation of the massive star. The γ-ray spectra emerging from the cascade strongly depends on the location of the observer with respect to the orbital plane of the binary. Here we apply this model to investigate the possible γ-ray emission features from another compact massive binary of the microquasar type, Cyg X-1. We conclude that the observational constraints at lower energies (from MeV to GeV) suggest that the spectrum of electrons injected in the jet is likely steeper than in other TeV γ-ray microquasars. The cascade γ-ray spectrum produced by electrons with such a spectrum in Cyg X-1 should be below the sensitivities of the MAGIC and VERITAS class Cherenkov telescopes. However, if the electron spectrum is flatter, then the highest TeV γ-ray fluxes are predicted at ∼7 h before and after the phase when the compact object is in front of the massive star. We suggest that Cherenkov telescopes should concentrate on these ranges of phases since the TeV flux can vary by a factor of ∼20 with the period of the Cyg X-1 binary system. Moreover, the model predicts clear anticorrelation of the GeV and TeV γ-ray emission. This feature can be tested by the future multiwavelength observations with the AGILE and GLAST telescopes in the GeV energy range and the MAGIC and VERITAS telescopes in the TeV energy range.

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We report results of the spectroscopy for V725 Tau (HDE 245770, BD +26• 883), commonly known as Flavia's star, which is the optical counterpart of the X-ray pulsar A0535+26, carried out at the 1.5 m Loiano telescope during one run of observations of this star spread over a period of years. The HeI emission lines clearly show doubling, which is good evidence for the presence of a disc. In this paper we critically discuss the possibility that this disc is a temporary accretion disc around the neutron star, a view that contrasts to the usual interpretation, which considers that this sort of doubling in the HeI emission lines is due to a disc formed by gas expelled from the Be star. In the former case the outer radius of the accretion disc could range from 5.9 × 10 10 cm to 2.1 × 10 11 cm, taking the most probable range into account for the orbital inclination of the binary system, from 35• to 39• . The presence of such a temporary accretion disc around the neutron star was predicted by Giovannelli & Ziółkowski (1990).

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