  Context: Some high mass X-ray binaries (HMXB) have been recently confirmed as
gamma-ray sources by ground based Cherenkov telescopes. In this work, we
discuss the gamma-ray emission from X-ray transient sources formed by a Be star
and a highly magnetized neutron star. This kind of systems can produce variable
hadronic gamma-ray emission through the mechanism proposed by Cheng and
Ruderman, where a proton beam accelerated in the pulsar magnetosphere impacts
the transient accretion disk. We choose as case of study the best known system
of this class: A0535+26. Aims: We aim at making quantitative predictions about
the very high-energy radiation generated in Be-X ray binary systems with
strongly magnetized neutron stars. Methods: We study the gamma-ray emission
generated during a major X-ray outburst of a HMXB adopting for the model the
parameters of A0535+26. The emerging photon signal from the disk is determined
by the grammage of the disk that modulates the optical depth. The
electromagnetic cascades initiated by photons absorbed in the disk are
explored, making use of the so-called "Approximation A" to solve the cascade
equations. Very high energy photons induce Inverse Compton cascades in the
photon field of the massive star. We implemented Monte Carlo simulations of
these cascades, in order to estimate the characteristics of the resulting
spectrum. Results: TeV emission should be detectable by Cherenkov telescopes
during a major X-ray outburst of a binary formed by a Be star and a highly
magnetized neutron star. The gamma-ray light curve is found to evolve in
anti-correlation with the X-ray signal.
