X-Ray Transient Sources (Multifrequency Laboratories) The Case Of The Prototype A0535+26/Hde 245770

The goal of this paper is to discuss the behaviour of the X-ray transient source A0535+26 which is considered for historical reasons and for the huge amount of multifrequency data, spread over a period of 35 years, as the prototype of this class of objects. Transient sources are formed by a Be star - the primary - and a neutron star X-ray pulsar the secondary - and constitute a sub-class of X-ray binary systems.We will emphasize the discovery of low-energy indicators of high-energy processes. They are UBVRI magnitudes and Balmer lines of the optical companion. Particular unusual activity of the primary star - usually at the periastron passage of the neutron star - indicates that an X-ray flare is drawing near. The shape and intensity of X-ray outbursts are dependent on the strength of the activity of the primary.We derive the optical orbital period of the system as 110.856 +/- 0.02 days. By using the optical flare of December 5, 1981 (here after 811205-E) that triggered the subsequent X-ray outburst of December 13, 1981, we derive the ephemeris of the system as JD Popt-outb = JD 0 (2, 444, 944) +/- n(110.856 +/- 0.02). Thus the passage of the neutron star at the periastron occurs with a periodicity of 110.856 +/- 0.02 days and the different kinds of X-ray outbursts of A0535+26 following the definitions reported in the review by Giovannelli & Sabau-Graziati (1992) - occur just after similar to 8 days. The delay between optical and X-ray outbursts is just the transit time of the material coming out from the optical companion to reach the neutron star X-ray pulsar.The occurrence of X-ray "normal outbursts", "anomalous outbursts" or "casual outbursts" is dependent on the activity of the Be star: "quiet state: steady stellar wind", "excited state: stellar wind plus puffs of material", and "expulsion of a shell", respectively. In the latter case, the primary manifests a strong optical activity and the consequent strong X-ray outburst can occur in any orbital phase, with a preference at the periastron passage of the neutron star, because of its gravitational effects on the Be star.