Abstract
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 = JD0 (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 ∼ 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.
Highlights
The trivial definition of X-ray binaries (XRBs) is that they are binary systems emitting X-rays
It has been largely demonstrated that X-ray binary systems emit energy in IR, Optical, UV, X-ray, Gammaray energy ranges, and they sometimes show valuable radio emission
We think we have clearly demonstrated that the optical orbital period of the A0535+26/Flavia’ star system is Porb−opt = 110, 856 ± 0.02 days
Summary
The trivial definition of X-ray binaries (XRBs) is that they are binary systems emitting X-rays. HMXBs can be divided into two sub-classes Hard Transient X-ray Sources (HXTS) in which the neutron star is eccentrically (e ∼ 0.2–0.5) orbiting around a V-III luminosity-class Be star (Porb > 10 days); they show strong variable pulsed hard Xray emission (LXmax/LXmin > 100) with KT ≥ 17 keV; Permanent X-ray Sources (PXS), in which the neutron star or black hole is circularly orbiting (e ∼ 0) around a giant or supergiant OB star (Porb < 10 days); they show an almost steady permanent pulsed hard Xray emission (LXmax/LXmin 100);
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