A 0535 + 26/HDE 245770: A typical X-ray/Be system

Abstract

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.

High-mass X-ray binaries in the Milky Way

“…Besides giant outbursts not related to the specific orbital phase, the source demonstrates also normal (Type I) outbursts linked to the periastron passages of the neutron star (see, e.g. Giovannelli and Graziati, 1992). The binary system consists of a B0IIIe star HDE 245770 at the distance of ∼ 2 kpc (Steele et al, 1998) and a neutron star rotating with a spin period ∼ 103 s. The orbit is highly eccentric (e ∼ 0.47) with a period of ∼ 111.1 days (Finger et al, 1996b).…”

X-Ray and Optical Observations of a 0535+26

“…X-ray flux measurements by distinct missions from 1970 until 1989 were obtained from Giovannelli & Graziati (1992 In addition, in this work we present results for the continuous monitoring of A 0535+26 by Fermi GBM (Meegan et al 2009) since 2008 June 11. The GBM is an all-sky instrument sensitive to X-rays and gamma rays with energies between ∼8 keV and ∼40 MeV.…”

“…Figure 1 shows a long-term X-ray/optical overview of A 0535+26/HDE 245770. The pre-BATSE X-ray flux measurements found in Giovannelli & Graziati (1992) large variety of instruments, operating in different energy bands, and were presented only in Crab units. Due to the fact that the conversion of those measurements to real flux values is not straightforward and in order to show a global picture of this system, all the X-ray flux/count rates were converted into Crab units and then normalized using the maximum value for the whole data set (that is, BATSE).…”

“…The optical counterpart is the O9.7 IIIe star HDE 245770 (Giangrande et al 1980). An exhaustive review of observations at different wavelengths from 1970 until 1989 is found in Giovannelli & Graziati (1992).…”

Very high energy γ-ray emission from X-ray transients during major outbursts

“…This motivated extensive observational campaigns that have made of it the best known system in its class, and a good subject for the model discussed here. A complete review of A0535+26 is given by Giovannelli & Graziati (1992).…”