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Astron. Astrophys. 344, 277-281 (1999)

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1. Introduction

The A1118-61 hard X-ray transient underwent a major outburst only twice: in 1974, when it was discovered by Ariel-5 satellite, and from December 1991 to February 1992 (Bildsten et al., 1997; for a detailed discussion of the system, see also Coe et al., 1994). The peculiar star Hen 3-640 (V=12.1, spectral type O9.5 IVe), optical counterpart of A1118-61 (Chevalier & Ilovaisky, 1975; Motch et al., 1988) has been found to show long term minor photometric (0.1 mag; Wray, 1966) and spectral variability (Motch et al., 1988). Previous spectroscopic studies (Villada et al., 1992; Polcaro et al., 1993; Coe et al., 1994) have demonstrated also the presence of strong spectral variability in Balmer line equivalent width on a short time scale (of order of a few hours): this behaviour has been suggested as due to the presence of the compact companion of Hen 3-640, the 405 s pulsar A1118-61.

The physical mechanism producing the X-ray outbursts in binary systems is the sudden increase of the mass accretion on the neutron star, that can be due to an activity enhancement of the primary emission line star. Other possibilities are that, close to the periastron passage, gravitational effects can trigger the release of blobs of material from the optical star or the neutron star can cross the equatorial disk of the B star as demonstrated in the case of A0535+26/HDE245790 (e.g. Giovannelli & Sabau-Graziati, 1992). A combination of these effects is also possible. Indeed Hen 3-640 very probably usually does not fill up its Roche lobe and the Hen 3-640/A1118-61 system has probably a long orbital period (Motch et al., 1988; Lund et al., 1992) and the reason of the X-ray outbursts generation is still unclear. Actually, the intriguing spectra recorded after the January 1992 outburst show that the system underwent a very complex event (Coe et al., 1994 and references therein).

Hen 3-640 has been repetitively observed within the framework of the collaborative program between the Observatorio de la Universidad de Cordoba and the Istituto di Astrofisica Spaziale (CNR) devoted to the monitoring of the optical counterparts of hard X-ray transients.

The 1990, 1992 and 1993 observations have been previously discussed in details (Villada et al., 1992; Polcaro et al., 1993; Coe et al., 1994; Giovannelli et al., 1994a); we present here the scenario that we derive from our 12-year observation campaign.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999
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