Long term spectral variability of Hen 3-640 (A1118-61)

We present spectroscopic observations of the peculiar emission line star Hen 3-640, the optical counterpart of the hard X-ray source A 1118-61, from 1985 to 1997. Our data confirm strong spectral variability (both on long and short time scales) of the star and the correlation between the hard X-ray flux and the Hα equivalent width. We also found a significant Hα/Hβ flux ratio variability during the last 12 years and a clear evidence of a general decrease of the ionization level after the 1992 outburst. The derived scenario is that of a large extended envelope surrounding Hen 3-640, expanding because of processes intrinsic to the optical star; when the density around the compact object reaches a value large enough to generate a steady accretion disk around the A1118-61 pulsar, the mass transfer efficiency and thus the X-ray flux sharply increases. Our scenario does not foreseen any exact periodicity in the X-ray outbursts of the systems.