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


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

M. Villada 1, C. Rossi 2, V.F. Polcaro 3 and F. Giovannelli 3

1 Observatorio de la Universidad de Cordoba, Cordoba, Argentina
2 Universita' di Roma "la Sapienza"
3 Istituto di Astrofisica Spaziale, CNR, Area di Ricerca di Roma Tor Vergata, I-00133 Roma, Italy

Received 20 May 1998 / Accepted 19 November 1998

Abstract

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 H[FORMULA] equivalent width. We also found a significant H[FORMULA]/H[FORMULA] 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.

Key words: X-rays: stars – stars: individual: Hen 3-640 (A1118-61) – stars: emission-line, Be

Send offprint requests to: V.F. Polcaro (polcaro@saturn.ias.rm.cnr.it)

© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999

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