The peculiar optical companion

to the eclipsing millisecond Pulsar PSR 1740-5340

in the globular cluster NGC 6397

A team of researchers of the Bologna Astronomical Observatory (Francesco R. Ferraro, Nichi D'Amico, Andrea Possenti & Elena Sabbi) discovered the optical counterpart to the companion of the eclipsing millisecond Pulsar PSR J1740-5340 in the inner region of the high-density Galactic Globular Cluster NGC6397.

The research has been published in a paper appeared in the 1 November 2001 issue of the Astrophysical Journal Letters.

By using high resolution HST images and wide-field images obtained at ESO, the team identified the companion as a relatively bright variable star. The brighteness and the colour of the companion exclude that it is a white dwarf (the usually seen companion to a millisecond pulsar). It may be a Main Sequence star acquired by exchange interaction in the cluster core or, alternatively, it may be the swelled star that spun up the MSP. In both cases, now it is a star highly deformed by the tidal effect of the pulsar, and it is still overflowing its Roche lobe. In fact, both radio and optical observations suggest that the pulsar is orbiting within an extended envelope of matter released from the companion, as depicted in this artistic rendition.

This is indeed the only system discovered so far in which both the companion is still losing mass and the millisecond pulsar is active. However the pulsar's powerful emission prevents gas from accreting onto the neutron star, shooting it outside the binary system. See a sketch of this sweeping mechanism.


  • - European Southern Observatory
  • - Bologna Astronomical Observatory
  • - ZAF @ Bologna Astronomical Observatory
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Last Update:   22 Feb 2002

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