Research Note

LS I +61303 in the context of microquasars

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: mmassi@mpifr-bonn.mpg.de

Received: 18 December 2003
Accepted: 27 April 2004

Abstract

LS I +61°303 is one of the most observed Be/X-ray binary systems because, peculiarly, it has periodical radio and X-ray emission together with strong, variable gamma-ray emission. This source remains, however, quite enigmatic. Some properties of this system can be explained assuming that the unseen companion is a non-accreting young pulsar with a relativistic wind strongly interacting with the wind of the Be star. On the contrary, other properties of LS I +61°303 fit a model where the companion is accreting even with two events of super-critical accretion along the orbit. The very recent discovery of a radio jet extending ca. 200 AU from both sides of a central core has definitely proved the occurrence of accretion-ejection processes in this system. Therefore it is of great interest to combine this result with previous observations at other wavelengths within the framework of the two-peak accretion (ejection) model. Concerning the first ejection, we show that the observed gamma-ray variations might be periodic with outbursts confined around the periastron passage (i.e. where the first accretion-rate peak occurs and high-energy emission but no radio emission is predicted). Concerning the second ejection, with radio bursts, we point out that it can be also traced in the X-ray data, both in episodes of hardening of the X-ray emission and in a transition from soft- to hard-states at the onset of radiobursts. Both hardening and transitions between spectral states are related to the dramatic change in the structure of the accretion disk preceeding the ejection. Finally, we explore the nature of the accretor and we conclude that on the basis of the present optical data a black hole cannot be ruled out.