Feasibility study of Lense-Thirring precession in LS I +61°303

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

Received: 2 December 2009
Accepted: 18 March 2010

Abstract

Context. Very recent analysis of the radio spectral index and high energy observations have shown that the two-peak accretion/ejection microquasar model applies for LS I +61°303.

Aims. The fast variations of the position angle observed with MERLIN and confirmed by consecutive VLBA images must therefore be explained in the context of the microquasar scenario.

Methods. We calculate what could be the precessional period for the accretion disk in LS I +61°303 under tidal forces of the Be star (P_tidal-forces) or under the effect of frame dragging produced by the rotation of the compact object (P_{Lense-Thirring}).

Results. P_tidal-forces is more than one year. P_{Lense-Thirring} depends on the truncated radius of the accretion disk, R_tr. We determined R_tr = 300 r_g for observed QPO at 2 Hz. This value is much above the few r_g, where the Bardeen-Petterson effect should align the midplane of the disk. For this truncated radius of the accretion disk P_{Lense-Thirring} for a slow rotator results in a few days.

Conclusions. Lense-Thirring precession induced by a slowly rotating compact object could be compatible with the daily variations of the ejecta angle observed in LS I +61°303.