3 The precessing jet

The asymmetry in the supernova, in which the neutron star is formed, causes the angular momentum axis of the binary to make an angle $\nu$with the spin axis of the black hole. This misalignment causes the accretion disc around the black hole to process (see Larwood 1998).

The magnetic field of the black hole is anchored in the disc. Energy can then be extracted from the black hole by slowing down its rotation via the magnetic field which exert torque on the induced current on the black hole horizon (Blandford & Znajek 1977; Thorne et al. 1986). The radiation is liberated in a narrow beam with an opening angle of $\ {\raise-.5ex\hbox{$\buildrel<\over\sim$}}\6^\circ$ (Fendt 1997). Such a narrow beaming is supported by the results of the ROTSE (Robotical Optical Transient Search Experiment) telescope (see http://www.umich.edu/rotse/ for more details). Since the magnetic field is anchored in the disc the radiation cone precesses with the same amplitude and period as the accretion disc.

The intrinsic time variation of a single gamma-ray burst has a short rise time followed by a linear decay (Fenimore 1997). We construct the burst time profile from three components: an exponential rise, a plateau phase and a stiff decay (see PZLL for details). Rapid variations within this timespan are caused by the precession and nutation of the beam. This results in a model with eight free parameters; three timescales for the profile of the burst, the precession and nutation periods, the precession angle and the direction (two angles) from which the observer looks at the burst.

Figure 1 gives the result of fitting the model to the gamma-ray burst with BATSE trigger number 1609 in the energy channel between 115 keV and 320 keV. The fit was performed by minimizing the $\chi^2$ with simulated annealing (see PZLL). The light curves computed with this model show similar complexities and variability as observed gamma-ray bursts (see Fig. 1).

Acknowledgements

We are grateful to Gerry Brown and Jun Makino for a great time, numerous discussions and financial support. This work was supported in part by the U.S. Department of Energy under Grant No. DE-FG02-88ER40388 and by NASA through Hubble Fellowship grant #HF-0112.01-98A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS 5-26555.