How pulsars get their kicks




A team studying a binary pulsar system has found the first instance of classical
spin-orbit coupling known outside the Solar System. The system seems likely
to have attained its present state because the supernova explosion that
created the pulsar was asymmetric. This may explain how many pulsars get
the initial 'kick' that sends them streaking through the Galaxy at high
velocities.



PSR J0045-7319 is a radio pulsar in orbit around a B-star companion:
it is one of only two such systems known, both found with the Parkes telescope.
Five years of pulsar timing have shown that it is not travelling in a simple
Keplerian orbit. Instead, the orbit shows two precession effects: classical
periastron advance, caused by the oblateness of the B star; and precession
of the inclination angle of the orbital plane &shyp; the plane is becoming
more nearly edge-on to us. This second effect must arise from coupling between
the orbital angular momentum and the spin angular momentum of the B star.




But if the orbital plane is precessing, the B star's spin axis must
be inclined relative to the vector of orbital angular momentum. (In this
system it is tilted by between 25° and 41°.) How could this state
of affairs come about? It is almost certain that the system's angular momenta
were all aligned before the pulsar was born. It is highly unlikely that
they were misaligned by an encounter with a third body. And so it seems
probable that, at its birth, the pulsar was kicked out of the system's original
orbital plane and into a new one, re-aligning the vector of orbital angular
momentum. The birth 'kick' must have been at least of the order of 50 km
s-1, and more probably around

100 km s-1. The only likely source of this kick is an asymmetric supernova
explosion.



Evidence for a birth 'kick' to neutron stars is satisfying, because
this mechanism has been invoked to explain not only large pulsar velocities
but also the absence of companions to OB runaway stars &shyp; and even a
Galactic halo of neutron stars, mooted as a possible source of gamma-ray
bursts.





V. M. Kaspi (IPAC/Caltech/JPL); M. Bailes (Uni. Melbourne); R.
N. Manchester (ATNF); B. W. Stappers (MSSSO); J. F. Bell (Jodrell Bank)



More:



  • ATNF "Pulsar
    - Supernova Remnant" Project
  • Reference: KASPI, V.M., BAILES, M., MANCHESTER, R.N., STAPPERS, B.W.
    & BELL, J.F. "Evidence from a precessing pulsar orbit for a neutron-star
    birth kick". Nature, 384, 584-586 (1996).












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