Pulsars with Bounce

Astronomers at the Max Planck Institute for Astrophysics in Munich and the University of Chicago have a new explanation for the curious high speeds of some pulsars moving through interstellar space. In gravitating themselves to death, some stars might suffer an asymmetric supernova. Since momentum must be conserved at all times, the imbalance in the explosion debris would be taken up by the remnant of the star, namely the spinning neutron star, or pulsar, that is left behind. Or rather, the pulsar won't be left behind, but will be kicked out into space away from the original stellar position with enough velocity (as much as 1000 km/sec) to be measurable by telescopes on Earth.

For some time, one explanation for the pulsar velocities has been the idea that the emission of neutrinos from the newly formed neutron star causes the acceleration. Even a 1% asymmetry in the emission could result in pulsar speeds as great as 300 km/sec, but this line of thinking necessitates the presence of extreme conditions, such as magnetic fields of 10¹⁶ gauss.

Thomas Janka (thj@mpa-garching.mpg.de) and his colleagues believe the observed effects are better explained if the asymmetries come not from neutrino emission but from the way matter reacts with neutrinos shooting into (and heating) the infalling stellar layers that are about to be flung back out into space during the supernova explosion.

In other words, the irregularities arise not from particle physics but from the purely hydrodynamic effects of a gust of neutrinos plowing into a layer of material, a process in which small instabilities in a shock front can quickly grow much larger. (Scheck et al., Physical Review Letters, 9 January 2004; see colorful illustrations at www.mpa-garching.mpg.de/mpa/research/current_research/hl2003-10/hl2003-10-en.html)