Neutron Stars as Cannonballs

Scientists at the Max-Planck-Institute for Astrophysics in Garching and the University of Chicago have substantiated an explanation for the high space velocities of observed pulsars. Their computer models confirm the likely connection with asymmetries during supernova explosions.

Abbildung 1 Abbildung 2 Abbildung 3

Fig. 1: The Crab Nebula with the Crab Pulsar at its center (Credit: linkPfeilExtern.gifESO).

Fig. 2: X-ray image of the PUPPIS A supernova remnant taken by the ROSAT satellite. In the inset close-up view, a faint pinpoint source of X-rays is visible which is most likely the young neutron star, moving with about 1000 km/sec in the opposite direction than the hot, bright gaseous relics of the supernova explosion. (Credit: linkPfeilExtern.gif S. Snowden, R. Petre (LHEA/GSFC), C. Becker (MIT) et al., ROSAT Project, NASA).

Fig. 3: The linkPfeilExtern.gifGuitar Nebula. It is produced by the bow shock and the wake left behind by a neutron star which is travelling through the interstellar medium at an extraordinarily high speed of about 1600 km/sec. (Image taken with the 5-m Hale Telescope of the Palomar Observatory. Source: S. Chatterjee & J.M. Cordes, linkPfeilExtern.gifAstrophys.J. 575, 407 (2002))

Fig. 4: Asymmetric gas distribution in the interior of an exploding star one second after the start of the explosion. The pictures show the shock wave of the explosion for different computer simulations. The neutron star is (not visible) at the center. The vortices and inhomogeneities of the gas flow grow from random fluctuations and develop differently in each model (click on the pictures for bigger versions).

These are movies of the two uppermost simulations::
linkPfeil.gifWeak explosion, 4.5 MB, DivX-Format
linkPfeil.gifStrong explosion, 4.5 MB, DivX-Format

(For Playback under Windows you need the free DivX5-Codec from Under Linux e.g. mplayer or xine can be used.)

Fig. 5: One second after the start of the supernova-explosion the neutron star has gained a velocity up to more than 500 km/s. Because of the persistently large acceleration in some models (indicated by the length of the colored arrows) the final maximum speed can even be higher.

Stars with more than ten times the mass of our Sun end their lives in spectacularly powerful supernova explosions. While the major part of the stellar gas is violently ejected, the core of the star collapses by its own gravity to form a neutron star. The latter has a mass of roughly 1.5 times the Sun, but its diameter is only about 20 kilometers. The matter in its interior is therefore more dense than in atomic nuclei.

Some of the known neutron stars are found inside the gaseous remnants of past supernova explosions. The most famous example is the `pulsar' within the Crab Nebula (Fig.1). Because it spins around its axis about 33 times per second, we receive on Earth characteristic, regular pulses. Such rotating neutron stars were therefore named pulsars. Other neutron stars, however, move away from the site of their formation with very high speed (Fig.2). Typical velocities are several hundred kilometers per second, but some pulsars propagate through interstellar space with more than 1000 kilometers per second (Fig.3). This is much faster than the motion of ordinary stars in our Galaxy. Therefore many neutron stars can escape from the gravitational pull of the Milky Way.

The orgin of the pulsar motions has long been a mystery. There is, however, no lack of ideas, partly invoking very speculative or exotic physics phenomena. A connection with observed anisotropies of supernova explosions had so far not been demonstrated convincingly.

A team of scientists at the Max-Planck-Institute for Astrophysics in Garching and the ASCI Flash Center of the University of Chicago has now discovered a simple and natural cause for such a connection. In computer simulations the team found that stochastic, little perturbations in the star can amplify to huge anisotropies by the rapid growth of fluid instablities during the launch of the explosion (Fig.4, movies). The explosion shock wave and the ejected matter therefore develop global deformation and the neutron star can be kicked to very high velocities of several hundred kilometers per second within just a second (Fig.5).

For the first time the computer models allow one to understand the measured pulsar motions without making use of additional assumptions. Interestingly, the results seem to support a theory which has been favored for a long time to explain the beginning of the supernova explosion (siehe linkPfeil.gif"How do Massive Stars Explode?") but which could so far not be confirmed by detailed numerical simulations (siehe linkPfeil.gif"Supernova Simulations Still Defy Explosions" ): The explosion is caused by the action of neutrinos. These neutral, weakly interacting elementary particles are radiated by the hot neutron star in huge numbers. They heat the gas in the stellar interior and create the pressure by which the explosion is started. This heating leads to violent buoyancy (see movies) until the expansion of the stellar gas occurs in a generically anisotropic manner. The mechanism of the explosion, the observed asymmetries of supernovae, and the pulsar proper motions are therefore all linked to one effect.

(Hans-Thomas Janka, Konstantinos Kifonidis,
Ewald Müller, Leonhard Scheck, Tomek Plewa)

L. Scheck, T. Plewa, H.-Th. Janka, K. Kifonidis, and E. Müller:
"Pulsar Recoil by Large-Scale Anisotropies in Supernova Explosions",
astro-ph/0307352, Phys. Rev. Lett., submitted