Gamma-Ray bursts (GRBs) are bright and very energetic flashes of gamma radiation with durations between some thousandths of a second and many minutes. According to their duration, the roughly 3000 bursts detected so far are divided into two classes. The so-called long bursts emit gamma radiation for more than two seconds, whereas the short bursts last less than two seconds. While the long bursts are relatively well investigated and some of them could be linked to the explosive death of massive stars, the origin of the short bursts has remained enigmatic, because they are more challenging to observe. Their short duration makes it difficult to catch the gamma radiation and to determine their position on the sky sufficiently accurately to look for "afterglow" signals in X-rays or optical light and thus to measure their cosmological redshift and distance from the earth. Only recently such precise localizations have been achieved in a few cases by the HETE satellite and the Swift Gamma-Ray Burst Explorer. The bursts are hosted by elliptical galaxies between two and six billion light years away and seem to come from an old population of stellar objects, in agreement with the hypothesis that they might be produced by merger events of binary neutron stars (Figure 1).

Neutron stars are the extremely compact remnants which are left behind when massive stars explode as supernovae at the end of their lives. In a neutron star more than a solar mass of matter is compressed into a sphere of roughly 20 kilometers diameter. The density in a neutron star is therefore higher than that in atomic nuclei. Sometimes such neutron stars are born in binary systems and orbit around each other with periods of less than one day. The most famous example is the binary pulsar PSR 1913+16, whose discovery was rewared with the Nobel Prize in Physics to Russell A. Hulse and Joseph H. Taylor in 1993. This discovery was spectacular because it confirmed a prediction from Einstein's Theory of Gravity: Such compact binary systems cannot live forever, but their orbital distance shrinks continuously by the emission of gravitational waves. Unavoidably, the two stars get closer and closer, swirling more and more rapidly around each other, reaching velocities close to the speed of light. Finally, they crash violently into each other and get destroyed. What then happens is still a matter of intense research and requires complex computer simulations which solve the equations of Einstein's Theory of General Relativity.

Researchers at the Max-Planck Institute for Astrophysics have developed such computer models which allow them to study these events with more realism than before. The relativistic simulations in particular include a detailed description of the properties of dense and hot neutron star matter. They reveal that the two colliding neutron stars merge into one very compact, rapidly spinning object, in which temperatures of several hundred billion degrees are reached (Figure 2). The core of this object is going to collapse to a black hole within only fractions of a second, leaving behind some matter in a toroidal gas cloud that girds the equator of the spinning black hole. Fictitious centrifugal forces like the ones experienced on a merry-go-round, prevent this doughnut-shaped gas torus from being immediately pulled into the black hole by its enormous gravity. But friction will decelerate the gas motion and will unavoidably drive the gas closer to the black hole like swirling water to the sink. Spiralling inward to the black hole, the gas heats up and can release huge amounts of gravitational binding energy. The energy output increases with the mass that is accumulated in the gas torus and then swallowed by the black hole after many revolutions.

Determining the amount of matter that can achieve escaping the immediate collapse into the black hole was one of the main goals of the computer simulations. The models of the astrophysicists reveal that, depending on the ratio of the two neutron star masses and the total system mass, the black hole is initially girded by a few hundredths to more than a tenth of a solar mass of gas (Figure 3), which is well sufficient to explain the energy output of the short gamma-ray bursts whose distances could recently be determined. The simulations therefore clearly support the possibility that these gamma-ray bursts were the death throes of colliding binary neutron stars, a long-standing hypothesis whose ultimate solidification will require many more observations of well-localized short bursts hand in hand with further theoretical exploration.

R. Oechslin, H.-Th. Janka

Literature:

R. Oechslin, H.-Th. Janka, Torus Formation in Neutron Star Mergers, submitted to MNRAS, astro-ph/0507099