Beacon of the Universe
The American Vela spy satellite was originally supposed to detect surface nuclear tests forty years ago. The short gamma-ray pulses he registered, however, came from a different direction - space. Since then, interpreting their origin has demanded a great deal from the imagination of astrophysicists. Thanks to a happy coincidence, the riddle is now cleared up a little further.
If you turn on the TV in the middle of an exciting thriller, it's difficult to get into the plot. In cosmic cinema it is quite similar: Huge explosions in space should be viewed from the start in order to unravel their mysteries. For the first time, astronomers have now managed to sit down at the telescopes in time for the presentation on February 18, 2006.
On this day, the Swift satellite detected an unusual gamma-ray burst about 440 million light-years from us in the constellation Aries. Unusual because it lasted about thirty minutes, about a hundred times longer than previous gamma-ray bursts. In such scenarios, more energy is released in a few seconds than our sun would emit in about ten billion years.
Four international groups of researchers immediately focused their telescopes on the object they named GRB 060218. By following the spectacle in different spectral ranges, they were able to measure the total energy emitted. The maximum intensity of the emitted radiation was not in the high-energy range of gamma rays, but in the longer-wave X-ray range. Accordingly, GRB 060218 is also referred to as an X-ray flash.
Consequently, the scientists captured the object on X-ray images, during which they observed a directed jet of matter erupting from GRB 060218. The hot gas ejected from the core of the explosion at nearly the speed of light - a characteristic of gamma-ray bursts.
They discovered something else in these images: the cone-shaped jet of matter was accompanied by a gas shell with a temperature of two million degrees, which expanded almost spherically. Such a signature is characteristic of a supernova explosion. These usually occur at the end of the life of massive stars, when their cores collapse under their own gravity. The blast from the exploding celestial body then hurls stellar material into the area.
The optical luminosity of the object also supported the occurrence of a supernova. This rose again after the optical afterglow of the X-ray flash faded after two days. The researchers believe that this phenomenon can be attributed to the expanding gas envelope of the supernova explosion, which was now emitting visible light. The almost spherical pressure wave could also be observed in the radio range.
By analyzing the data collected over days and weeks and developing computer models of the explosion, the astrophysicists were able to draw valuable conclusions about what was happening. They found out that the supernova was weaker than a so-called hypernova, which is associated with ordinary gamma-ray bursts. On the other hand, it was too luminous for a classic supernova that does not produce a gamma-ray burst. Paolo Mazzali from the Max Planck Institute for Astrophysics in Garching believes that the mass of the star may be responsible for the different behavior. Based on the material catapulted out, the researchers calculated that the star had twenty times the mass of the sun.
The explosion of such a star should produce a neutron star - stars that combine several solar masses with only a few kilometers in diameter. The researchers suspect a magnetar, a neutron star with an extremely strong magnetic field. Until now, astronomers assumed that the gamma-ray bursts can only form during a hypernova explosion, in which a star collapses into a black hole.
The series of star types that can produce a gamma-ray burst or its brother, the X-ray flash, should therefore have been extended to include relatively low-mass stars. However, the results do not yet explain why stars emit X-ray flashes and others do not.
But astronomers shouldn't be short of research objects, since there are far more low-mass than high-mass stars in the universe. However, due to their lower brightness, X-ray flashes are more difficult to detect than the high-energy gamma-ray bursts. Thus, GRB 060218 is one of the rare premieres in the cosmic cinema - at least until the "eyes of the astronomers" become sharper.
So it remains to be seen when the universe next invites us to such an exclusive performance.
