The Mysteries of the Extreme Universe
17 September 1998Flares from the far reaches of the Universe, giant black holes in theheart of galaxies, clouds of radioactive material in our Milky Way,extremely dense remnants of dead stars and starquakes on bizarre,magnetised objects: the list of phenomena that emit powerful gamma raysis long and full of mysteries.
The extreme Universe with its exceptional and intriguing sources of gamma rays was the topic of the Third Integral Workshop, named after ESA's International Gamma Ray Astrophysics Laboratory to be launched in 2001. 270 scientists from Western and Eastern Europe, Russia, the United States and Japan gathered from 14-18 September 1998 in Taormina, Italy to discuss recent results in high-energy astronomy.
On 27 August 1998 several spacecraft detected a spectacular burst of gamma rays. Only once before, on 5 March 1979, had a similar event been registered. Data on the extremely strong flare, presented at the Integral Workshop, revealed details. A light curve based on observations by ESA's Ulysses spacecraft showed that the unusual event happened very quickly. The light curve had a fast rise within one second and an exponential decay lasting more than 300 seconds. During the decay strong pulses with a clear periodicity of about 5 seconds were detected.
The source of the exceptional gamma-ray burst is called SGR 1900+14. SGR stands for soft gamma-ray repeater. Soft doesn't mean feeble. These gamma rays are just less energetic than those emitted by other strange objects, but still enormously powerful. The name 'repeater' shows that the source has been observed before. It was first detected in 1986. After a period of silence, in May 1998 SGR 1900+14 entered an extraordinary new phase of activity with a previously unobserved frequency and intensity of bursts that culminated in the 27 August event.
Starquakes on magnetars
Only a few soft gamma ray repeaters are known. Since their first discovery in 1979 astronomers have been wondering what the source of the strange radiation signals could be. Now they think they have found a solution for the mystery as experts at the Integral Workshop stated. Soft gamma- ray repeaters like SGR 1900+14 seem to be a class of bizarre new stars called magnetars. Unimaginably dense, these stars have a super-strong magnetic field a thousand trillion times stronger than that of Earth.
Like neutron stars magnetars are the compact cinder that remains when ordinary stars explode in a supernova. Under enormous pressure protons and electrons combine to make neutrons. But magnetars have a much stronger magnetic field than normal neutron stars. Astronomers think that on magnetars the stellar equivalent of an earthquake could happen: the motion of the magnetic field heats up the stars surface, a solid crust made of iron. With a violent shake, the crust relieves the tension by cracking apart: a starquake that blasts the Universe with gamma rays.
Huge blasts in distant galaxies
Unlike the soft gamma-ray repeaters, hard gamma-ray bursts are one-off events, going off like a cosmic firecracker and then never heard again. About once every day satellites register a hard gamma-ray burst somewhere in the sky. Until last year, the distance scale of these gamma ray bursts was completely unknown. But in February 1997 astronomers found for the first time an optical counterpart of the gamma radiation and were able to measure its distance. Two other bursts have been located in the distance since then. All three bursts came from galaxies very far away.
At the Integral Workshop scientists pointed out that they now are sure that the gamma-ray bursts are generated in distant galaxies. The scientists found out that most gamma-ray bursts were generated when the Universe was only about half its present age, 8 billion years ago, but they still don't know what the source of these blasts is. Are these gamma-ray bursts generated when a neutron star merges with another neutron star or a black hole? Or are they perhaps caused by extremely energetic supernovae? Integral will help to solve this mystery.
"The workshop demonstrated that Integral will be the right mission at the right time to advance significantly our knowledge of the high-energy universe", says Project Scientist Christoph Winkler. Besides the gamma-ray bursts the spacecraft will monitor many other phenomena, for instance gamma-ray beams coming from exceptional so-called blazars and pointing at the Earth like a searchlight, or the intense sources of gamma rays lying near the centre of our own galaxy. The European satellite will also look at neutron stars or candidates of stellar black holes in the Milky Way and the formation of new elements like Aluminum, Cobalt or Titan - indeed this is one of the most important tasks of Integral.