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Signals from the Beginnings of the World Fundamental forces in space: For a few seconds, a gamma-ray burst radiates as brightly as the whole universe together, the radiation emanating to the outside via two jets. Events such as these still hide their secret: is it the explosion of an extremely massive star, a neutron star falling into the gravitational maelstrom of a black hole, or the fusion of two neutron stars or black holes? xxx 46 MaxPlanckResearch 3 | 09 PHYSICS & ASTRONOMY_Gamma-Ray Bursts Signals from the Beginning of the World A star exploded just 625 million years after the Big Bang, but the radiation of this event didn’t reach Earth until last spring. This gamma-ray burst was named GRB 090423. It is the most distant astronomical object yet discovered. Jochen Greiner and his colleagues at the Max Planck Institute for Extraterrestrial Physics in Garching investigate such cosmic ‘ignition sparks’ at the edge of space and time. TEXT HELMUT HORNUNG t must have been a violent catastro- In the years that followed, scientists into radiation with incredibly high ef- phe. Somewhere in the early uni- investigated the gamma-ray burst phe- ficiency. Over the years, astrophysicists verse, a star blew up – a heavyweight nomenon with instruments designed have put forward at least 150 different with several times the mass of our specifically for this purpose. theories – most of which have since Sun. In the course of this detona- From 1991 until its controlled crash been dismissed. Ition, within less than ten seconds, nine years later, the Compton space The issue is further complicated by as much energy was released as the observatory registered about 2,000 two sub-classes that were discovered Sun has produced during its entire gamma-ray flashes (MAXPLANCKRESEARCH through Compton measurements: gam- 10-billion-year lifetime. The radiation 3/2007, p. 60 ff). The scientists learned ma-ray bursts with a short duration of signal propagated away from the site that the gamma-ray bursts are random- less than one to two seconds, and those of this inferno at the speed of light. ly distributed and arrive from all direc- that typically last between ten and one After travelling for 13 billion years, tions without recognizably predomi- hundred seconds. The short events are the flash finally arrived near Earth: on nant sites of origin in the sky. possibly created during the fusion of April 23, 2009, it flared up in the Leo two neutron stars that had been orbit- constellation – astronomers observed GAMMA-RAY BURSTS – SIGNS OF ing each other for some time; alterna- what is known as a gamma-ray burst, AN INTERGALACTIC BATTLE? tively, such a remnant star might have GRB 090423. an even more compact partner – a black The first such flash was registered What is happening in space? Could this hole whose attractive force it cannot re- more than 40 years ago. On July 2, be “star wars”? Is it the detonation of sist, finally plunging into its gravita- 1967, American Vela espionage satel- bombs from highly developed civiliza- tional sink. lites detected gamma rays that could tions? The distance to the sites of the Long gamma-ray bursts constitute not have originated from secret above- gamma-ray bursts was long the subject by far the larger population. In the ground nuclear weapons tests that the of great controversy. It has now become presently accepted scenario, they arise orbiting spies were watching for. The clear that the bursts come from very far when a very massive star collapses at measurements they recorded were not away, from distances of billions of the end of its evolution, after the fuel published until 1973 – with the con- light-years. A mechanism must thus be for nuclear fusion in its core has run clusion that these signals must have found to explain the immense energy out and the reactor fizzles. As radiation Computer graphic: Mark Garlick/SPL – Agentur Focus come from the depths of the universe. generation, such as conversion of mass pressure from the inside decreases, 3 | 09 MaxPlanckResearch 47 +90 +180 -180 Gamma-ray bursts appear randomly distributed across the sky. This is shown by sky -90 charts such as those from the Compton satellite observatory measurements. gravity wins and the star contracts. Fi- side, the remains of the collapsing star nally, electrons and protons are so disappear behind the event horizon tightly compressed that they merge to of a cosmic mass swallower: a black form neutrons. This reduces the pres- hole is born. sure in the core even further and final- Astrophysicists are working on the ly the entire star collapses. Central den- theoretical details of this scenario. sity then reaches that of an atomic What causes gas to stream into a cen- nucleus – about 100 million tons per tral compact object at high velocity and cubic centimeter (MAXPLANCKRESEARCH thus convert its surroundings into a 2/2008, p. 20 ff). source of high-energy light and plasma jets? If you want to understand this, SCRAPS OF A STAR DISAPPEAR there is only one thing to do: keep a BEHIND THE EVENT HORIZON close eye on nature during such a proc- ess. This is where gamma-ray bursts of- Scientists still do not know exactly how fer an ideal starting point, because they arrival at this density limit transforms represent the most direct messengers of the implosion into an explosion. The such spectacular cosmic events. On April 23, 2009, a gamma-ray burst shockwave forming at this “wall” ap- There are two practical paths of flared up in the Leo constellation pears to be insufficient: neutrinos – progress: Gamma rays of the burst can (orange circle) – a very special event. Never before had the scientists looked electrically neutral elementary particles be measured directly in order to infer so far back into the history of the that hardly interact with other particles the source and the emission mecha- universe: the light started its journey – play a role, as does the stellar rotation, nism from its time profile and energy 625 million years after the Big Bang. which accelerates during the collapse spectrum. Alternatively, one can ob- and channels the collapsing flow of serve the afterglow of the eruption in matter into a fast-rotating disk. the X-ray spectrum, and also in the vis- Moreover, a jet traveling at almost ible and infrared spectra. the speed of light is ejected from the The prompt gamma-ray burst phase two polar regions of the collapsed star. is measured through the gamma-ray Within this jet of plasma, shockwaves eyes of the Swift, INTEGRAL and Fermi are formed by collisions of faster parti- satellites; the afterglow is registered at cles with slower ones, and these, in lower wavelengths by instruments in- turn, generate gamma rays. Deeper in- cluding those aboard Swift, as well as Chart: NASA; photo: NASA – Swift Stefan Immler 48 MaxPlanckResearch 3 | 09 PHYSICS & ASTRONOMY_Gamma-Ray Bursts t = 1.83 ms t = 2.59 ms t = 3.78 ms t = 6.60 ms t = 7.56 ms t = 11.34 ms Catastrophe in the computer: Scientists now use sophisticated software to model candidate mechanisms for gamma-ray bursts. The sequence shows a so-called merger, two neutron stars that collide and merge within a few milliseconds to form a flattened disk of matter accreting onto a newly formed black hole. the GROND instrument at the Max that X-ray glow’s intensity: “Instead of Physics. While the cause of the plateau Planck Society’s 2.2-meter telescope at the expected exponential decay, we phase is not known, Greiner and his La Silla in the Chilean Andes. GROND find a much steeper decline within the colleagues take the sudden flaring as a was developed entirely at the Max first few minutes. During the first few sign of interactions within the explo- Planck Institute for Extraterrestrial hours, the emission often reaches a sion cloud. Physics, and the detectors for one phase of constant intensity. Sometimes Known as Fermi (originally GLAST of the Fermi onboard instruments it then flares up again for about half an for Gamma-ray Large Area Space Tele- were contributed by the members of hour, during which the X-ray bright- scope), the most powerful gamma-ray the high-energy astrophysics group at ness changes by a factor of up to 100,” observatory to date arrived at its obser- this institute. says Jochen Greiner from the Max vation post in its Earth orbit in July Swift was launched into space in Planck Institute for Extraterrestrial 2008. On board, the main telescope LAT November 2004 and has since used its several telescopes to monitor the sky for gamma bursts. Its measurements can also detect gamma-ray burst after- LIKE RAISINS IN RISING DOUGH glows. Scientists didn’t discover this until 1997, with the Italian BeppoSax Cosmological redshift is a measure of The redshift (abbreviated z) is defined satellite – 30 years after the incidental the distance of an object. Galaxies virtu- as the ratio of the observed wave- registration of the first burst of this ally swim in space-time, which expands length to the wavelength of the corre- type. This afterglow is believed to result with the expansion of the universe. sponding emission process at rest in from the interaction of the star’s out- Galaxies are carried along within, the laboratory. Astronomers use red- ward-moving explosion front with sur- just like raisins in rising dough. This shift to measure the distance of the rounding matter, and can be observed motion is imposed onto any electro- object and thus the point in time for hours to days in a wide range of magnetic radiation moving through the when the object emitted its radiation.
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