Fundamental forces in space: For a few seconds, a gamma-ray burst radiates as brightly as the whole 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 , a neutron star falling into the gravitational maelstrom of a , or the fusion of two neutron 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 , but the radiation of this event didn’t reach 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. I tion, 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 . 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- – 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 spectra. the 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 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 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 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 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 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. wavelengths, from X-ray, universe: if space-time increases by a Accordingly, z = 1 corresponds to a dis- and optical radiation to radio waves. certain factor as the radiation propa- tance of 5.9 billion light-years; the uni- Within one minute of detecting a gates, this is reflected in a corresponding verse was just under half its present new gamma-ray burst, Swift can turn increase in the radiation wavelength, age of 13.7 billion years when that its X-ray telescope to the direction de- resulting in a shift toward the redder radiation was emitted. termined in the sky. From measure- wavelength region of the spectrum. ments early in the mission, the scien-

Computer graphics: University of Bremen – Stephan Rosswog tists were surprised by the variations of

3 | 09 MaxPlanckResearch 49 PHYSICS & ASTRONOMY_Gamma-Ray Bursts

» Astronomers are finally observing the afterglow of a gamma-ray burst in the infrared as well. This opens up new perspectives and marks a methodological breakthrough.

(Large Area Telescope) and the GLAST tillate.” Light-sensitive photo-multipli- The GROND detector developed by Jo- Burst Monitor (GBM) with its 14 detec- er tubes convert this scintillation light chen Greiner radically improves this tors share the incident gamma-ray light. into an electrically usable signal. The situation: “The instrument combines LAT scans the gamma-ray spectrum central GBM board computer processes the detection of the afterglow with a at high energies of between 20 mega- this signal to derive an initial and still rough determination of the distance. electronvolts and 300 giga-electron- crude arrival direction. “This is then We can now determine the distance of volts. The GBM detectors developed at transmitted to both the LAT instrument a gamma-ray burst within minutes, or the Max Planck Institute for Extraterres- and the ground station, from where the within one to two hours at worst, to trial Physics, on the other hand, moni- burst alarm is reported to a large within an accuracy of about 5 percent,” tor radiation at lower energies of number of observatories worldwide,” says the scientist. The successful new between 15 kilo-electronvolts and 1 says Max Planck scientist Andreas von strategy has caught on: “During the last mega-electronvolt, and between 150 ki- Kienlin. The goal is to monitor the af- two years, GROND has enabled us to lo-electronvolts and 30 mega-electron- terglow of a gamma-ray burst in a wide discover as many bursts at a redshift volts, respectively, for its two different range of possible wavelengths. larger than four as we had discovered detector types. The gamma-ray monitor in ten years before.” has an omnidirectional field of view, RISING BEYOND THE LIMITS GROND is an abbreviation bor- and raises an alarm as soon as it detects rowed from Tolkien’s book Lord of a gamma-ray burst. The satellite with its Only recently was it possible to open the Rings and stands for Gamma-Ray main instrument then turns to the di- one of these wavelength windows: “We burst Optical and Near-infrared Detector. rection estimated from the GBM data. are finally observing the afterglow in GROND is basically a camera that can The detection of gamma-rays with the infrared region systematically as take measurements in visible and the LAT functions differently than with well,” says Jochen Greiner. “I feel this in infrared bands. The instrument, de- an optical telescope, because light of marks a breakthrough in method be- veloped and built in collaboration such high energy simply cannot be de- cause it allows the redshift to be deter- with the Thuringian State Observato- flected into a focal point. Instead, the mined directly from the gamma-ray ry in Tautenburg, achieves this using instrument uses the pair production ef- burst signal – instead of later measuring six beam splitters to divide the fect: inside the detector, the massless the redshift of the galaxy where the ex- light into seven color channels. Each light particle – the photon – transforms plosion appeared.” Moreover, it ex- channel has its own detector, so that into a pair of oppositely charged mas- pands the accessible redshift region to the seven channels are exposed simul- sive particles – a negatively charged earlier cosmic times. Quasars, young taneously. electron and its positively charged an- galaxy cores that are so bright they can tiparticle, the positron. “With the aid be seen at great distances, no longer SWALLOWED BY of a tracking detector, we use the inter- mark the limits. action of the charged particles with the Determining the distance is of The distance to an object in the deep individual detector units to determine crucial importance in the study of universe is determined from its cosmo- their path, from which the initial ener- gamma-ray bursts. As years have gone logical redshift (see box on page 49). gy and direction of the gamma-ray by, it has been possible to increasing- The so-called absorption edge of radia- photon can be determined,” says Gott- ly improve the sensitivity of detectors. tion from intergalactic hydrogen is an fried Kanbach from the Garching-based Until recently, however, scientists important marker. This occurs normal- high-energy astrophysics group, ex- monitored the gamma-ray burst after- ly at a wavelength of 120 nanometers: plaining the principle they employ to glow mainly in the visible spectral radiation of shorter wavelengths is take the measurements. band. This involved first recording im- swallowed (absorbed) by the hydrogen GBM houses 14 crystal detectors, 12 ages of the corresponding region in atoms and the object disappears. The made of sodium iodide (NaI) and 2 the sky in order to detect the exact po- cosmological redshift moves this ab- made of bismuth germanate (BGO). sition of the newly appearing fireball. sorption edge to longer wavelengths. These are scintillation crystals, so The redshift was not measured until GROND allows this position of the named because, upon the interaction the following night with the aid of a shifted edge to be determined so that of a gamma-ray photon, they produce spectrograph pointed precisely in that the recession velocity and thus the dis-

a brief flash of optical light – they “scin- direction. tance of the object can be derived. > Photos: MPI for Extraterrestrial Physics – Andreas von Kienlin (top)/NASA (bottom)

50 MaxPlanckResearch 3 | 09 The most sensitive gamma-ray observatory to date, the Fermi satellite (bottom) launched in July 2008, also carries on board an instrument called GBM. Its most important components are twelve sodium iodide and two bismuth germanate detectors. These and their power supply were built by the Astrium company under the direction of the Max Planck Institute for Extraterrestrial Physics. The digital processing unit was provided by NASA’s Marshall Space Flight Center.

Digital processing 12 x Sodium iodide detectors

Bismuth germanate detectors 2 x Power supply

3 | 09 MaxPlanckResearch 51 CCDs IR/VIS beam splitter Focal point

Deflection mirror 1

Collimator lens 1

Entrance window Collimator lenses CCDs

GROND is essentially a cameramera that measures Deflection mirror 2 in both the visible and the iinfrarednfrared bands. The schematic diagram of tthehe optics shows the entrance window at the topp with the beam splitter for the visible spectrumtrum behind it, and IR detector J channel below, the system of lensess foforr the three infrared channels. GROND splits thee incident light intointo a total of seven channels thathat are eexposedxposed simultaneously. The instrumentment waswasd developedeveloped by Jochen Greiner of the Max Planck Institute for Extraterrestrial Physics. IR detector K channel IR beam splitter IR optics for J channel

Something that sounds so easy still has says Greiner. “Instead of moving the advancing into ever earlier epochs of its technical difficulties – and this is telescope, we internally move the im- the universe. GRB 090423 also marks why nowhere in the world is there an- age through a wobble mirror, like the earliest existence of a cosmic object other such instrument that operates in squinting in a circle as if we were roll- ever observed. the visible and the near infrared spec- ing our eyes.” tra simultaneously. The infrared detec- GROND is one of three instruments THE CYCLE OF THE ELEMENTS tors must be cooled to a temperature of installed at the 2.2-meter telescope IS INITIATED minus 210 degrees Celsius, for exam- owned by the Max Planck Society and ple. The optical detectors, on the other the European Southern Observatory on The first generation of stars, probably hand, must not be colder than minus the summit of La Silla in the Chilean born a few hundred million years after 120 degrees Celsius, even though they Andes. GROND’s controlling computer the Big Bang out of gigantic clouds of are separated from the former by less is connected directly to the ground sta- gas, were composed mainly of hydro- than 10 centimeters. The lens system tion of the Swift satellite. When it de- gen. In other words, they were very de- positioned in front of the infrared de- tects a gamma-ray burst during the ficient in metals. Only nuclear fusion tectors must also be cooled to minus Chilean night, GROND is activated reactions inside these so-called Popula- 200 degrees so as not to generate addi- without human intervention: within a tion III stars produced the first amounts tional background radiation. Finally, short time, the current observation is of heavier elements, such as carbon, ni- exposures in the near infrared must interrupted, the telescope moves to the trogen and oxygen, and spread them typically be as short as only about 10 position transmitted by Swift, and a tilt into the vastness of space when they seconds – otherwise the detector will mirror is placed into the beam path. exploded. “This gave rise to new gener- be dazzled by the bright, ever-present Some two to five minutes after the ations of stars, and the cosmic cycle of celestial background. alarm has gone off, GROND starts tak- the elements started, with its ever in- In order to record sources that are ing the first images. “In the case of GRB creasing enrichment of heavy ele- fainter than the normal celestial 090423, though, we had to wait 15 ments,” explains Max Planck scientist brightness in the near infrared, a tele- hours because the object had already Roland Diehl. Our Sun, being a relative- scope is usually pointed to a slightly set in the sky over La Silla,” recalls ly metal-rich star, thus belongs to one offset area of the sky after each 10-sec- Greiner with regret. of the later stellar generations. Did the ond exposure. This is not possible Nevertheless, the observation of the star whose gamma-ray burst was regis- with GROND, because its 45-second burst of April 23 was a great success. tered on April 23 belong to this very readout time of the optical detectors The explosion of a high-mass star with first generation of stars, or to a some- is much longer: that would entail a redshift of z = 8, dating it to about 13 what later generation? A fascinating pausing for 45 seconds every 10 sec- billion years ago, is the most distant ob- question, and one that remains to be onds – an extremely inefficient obser- ject astronomers have seen so far. With answered. Just like the fundamental vation method. “We have circumvent- the finite speed of light, this means question: What was the universe made

ed this problem with a small trick,” that, with increasing distance, they are of at its beginnings? > Graphics from: Jochen Greiner et al. 2008, PASP 120, 405

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THE BLUE LUMINESCENCE

Two other telescope installations in which the Max Planck particles is thus generated in a cascade. The collective radi- Society has a stake through its Institute for Nuclear Physics ation from polarized atoms of the atmosphere as they expe- in Heidelberg and its Institute of Physics in Munich also have rience this electromagnetic particle shower emerges as blue the gamma sky in their sights: H.E.S.S. and MAGIC. But light – the Cherenkov radiation – for several billionths of neither of them measures gamma rays directly. The Earth’s a second at an altitude of around eight kilometers. It is atmosphere is not transparent to this extremely energy-rich precisely this blue light that is recorded by H.E.S.S. (High light – which is fortunate for life on Earth. Instead, these in- Energy Stereoscopic System) and MAGIC (Major Atmospheric struments register what is known as Cherenkov radiation. Gamma-ray Imaging Cherenkov telescope). This radiation flash is generated when a particle shower Astronomical targets of the installations include the caused by incident gamma rays in the upper atmosphere center of the Milky Way, rapidly rotating neutron stars propagates at a velocity exceeding the local speed of light (pulsars) and the remnants of supernovae. The two MAGIC in that medium. The initial interaction produces pairs – an telescopes at La Palma on the Canary Islands (photo) also electron and its antiparticle, the positron – which propagate specifically target gamma-ray bursts with the “stereoscopic in the atmosphere and are deflected in the electric fields of view” of the atmosphere through their 17-meter mirrors; atomic nuclei. This generates secondary gamma-ray pho- H.E.S.S. in South Africa is composed of four mirrors, and also tons, which in turn produce new electron-positron pairs targets such events when a satellite raises an alarm and the upon passing an atomic nucleus. A shower of high-velocity installation’s sky visibility renders it active. Photo: MPI of Physics – Robert Wagner

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Gamma-ray bursts play an important initially measured by GROND to with- Gamma-ray bursts may show how the role in the study of this question, since in an accuracy of 5 percent. “The rela- very first “megastars” may have formed they light up their surroundings, and tively short time from one redshift a further 400 to 500 million years ear- matter thus leaves behind characteris- record to the next gives us hope that lier, at of 25 to 30. But the tic fingerprints in the form of absorp- further distance records will be set in warm atmosphere of the Earth sets a tion lines in the spectra. Studying the the near future,” says Greiner. redshift limit of about 13 that is difficult abundances of metals in this very early to overcome with today’s telescopes, time of the universe also promises val- OBSERVATIONAL COSMOLOGY IS and corresponds to a universe age of uable conclusions about details of star STANDING AT A CROSSROADS 330 million years. To look beyond, one formation under the largely unknown would need either much larger tele- conditions of that time. The wide-ranging importance of gam- scopes or a new generation of gamma- Jochen Greiner points out yet an- ma-ray bursts for observational cosmol- ray satellites. Scientists at the Max Planck other aspect: “The source of a gamma- ogy is now definitely being recognized. Institute for Extraterrestrial Physics ray burst is directly related to the for- After all, looking into earlier and earlier proposed such a mission to the Europe- mation of a black hole. What type of epochs of the universe is one of its main an Space Agency and the German Aer- radiation just manages to escape such objectives. Extensive long-term observa- ospace Center back in 2007. They now an extreme gravitational field as it is tions, such as those with the Hubble hope that the current measurements of building up? What are the processes Space Telescope, have been conducted GRB 090423 will give their proposal that cause matter to disappear behind and hundreds of nights with the tele- more weight – and that the sponsors the event horizon of such a compact scopes of the 8-to-10-meter class have may also succumb to the fascination of object?” Gamma-ray light is the most been spent in recent years on galaxies the chase for cosmic records. energetic form of electromagnetic radi- and on the nuclei of active galaxies in ation and is thus very well suited to order to overcome the magic limit of shed light on these processes. redshift 7, which corresponds to a uni- GLOSSARY So what is the next move for gam- verse age of 780 million years. A 2.2- ma-ray burst astronomy? The previous meter telescope has now been sufficient Nuclear fusion Stars are nothing more than huge nuclear record holder was at a redshift of 6.7. It to make the leap. “This means,” says fusion reactors. In the centers of these was detected by the Swift satellite in Jochen Greiner, “that observational cos- globes of gas, and in their explosions September 2008, and the distance was mology is standing at a crossroads.” (supernovae), fusion processes occur at high pressure and temperatures of several million to several billion degrees, and 19 atomic nuclei fuse to form new nuclei. Stars such as our Sun spend most of their lives fusing hydrogen into helium.

20 Black hole g‘ r‘ i‘ z‘ J H K An astronomical object whose gravitation- al attraction is so strong that not even radiation can escape. Black holes occur in 21 two “weight classes”: as massive as stars, and millions of times more massive. Stellar 10 black holes are created at the end of the of a massive star. 22 ] Spectrograph AB This instrument – in the simplest case a prism, used by Isaac Newton in his experiments back in the 17th century – dis- 23 [μJy] v

F perses light into its different wavelengths. Dark absorption lines appear in a color

Brightness [mag spectrum, with specific lines being able to 24 1 characterize the source, allowing them to be attributed to specific chemical elements. A term introduced by Walter Baade in 1944 25 for stars that are similar in terms of their GRB 090423 composition (“”), their spatial distribution and their age. A distinction is 8.0 +0.4 z= -0.8 made between three populations: the first 26 stars are called Population III, while those 4000 6000 8000 10000Wavelength [Å] 20000 that formed later but relatively early in a gal- axy’s evolution are said to belong to Energy distribution of GRB 090423 as measured by GROND. The series of images shows the sky region Population II, and younger stars that formed of the gamma-ray burst in each of the seven filter bands, sequenced from the shortwave blue to the in more mature epochs of galaxy evolution longwave red spectrum (from left). In the four visible bands, the gamma-ray burst remains invisible, – such as the Sun – are called Population I. emerging suddenly and clearly only in the J band. The curve below it represents the intensities derived

from the images. The distance can be determined from the position of the intensity jump. Photos and diagram: Jochen Greiner

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