The Mystery of the Heavenly Bursts
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62 KNOWLEDGE FROM KNOWLEDGE Signs in in the recent heavens: years, depths space of been have detected fast radio bursts coming from the by telescopesby like the the at one Arecibo Observatory. IMAGE: JasoN HEssELS-ASTRON / DANIELLE FUTSELaaR-ARTsoURCE.NL / B.P. IRWIN / D. VAN DE WaTER / SHUTTERSTOCK Max Planck Research · 1 | 2020 PHYSICS & ASTRONOMY THE MYSTERY of THE HEAVENLY BURSTS TEXT: HELMUT HORNUNG A cosmic lightning storm is playing out all around us. At any given moment, somewhere in the sky, a burst of radiation flashes and then fades away. Only observable with radio telescopes, these bursts last one-thousandth of a second and are one of astro- physics’ greatest mysteries. Scientists 63 rather doubt this is evidence of warlike aliens fighting “star wars” in the “We now know of more than a hundred,” radio flash and the derived frequency vastness of space. Experts have named says Laura Spitler. Since March 2019, of such events closely matched the them “fast radio bursts” – but where Spitler has headed a Lise Meitner data obtained by astronomers for all Research Group on fast radio bursts previously recorded bursts. do they come from? (FRBs) at the Max Planck Institute for Radio Astronomy. Spitler has The findings did indeed confirm sta- dedicated herself to these transient tistical estimates that about 10,000 flickers in space for many years. In of these unusual cosmic phenome- 2014, an international team under her na flare up somewhere in the sky leadership discovered the first FRB every single day. This astonishingly in the Northern Hemisphere in the large figure was reached by calcula- The Parkes Observatory in Australia is a constellation of Auriga. At the time, ting how much of the sky would have gigantic dish of metal mesh gazing at the astronomers were making obser- to be observed and for how long, in the sky. With a diameter of 64 meters, vations in Puerto Rico using the dish an attempt to explain the compara- this radio telescope was once the of the Arecibo Observatory. This tively few observations made to largest fully mobile radio telescope in dish, with a diameter of 305 meters, date. the Southern Hemisphere. It regis- is constructed and supported in a tered a very mysterious radio burst in natural depression and can only fo- The Arecibo observation also removed 2001, and nobody noticed it! It wasn’t cus on a relatively small section of the last doubts about whether the until five years later that astrophysi- the sky. radio bursts really come from the cist Duncan Lorimer and his student depths of the universe. After the dis- David Narkevic stumbled upon the “Statistically, only seven eruptions covery of the first bursts, scientists signature of the signal in the tele- should occur per minute, dispersed had suggested that they were being scope data virtually by chance. Even over the entire sky. So it takes a lot generated way beyond the Milky then, the specialists couldn’t make of luck to align your telescope to the Way. This was deduced utilizing an sense of the phenomenon. But this right position at the right time,” said effect known as “plasma dispersion.” was not the only “Lorimer burst” to Spitler after announcing her disco- Radio signals that travel a long dis- be discovered. very. The properties of the observed tance through the universe collide Max Planck Research · 1 | 2020 KNOWLEDGE FROM SUMMARY Fast radio bursts last only a thousandth of a second and were discovered by chance almost 15 years ago. They originate from galaxies Ross at great distances and appear NG NG I to be produced in different environments. : HENN O T Astronomers now know ho P that more or less regularly repeating radio bursts,known as “repeaters”, also exist. The source of the radio bursts is still unknown. Pursuing the mystery: Laura Spitler researches fast radio bursts (FRBs) at the Max Planck Institute for Radio Astronomy. 64 with a large number of free elec- Matter is so densely packed in a neu- called FRB 121102, was reported on- trons in interstellar space. Plasma tron star that on Earth’s surface, one line in 2014. “This disproved all of dispersion decreases the velocity of teaspoonful of it would weigh about the models explaining FRBs as the lower-frequency radio waves pro- as much as Germany’s Zugspitze result of a catastrophic event,” says ducing a characteristic spread in the massif mountain. Neutron stars ro- Spitler. frequencies of the signal. The dis- tate rapidly around their axis, and persion of the radiation burst dis- some of them have exceptionally FRB 121102 (which was also discovered covered at the Arecibo Observatory strong magnetic fields. Explanations at the Arecibo Observatory) was was three times larger than would be for FRBs have varied. They may be subsequently monitored by the re- expected from a source within the generated during supernovae them- searchers using the Very Large Ar- Milky Way. selves, or perhaps during the fusion ray in New Mexico. After 80 hours of two neutron stars in a compact of observation time, they registered But where do the radio bursts come double system as the magnetic fields nine bursts and determined the from? Are they really from aliens us- of both stars collapse simultaneously. source’s position to within one sec- ing huge light sails to propel space Or perhaps a neutron star collapses ond of arc. At this position in the probes? This idea was originally put still further to create a black hole and sky, there is a permanently radiating forward by Abraham Loeb – a re- this generates a burst. radio source; optical images show a searcher at the renowned institution faint galaxy about three billion light- of Harvard University no less – and Such scientific scenarios initially sound years away. With a diameter of only was recently seized on by the me- plausible. However, they have one 13,000 light-years, this galaxy is a dia. But most astrophysicists believe flaw: they predict the occurrence of dwarf; the Milky Way is about ten the bursts have a natural source and only one radio burst in each event. times larger. “However, many new have come up with diverse explana- “If the radio burst is generated stars, and perhaps even particularly tions – all of them relatively bizarre. during a cataclysmic event that de- large ones, are being born in the gal- Many of these potential scenarios stroys the source, then we would ex- axy, and this might be a clue as to involve neutron stars. Neutron stars, pect to see only one burst per source,” the source of the radio bursts,” says only 20 kilometers in diameter, are says Laura Spitler. Indeed, only sin- Spitler. the remnants of the massive explo- gle bursts were observed in the years sions, known as supernovae, of high- right after their discovery – until She is referring to the possibility that mass stars. news of the first “repeater” burst, FRBs may be pulsars – cosmic light- Max Planck Research · 1 | 2020 PHYSICS & ASTRONOMY houses that regularly emit radio ra- rarer. Or perhaps they are younger tronomical,” this object is the closest diation. This brings neutron stars and more energetic than the Crab source ever observed. The findings back into play, because pulsars are Pulsar,” says Spitler. “FRB 121102’s also revealed that stars are evidently rapidly rotating neutron stars that home galaxy fits this model, as it born at a high frequency in the vicin- have strong magnetic fields. If the has the potential to produce just the ity of the burst. rotational and the magnetic axes right type of stars that become neu- of such an object are misaligned, a tron stars at the end of their lives.” Its position in the galaxy differs from light-house like, bundled radio beam But whether this model is correct is that of all the other bursts investi- can be produced. Each time this ce- still literally “written in the stars.” gated so far. FRBs, in conclusion, lestial spotlight sweeps across the Insight into their cause is not get- can apparently flare up in all kinds Earth, astronomers measure a short ting any easier, but observations are of cosmic regions and diverse envi- pulse. ongoing. ronments. “This is just one of the reasons why we still do not know The bursts from most radio pulsars In the summer of 2019, for instance, whether all bursts have the same are too weak to be detected from a the European VLBI Network of ra- type of source or are generated by great distance. However, short and dio telescopes examined another re- the same physical processes,” says extremely strong “giant pulses” are peater. During the five-hour obser- Spitler. “Their origin remains a another matter. Born in a supernova vation period, FRB 180916.J0158+65 mystery.” explosion observed in 1054 AD, the emitted no fewer than four radia- Crab Pulsar is a textbook example of tion bursts, each lasting less than this type of astronomical object. Its two milliseconds. Recently, the re- giant pulses would be visible even searchers also identified a roughly from neighboring galaxies. 16-day rhythm: for four days at a time, the source emits one or two “One promising model suggests that bursts every hour then goes silent FRBs are similar in nature to the for twelve days. The source of this Crab Pulsar, but they originate from radio burst is in a spiral galaxy about extragalactic neutron stars that emit 500 million light-years away. Even much larger pulses and are much though such a distance is truly “as- GLOSSARY 65 EUROPEAN VLBI NETWORK A network of radio telescopes located primarily in Europe and Asia.