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By Daniel Clery
PENTICTON, CANADA-The reports of the Dominion Radio Astrophysical Observatory here require old techniques: pad and pen. Upon my arrival, I have to turn off my DVR and my cell phone and put them in a shielded room with a Faraday cage, a wire mesh that prevents spurious electromagnetic signals from escaping. The goal is to prevent interference with the newest observatory radio telescope, the Canadian experience of mapping the intensity of hydrogen (CHIME).
On a clear and cold day in January, Nikola Milutinovic is standing on the vertiginous portico that runs along one of the four dish-shaped dishes of CHIME, 100 meters long. Milutinovic, a scientific engineer at the University of British Columbia (UBC) in Vancouver, analyzes their reflective surfaces in search of snow, which usually passes through wire mesh, but sometimes removes and freezes. Snow-covered hills surround it, protecting CHIME from mobile phone towers, TV transmitters and even microwave ovens from nearby towns. "If you turn on a mobile phone on Mars, CHIME could detect it," he says.
CHIME's career is neither so weak nor so close. The telescope is smaller and cheaper than other radio observatories. But luckily, as well as by design, his abilities are ideal for probing what might be the most convincing new mystery of astronomy: signals from the distant universe, called fast radio bursts (FRB). Discovered in 2007, the FRBs are so brilliant that they appear in the data as a peak in the nearby Canadian Rockies – as long as a telescope is watching and its electronics are fast enough to detect legumes, which do not last only a few thousandths. a second.
Just days before my visit, CHIME – still in its recovery phase – made the headlines in the world press for pocketing 13 new FRBs, bringing the total to more than 60. Nearly many theories exist to explain them. One of the few things researchers know for sure, by the nature of legumes, is that they come from far beyond our milky way. But in an instant, each event is over, the astronomers no longer being repressed by the light and the frustrating efforts to find their origin.
Astronomers believe that what generates FRB must be compact to produce such short pulses and extremely powerful to be seen at such great distances. Think of neutron stars or black holes or something even more exotic. FRBs can be repeated – though, strangely, only two of the dozen known seem to do so. Repetition may exclude explosions, mergers or other punctual cataclysmic events. Or again, solitary and repeated FRBs can be different animals with different sources – theorists simply do not know it.
C. BICKEL /SCIENCE
What they need are numbers: more events and, above all, more repeaters, which can be located in a particular environment in a home galaxy. CHIME will provide this by scanning the sky with high sensitivity. Its hollows do not move, but they observe a strip of sky half a degree wide, which extends from one horizon to the other. As the Earth rotates, CHIME sweeps across the northern sky. Sarah Burke-Spolaor, an astrophysicist at West Virginia University in Morgantown, says her sensitivity and wide field of vision will allow her to see a volume 500 times bigger than the Parkes radio telescope in Australia, who discovered the first FRB and 21 others. "CHIME just has access to that all day, every day," she says.
Once the CHIME commissioning phase is completed later this year, scientists believe it could find up to two dozen BRAs a day. "In a year, it will be the dominant discoverer of the FRB," says Edo Berger, an astrophysicist at Harvard University.
The odd looking telescope has been a work of love for the small team behind him – work is the key word. A contractor assembled the dishes by lining the troughs with a steel wire mesh reflecting the radio. But researchers from the University of British Columbia, the University of Toronto and McGill University in Montreal have painstakingly collected everything else. That includes 1,000 antennas fixed under the gantry crane in every home, 100 km of cables and more than 1,000 computer processors housed in radiation-shielded shipping containers, next to the dishes.
"Everyone got their hands on the telescope," said Milutinovic, who monitors the telescope and its computer systems. It's not just a clerical job. Although he left alone two baby ospreys nestled on a high pole near the telescope, he enlisted environmental advocates to remove other birds installed in the telescope structure, as well as only from time to time a rattlesnake. When a moisture sensor in one of the computer containers turns off at night, Milutinovic travels to the deserted observatory in a 25-minute drive. He is worried about other nocturnal visitors. "I saw traces of coyote, and there is a bear lying around here."
In a field where leading telescopes are costing billions, the $ 20-million CANNEL should have an impact out of proportion to its price. "CHIME shows that you can build a telescope that makes the world's news at a great price," said Milutinovic.
Hunting with hydrogen
None of this was part of CHIME's original work description. In 2007, a group of Canadian cosmologists came up with the idea of building a cheap telescope to measure 3D distribution across the universe of hydrogen gas clouds, which glow weakly at radio frequencies. According to Keith Vanderlinde of the University of Toronto, the goal was to map the undulations of the density of matter created shortly after the big bang and map their expansion over cosmic history. A change in the rate of expansion would indicate to researchers something about black energy, the mysterious force supposed to accelerate the growth of the universe. "Any possible manipulation would be a huge advantage for physics," says Vanderlinde.
CHIME would also be an excellent machine for studying pulsars. Pulsars are neutron stars, dense ashes of collapsed giant stars, which project electromagnetic rays out of their poles while rotating like a celestial lighthouse, sometimes thousands of times per second. Astronomers on Earth detect beams in the form of metronomic pulses of radio waves. CHIME will monitor 10 pulsars at a time, 24 hours a day, to detect hiccups in perfect timing that may result from the passage of gravitational waves in the intermediate space.
During the design of CHIME, few people thought about the FRB because the first, discovered in 2007 in the archived data of the Parkes telescope, was such an enigma. The measurement of dispersion was high, which meant that the pulse was spread over several frequencies, since free electrons in the intergalactic space had slowed the low frequency radio waves of the burst disproportionately. The high dispersion measurement suggested that the burst came from billions of light-years far beyond our local group of galaxies.
The pulse was still bright, implying that the energy of the source was a billion times greater than that of a pulsar. However, its short duration meant that the source could not exceed 3,000 kilometers because the signals could not cross a larger object fast enough that it could act in unison and produce a single short pulse. A pulsar the size of a city could fit into this space. But how could a pulsar explode so powerfully?
Astronomers have been tempted to consider this first glow as a mirage. But this was not an anomaly: a new impulse was discovered in Parkes' archival data in 2012. Then, after an upgrade with new digital instruments, Parkes detected four more in 2013, all with high dispersion measurements, suggesting cosmically distant origins. This document "made me believe," says Victoria Kaspi, McGill astronomer, who was working on the integration of pulsar monitoring in CHIME.
The document also raised awareness: CHIME could also be adapted to search for BRIs. "Vicky called me and said," You know, that would also make a good FRB machine, "" recalls Ingrid Stairs, a Kaspi's associate at UBC.
Unlikely partners
The upgrade was not easy. The capture of the FRB requires a temporal and frequency resolution finer than the mapping of the hydrogen. CHIME data should be recorded every millisecond on 16,000 frequency channels, says Kaspi. To do this, one had to tinker with the correlator, the dreadfully parallel computer that roamed the 13 terabits of continuous data transmission every second since the 1024 antennas of CHIME, which is comparable to the global mobile phone traffic.
Astrophysicists critical for the time needed a result different from that of cosmologists of absolute sensitivity. Cosmologists, wishing to map cosmic clouds, could do without additional resolution. At the end of each day, they could download the data to a hard drive and send it to UBC for a restful treatment. But it was not an option for FRB fighters, who needed high-resolution data that would quickly overload a hard drive. Kaspi and his colleagues have developed algorithms to analyze in real time a few minutes of high-resolution data stored in a buffer. If an event is detected, the key containing 20 seconds of data is saved. If there is nothing, they are dropped. Searching for FRBs is "crushing and grasping science," says Paul Scholz, a member of the McGill team.
Victoria Kaspi, an astronomer at McGill University in Montreal, Canada, realized that CHIME would be an ideal net for capturing radio bursts.
CHRISTINNE MUSCHI
As test observations began in 2017, the team was upset about the number of FRBs that CHIME would see. CHIME observed at frequencies of 400 to 800 megahertz (MHz), lower than the frequency of 1.4 gigahertz used to detect most FRB. A 300 MHz survey on another telescope found nothing, and another survey from 700 to 800 MHz showed only one burst. "It was disturbing, especially in the lower part of the group," says Stairs.
These concerns dissipated in July and August 2018, when the team won gold with the 13 new FRBs, although sections of the telescope were sporadically decommissioned for adjustments. The Haul, published in Nature in January, included a repeater, only the second discovered. Kaspi declined to provide an update on the number of discoveries made by FRB since last summer, citing two unpublished articles in preparation. But she says that CHIME "meets expectations". "It's a bit like drinking in a fire hose, but in the good sense of the word," she says.
Theories abound
The theoreticians want everything that CHIME will deliver, and even more. Poverty of information allows ideas to go wild. "Almost every aspect of FRB is at stake for theorists," says Berger. An online catalog of FRB origin theories had 48 entries at the time of writing. Many theorists have initially proposed models based on violent collapse or fusion of compact objects, including white dwarfs, neutron stars, pulsars, and black holes. But the discovery of repeaters has shifted speculation to sources that would not be destroyed at the moment of generating a burst.
Active galactic nuclei, supermassive black holes in the center of galaxies, drive winds and radiation that could trigger an explosion by striking nearby objects – a gas cloud, a small black hole, or a hypothetical quark star. Or the bursts could come from more speculative phenomena, such as lightning in the atmosphere of neutron atmospheres or the interaction of hypothetical dark matter particles called axions with black holes or stars to neutrons. Amanda Weltman, a theorist at the University of Cape Town, South Africa, does not neglect even more fanciful ideas such as cosmic strings, hypothetical filamentary defects in the void of space left by the moment that followed the big bang. They "could emit fast radio bursts in many ways," she says.
But as the number of FRBs detected went from single digits to dozens, astronomers realized that bursts could be perfectly common, detectable by the thousands each day if the right-hand telescopes were watching. "It's a serious problem for many models," Berger says.
FRB 121102, the first repeat event detected, may be the most revealing FRB to date. The Arecibo telescope in Puerto Rico saw its first burst in 2012, but since then dozens of others have been seen coming from this place in the sky. In 2017, the Very Large Array at 27 Karl G. Jansky antennas in New Mexico revealed that the FRB resides on the outskirts of a distant dwarf galaxy and that the location coincides with a weak but persistent radio source. . This faint radio glow can come from a remnant of the supernova, an expanding gas ball from a stellar explosion that could have formed a black hole or neutron star that powers the FRB. In another clue, the radio wave polarization of the FRB spins rapidly, suggesting that they emanate from a strong magnetic environment.
Almost every aspect of [fast radio bursts] is at stake for theorists.
Brian Metzger, a theorist at Columbia University, believes that a young magnetar – a highly magnetized neutron star – resides in the center of the cloud and feeds bursts. In a scenario developed with his colleagues, his magnetic field serves as a reserve of gaseous energy that bursts from time to time, projecting a layer of electrons and ions almost at the speed of light, this which looks like a coronal mass ejection from our sun. steroids. When the torch reaches the remaining ionic clouds of the previous flares, the resulting shockwave strengthens the magnetic field lines of the clouds and causes the electrons to coalesce. Spiral concert. Just as the Earth's synchrotrons whip electrons around the racetracks to emit useful X-rays, these gyrations generate a coherent impulse of radio waves.
Magnetars are often invoked to explain such energy events, Metzger explains. "They are a catch-all for everything we do not understand, but here it is a little justified." Shriharsh Tendulkar, a member of McGill's CHIME team, wonders if objects like magnets could explain both transponders and single-burst FRBs. Single-burst FRBs can "start as repeaters, then slow down as [the source’s] Magnetic field weakens, "he says.
But according to Weltman, it is too early to declare the mystery solved. "There are so many clues here, but they do not yet reveal a single conclusive theoretical explanation," she says.
Knowledge in numbers
As observers amass new FRBs, different categories of events may emerge, perhaps offering clues as to what triggers them. FRBs can also come from specific types of galaxies, or regions within them, which might allow theorists to distinguish between active galactic nuclei and other compact objects. "We need statistics and context," Metzger said.
In the coming years, other FRB observers will be posted online, including hydrogen intensity analyzes and real-time analysis in South Africa and the Deep. Synoptic Array in California. Thanks to their wide range of dishes, both facilities will locate the FRB in the sky – something that CHIME can not do for the moment. "They all opt for localization because they know that CHIME is going to clean up the statistics," says Scholz.
The CHIME team, in order not to be outdone, is developing a proposal to add stabilizers, smaller troughs located hundreds of kilometers away, which will record the same events from a different angle and thus help researchers to locate them. "With all these new efforts, there will be substantial progress in the coming years," Metzger said.
For the time being, while the CHIME commissioning phase is coming to an end, Milutinovic's job is to ensure that he continues to do his job. "You want it to be boring," he says. "It's the weather that is causing us the most problems": the fall of the hollows, summer heat waves that tax the cooling system of the electronics. Then there is grass, a fire hazard. Every summer, the observatory invites pastoralists to graze their cattle on-site – not only to be close to neighbors, but also because cows emit less radio frequency interference than a lawn mower . But they can not graze in the vicinity of CHIME because they may chew cables. Milutinovic therefore relies on diesel-powered lawn mowers which, due to lack of spark plugs, pose fewer interference problems.
But he longs for an even better tool for cutting high-resolution grass. "We thought we had a CHICKEN goat."
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