Radio bursts traced to faraway galaxy

Astronomers have traced a series of brief, enigmatic bursts of radio waves to a galaxy far, far away and indeed a long time ago — some three billion years or so. But as much as you might be hoping or dreading it to be true, this is probably not E.T.

“We’ve joked about spaceship battles and death stars blowing up, but we think we can explain it with ordinary physics,” said Shami Chatterjee, a Cornell University astronomer. Shami is the lead author of a paper published in Nature this month that details the search for the source of the radio waves known as “fast radio bursts,” — intense pulses of radiation from the sky lasting only a few milliseconds.

These have been disappointing times for those yearning for some alien direction from Out There. Last summer, Russian astronomers reported that they had recorded a promising-sounding signal from a star in the Hercules constellation, but they dismissed it when it became public as a freak bit of random radio noise, the astrophysical equivalent of a cosmic butt dial. More recently, searches for radio signals from a set of stars with anomalous spectral features and another star known as Tabby’s Star that has shown suspicious variations in its light seem to have come up empty.

Unexpected discovery?
So at least for now, the skies appear to be bereft of intelligence. But the new results from the fast radio burster, known as 121102 — after November 2, 2012, the date it was first observed — need not discourage any aficionados of cosmic mystery. Most likely, Shami said in a telephone interview, the bursts could be caused by weird reactions between a neutron star — the dense spinning magnet left behind by a supernova explosion — and the debris from that explosion.

Or perhaps from some unexpected quirk of a supermassive black hole in the centre of the galaxy, a dwarf assemblage of stars some three billion light-years away in the constellation Auriga. There are problems with both explanations, however, he added.
Fast radio bursts have led astronomers on a merry chase ever since they were
discovered in 2007 in data recorded earlier by the Parkes radio telescope in Australia. Because they are so short and until recently have never been seen to repeat, these phantoms have been hard for astronomers to study. Usually, astronomers notice them after the fact. Moreover, radio telescopes have poor angular resolution, making it impossible to determine exactly what star or distant galaxy they came from.

The radio emissions themselves, Shami said, resemble the blasts from pulsars — the spinning neutron stars that emit clocklike pulses of radiation and whose discovery in 1968 did indeed elicit speculation about little green men. But the radio waves arrive on Earth dispersed or spread out in time by wavelength, which implies that they have travelled from far outside our galaxy.

That great distance also implies that they are enormously more powerful than pulsars in our galaxy, adding to the mystery of what they are and raising the question of why they are not seen within our own galaxy, the Milky Way. In all, 18 of the fast bursters have been spotted since they were first recognised in 2007 — a small number. If extrapolated to the whole sky, that means 5,000 to 10,000 of these flashes should happen every day.

Where are they? Lacking much evidence to the contrary, astronomers theorised that the bursts resulted from apocalyptic events like collisions of neutron stars. At one point, Shami said, there were more theoretical models of the bursts than observed bursts. “Many things go bang,” he explained.

The big break came in 2012 when the burst known as 121102 repeated itself. Subsequent observing campaigns with the Very Large Array of telescopes in New Mexico and the Arecibo radio telescope in Puerto Rico, USA recorded a total of nine bursts over 83 hours of observing time and a terabyte of data during a six-month period in 2016. That meant that whatever was causing 121102, at least, was not destroying it. “We definitely know, for this one case anyway, the radio burst is not cataclysmic,” Shami said.

In addition, papers also have been submitted to the Astrophysical Journal Letters by two other groups led by Shriharsh Tendulkar of McGill University, Canada and by Benito Marcote of the Joint Institute for VLBI in Europe, in Dwingeloo, the Netherlands.

Massive stars
Following up on the Very Large Array observations, Benito’s team on the European VLBI network was able to pinpoint the location of the burst to a faint dwarf galaxy in the Auriga constellation. Shriharsh and his colleagues then used the eight-metre Gemini North telescope on Mauna Kea in Hawaii to observe the galaxy and measure its distance. That distance, three billion light-years, confirmed the original supposition that the fast bursts come from far, far away. “The host galaxy is puny,” Shriharsh said during the news conference in Grapevine. That dwarf galaxy is only a hundredth of the mass of the Milky Way.

If this burster, 121102, is indeed typical of the bunch, the astronomers said, this might be a clue. Such galaxies are typically home to some of the most violent events in the universe, Shami said, things that go seriously boom in the night like certain kinds of gamma-ray bursts and superluminous supernova explosions that result in extremely magnetic pulsars known as magnetars.

These are the signatures of massive stars, of the deaths of massive stars, he said. But this only raises more questions. “The only one that repeats is from three billion light-years,” Shami mused. “Where are all the nearby ones?” he asked, noting that they should be even brighter, saturating our radio receivers. “It’s very curious,” he said.