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Scientists confirm the source of mysterious radio bursts in the Milky Way

All the signs point to a powerful type of star.

On April 28, space observatories detected a burst of activity from the center of the Milky Way that lasted for a fraction of a second.

The blip was a fast radio burst, a strong radio signal that lasts mere milliseconds. Although fast radio bursts are frequently detected coming from outside our own galaxy, their sources remain a mystery. But a team of astronomers believe they have identified the source of the first such radio burst detected in the Milky Way — a kind of hyper-magnetized star.

Their findings are detailed in three studies published Wednesday in the journal Nature.

An artist's illustration of a magnetar — a dense, spinning ember left behind after a supernova and wrapped in intense magnetic fields.

Fast radio bursts were first detected in 2007, but the signals came from a far enough distance that astronomers had a hard time identifying their source. But then a radio burst came in from a little closer to home.

Before 2007, the next closest source detected was at a distance of 490 million light years away from Earth.

This radio signal was different. It was so bright scientists could tell that it came from within our own galaxy, yet it shared the properties of fast radio bursts from very distant sources — putting its source also tantalizingly close at hand.

"This burst was so bright that, in theory, if you had a recording from the raw data from your cellphone's 4G LTE receiver, which does detect radio waves, and if you kind of knew what you were looking for, you might have found the signal that came about half way across the galaxy in your cellphone data," Christopher Bochenek, a graduate student at the California Institute of Technology and one of the authors on the new studies, said in a press briefing to discuss the results.

“This is the first time we’ve been able to tie one of these exotic fast radio bursts to a single astrophysical object.”

Tracing it back, scientists found that the burst, dubbed SGR 1935+2154, appeared to erupt from a source just 30,000 light-years away from Earth.

From their observations, the astronomers worked out that the radio pulses seem to have been produced by a magnetar, a dense, spinning and hyper-magnetized star. These strange stars are the remnants left after a supernova and are wrapped in intense magnetic fields. These fields are so strong, Bochenek said, they "squish atoms into pencil-like shapes."

“There’s this great mystery as to what would produce these great outbursts of energy, which until now we’ve seen coming from halfway across the universe,” Kiyoshi Masui, assistant professor of physics at Massachusetts Institute of Technology, who led the team’s analysis of the fast radio burst's brightness, said in a statement.

“This is the first time we’ve been able to tie one of these exotic fast radio bursts to a single astrophysical object.”

Scientists have long hypothesized that fast radio bursts are produced by magnetars, but this is the first time that they have been able to prove it with direct observations. Such a discovery is no mean feat — this project had a 10 percent chance of actually detecting this kind of fast radio burst, Bochenek said.

In the case of this particular magnetar, dubbed SGR 1935+2154, it is one of about 30 known magnetars in the Milky Way. These neutron stars occasionally go through spurts of activity, during which they emit radiation at different wavelengths.

The researchers were able to pinpoint the fast radio burst to a point in the sky within a fraction of a degree of this magnetar, which happened to be blasting out X-rays around the same time.

“There was some buzz in the astronomy community about this magnetar that had become active in the X-ray, and it had been mentioned within our collaboration that we should keep an eye out for something more from this magnetar,” Masui said. “If it was coming from any other object close to the magnetar, it would be a very big coincidence," he added.

Now that scientists have confirmed that fast radio bursts are produced by magnetars, the next cosmic mystery to crack is to figure out exactly how these bursts of emissions are produced by the strange stars.

Abstract, study one: Magnetars are highly magnetized young neutron stars that occasionally produce enormous bursts and fares of X-rays and γ-rays1 . Of the approximately thirty magnetars currently known in our Galaxy and the Magellanic Clouds, fve have exhibited transient radio pulsations2,3 . Fast radio bursts (FRBs) are millisecondduration bursts of radio waves arriving from cosmological distances4 , some of which have been seen to repeat5–8 . A leading model for repeating FRBs is that they are extragalactic magnetars, powered by their intense magnetic felds9–11. However, a challenge to this model is that FRBs must have radio luminosities many orders of magnitude larger than those seen from known Galactic magnetars. Here we report the detection of an extremely intense radio burst from the Galactic magnetar SGR 1935+2154 using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project. The fuence of this two-component bright radio burst and the estimated distance to SGR 1935+2154 together imply a burst energy at 400 to 800 megahertz of approximately 3 × 1034 erg, which is three orders of magnitude higher than the burst energy of any radio-emitting magnetar detected thus far. Such a burst coming from a nearby galaxy (at a distance of less than approximately 12 megaparsecs) would be indistinguishable from a typical FRB. However, given the large gaps in observed energies and activity between the brightest and most active FRB sources and what is observed for SGR 1935+2154-like magnetars, more energetic and active sources— perhaps younger magnetars—are needed to explain all observations.
Abstract, study two: Since their discovery in 20071 , much efort has been devoted to uncovering the sources of the extragalactic, millisecond-duration fast radio bursts (FRBs)2 . A class of neutron stars known as magnetars is a leading candidate source of FRBs3,4 . Magnetars have surface magnetic felds in excess of 1014 gauss, the decay of which powers a range of high-energy phenomena5 . Here we report observations of a millisecond-duration radio burst from the Galactic magnetar SGR 1935+2154, with a fuence of 1.5 ± 0.3 megajansky milliseconds. This event, FRB 200428 (ST 200428A), was detected on 28 April 2020 by the STARE2 radio array6 in the 1,281–1,468 megahertz band. The isotropic-equivalent energy released in FRB 200428 is 4 × 103 times greater than that of any radio pulse from the Crab pulsar—previously the source of the brightest Galactic radio bursts observed on similar timescales7 . FRB 200428 is just 30 times less energetic than the weakest extragalactic FRB observed so far8 , and is drawn from the same population as the observed FRB sample. The coincidence of FRB 200428 with an X-ray burst9–11 favours emission models that describe synchrotron masers or electromagnetic pulses powered by magnetar bursts and giant fares3,4,12,13. The discovery of FRB 200428 implies that active magnetars such as SGR 1935+2154 can produce FRBs at extragalactic distances.
Abstract, study three: Fast radio bursts (FRBs) are millisecond-duration radio transients of unknown physical origin observed at extragalactic distances1–3 . It has long been speculated that magnetars are the engine powering repeating bursts from FRB sources4–13, but no convincing evidence has been collected so far14. Recently, the Galactic magnetar SRG 1935+2154 entered an active phase by emitting intense soft γ-ray bursts15. One FRB-like event with two peaks (FRB 200428) and a luminosity slightly lower than the faintest extragalactic FRBs was detected from the source, in association with a soft γ-ray/hard-X-ray fare18–21. Here we report an eight-hour targeted radio observational campaign comprising four sessions and assisted by multi-wavelength (optical and hard-X-ray) data. During the third session, 29 soft-γ-ray repeater (SGR) bursts were detected in γ-ray energies. Throughout the observing period, we detected no single dispersed pulsed emission coincident with the arrivals of SGR bursts, but unfortunately we were not observing when the FRB was detected. The non-detection places a fuence upper limit that is eight orders of magnitude lower than the fuence of FRB 200428. Our results suggest that FRB–SGR burst associations are rare. FRBs may be highly relativistic and geometrically beamed, or FRB-like events associated with SGR bursts may have narrow spectra and characteristic frequencies outside the observed band. It is also possible that the physical conditions required to achieve coherent radiation in SGR bursts are difcult to satisfy, and that only under extreme conditions could an FRB be associated with an SGR burst.

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