Astronomers Detect Source of Powerful Radio Burst Within the Milky Way

On April 28^th, 2020, a supermagnetized stellar remnant, called a magnetar, emitted a concurrent mix of radio signals and X-rays that were never seen before.

This recent flare-up comprised the first-ever fast radio burst (FRB) observed from within the Milky Way galaxy and demonstrates that magnetars can create such powerful and enigmatic radio blasts that were previously observed only in other galaxies.

Before this event, a wide variety of scenarios could explain the origin of FRBs. While there may still be exciting twists in the story of FRBs in the future, for me, right now, I think it’s fair to say that most FRBs come from magnetars until proven otherwise.

Chris Bochenek, Doctoral Student in Astrophysics, California Institute of Technology

Bochenek also headed one study of the radio event.

A magnetar can be defined as a type of isolated neutron star, which is the crushed, city-size remnants of a star whose size is several times that of the Sun.

The powerful magnetic field of the magnetar makes it quite unique. The magnetic field can be up to a thousand times stronger than a regular neutron star and 10 trillion times stronger than a refrigerator magnet. This denotes a large storehouse of energy that, according to astronomers, probably powers the outbursts of magnetars.

A number of satellites, including NASA’s Wind mission, detected the X-ray part of the synchronous bursts. The radio component was identified by the Canadian Hydrogen Intensity Mapping Experiment (CHIME)—a radio telescope based at Dominion Radio Astrophysical Observatory in British Columbia and headed by Montreal-based McGill University and also by the University of British Columbia and the University of Toronto

Survey for Transient Astronomical Radio Emission 2 (STARE2)—a NASA-funded project—also identified the radio burst observed by CHIME.

Comprising three detectors in Utah and California and operated by the California Institute of Technology (Caltech) and NASA’s Jet Propulsion Laboratory based in Southern California, the STARE 2 project is headed by Bochenek, Shri Kulkarni from Caltech, and Konstantin Belov from JPL. The team established that the energy of the radio burst was similar to FRBs.

By the time these synchronous bursts took place, astronomers were already tracking their source for over half a day.

Then, late on April 27^th, 2020, the Neil Gehrels Swift Observatory of NASA detected a new round of activity from a magnetar known as SGR 1935+2154, or SGR 1935 for short, situated in the constellation Vulpecula. To date, this was the object’s most prolific flare-up ever observed—a storm of quick bursts of X-rays, with each lasting just less than a second.

The storm continued for hours, and was later detected at numerous times by Swift, NASA’s Fermi Gamma-ray Space Telescope, as well as by NASA’s Neutron star Interior Composition Explorer (NICER)—an X-ray telescope installed on the International Space Station.

Around 13 hours after the storm had abated and when the magnetar was no longer detected by Swift, NICER, and Fermi telescopes, one unique X-ray burst flared up.

The outburst was observed by the INTEGRAL mission of the European Space Agency, the Huiyan X-ray satellite of the China National Space Administration, and the Russian Konus instrument on Wind. When the half-second-long X-ray burst erupted, STARE2 and CHIME spotted the radio burst, which persisted just a thousandth of a second.

The radio burst was far brighter than anything we had seen before, so we immediately knew it was an exciting event. We’ve studied magnetars in our galaxy for decades, while FRBs are an extragalactic phenomenon whose origins have been a mystery. This event shows that the two phenomena are likely connected.

Paul Scholz, Researcher, Dunlap Institute for Astronomy & Astrophysics, University of Toronto

Scholz is also a member of the CHIME/FRB Collaboration.

Articles from both the CHIME/FRB Collaboration and the STARE2 research team were published in the Nature journal on November 4^th, 2020.

The distance of the SGR 1935 is yet to be clearly established, with estimates spanning from 14,000 to 41,000 light-years. If it is believed to be located at the closer end of this range, the X-ray part of the concurrent bursts would carry similar energy as that produced by the Sun over a month. But interestingly, it was not as strong as some of the flares observed in the magnetar’s storm eruption.

The bursts seen by NICER and Fermi during the storm are clearly different in their spectral characteristics from the one associated with the radio blast. We attribute this difference to the location of the X-ray flare on the star's surface, with the FRB-associated burst likely occurring at or close to the magnetic pole. This may be key to understanding the origin of the exceptional radio signal.

George Younes, Researcher, George Washington University

Younes is also the lead author of two articles exploring the burst storm, and these papers are being peer-reviewed.

The radio burst of the SGR 1935 was many times brighter than any radio bursts seen from magnetars in the Milky Way galaxy. If this phenomenon had happened in another galaxy, it would not have been identified from some of the weaker FRBs seen. Moreover, the radio pulse also arrived at the time of an X-ray burst, something that was never observed before in relation to FRBs.

When the observations were taken together, they strongly indicate that SGR 1935 magnetar created the Milky Way’s equivalent of an FRB, which implies that magnetars in other galaxies probably emit at least some of these signals.

For solid evidence of the magnetar connection, scientists would preferably like to detect an FRB beyond the Milky Way galaxy that corresponds with a burst of X-rays from the same source. Such a combination may only be feasible for neighboring galaxies, and this is the reason why STARE2, CHIME, and NASA’s high-energy satellites will continue to watch the skies.

On April 28, space- and ground-based observatories detected powerful, simultaneous X-ray and radio bursts from a source in our galaxy. Watch to see how this unique event helps solve the long-standing puzzle of fast radio bursts observed in other galaxies. Video Credit: NASA’s Goddard Space Flight Center.