Just two weeks ago, scientists announced the first detection of an unexplained astronomical signal originating from inside our own galaxy, the Milky Way.
Discovered in late April, the fast radio burst (FRB) was traced back to a churning type of dead star called a “magnetar” located some 30,000 light years away, providing an unprecedented opportunity to study the source of an FRB up close.
A few days later, on May 3, scientists at China’s Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST)—the world’s largest single-dish radio telescope—detected a second extremely faint burst from the magnetar. Now, an international team of researchers led by Franz Kirsten, a postdoctoral researcher at the Chalmers University of Technology in Sweden, has revealed that this magnetar, called SGR 1935+2154, spewed out two other bright bursts in rapid succession in late May.
This means that the energetic object in our galactic backyard may produce rare “repeater” bursts that could have a “similar physical nature to the sources of (repeating) extragalactic FRBs,” according to a study published on Monday in Nature Astronomy. In other words, it might help us explain what produces these mysterious repeating signals, which can sometimes come in patterns.
“We were very surprised to see such bright bursts and, of course, extremely excited too,” said Kirsten in an email. “Until our discovery nobody knew if this magnetar could generate bursts in between the really bright one detected earlier and the very faint one detected by FAST in China.”
Taken together, the full quartet of known bursts from the magnetar have a luminosity range that spans at least seven orders of magnitude, meaning that the most radiant burst is about 10 million times brighter than the dimmest one.
“This is remarkable and raises the question if one or several physical processes generate the different bursts,” Kirsten noted.
Since the first FRB was detected in 2007, scientists have tried to explain the origins of these bright, millisecond-long signals. This quest has been complicated by the fact that FRBs seemed to only show up in faraway galaxies, sometimes billions of light years from the Milky Way.
The large distances to FRBs “have made it hard to study their broadband emission mechanism and local environments,” Kirsten and his colleagues said in the study. “This limits the avenues to differentiate between competing models.”
“The localization of very nearby (tens of megaparsecs) FRBs could help, as would the discovery of an FRB source, at kiloparsec distances, in the Milky Way,” they added.
This help finally arrived on April 28, when an extremely bright burst from inside the Milky Way was independently detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Survey for Transient Astronomical Radio Emission 2 (STARE2). The FRB emanated from SGR 1935+2154, which is categorized as a magnetar due to its intense magnetic field.
Like many astronomers around the world, Kirsten and his colleagues quickly arranged for follow-up observations of the magnetar. The researchers eventually racked up hundreds of hours of observation time between the Westerbork Synthesis Radio Telescope in the Netherlands, the Onsala Space Observatory in Sweden, and a radio telescope at Nicolaus Copernicus University in Toruń, Poland.
The team’s efforts paid off on May 24, when the Westerbork observatory captured two rapid bursts from the magnetar, which erupted 1.4 seconds apart. The bursts were roughly 10,000 times fainter than the April event seen by CHIME and STARE2, though they were 1,000 times brighter than the FAST detection on May 3.
“We observed the star for so very long and then we got two bursts in very close succession,” Kirsten said. “This implies that bursts happen in a clustered fashion, i.e. whenever a burst occurs chances are high that you get another one very close in time.”
This is an important discovery, because it seems to match observations of some repeating fast radio bursts in other galaxies. Like the bursts from SGR 1935+2154, those extragalactic FRBs also show periods of enhanced activity, before sinking back into a dormant state
“Despite this similarity, we cannot draw any conclusions about the similarities of the individual emission mechanisms,” Kirsten cautioned. “The bottom line is that the magnetar shows characteristics in the burst occurrence that are also seen in repeating FRBs. This hints at magnetars being a potential candidate for the type of star that generates FRBs at cosmological distances.”
No doubt scientists will continue staring at SGR 1935+2154 to see what else the magnetar coughs up in the future. Astronomers are also keen to see if any of the other 30-odd local magnetars in the Milky Way pipe up with bursts of their own.
These observations could shed light on the bursts we keep witnessing in other galaxies, which display a wide range of properties that may be caused by many different types of sources and mechanisms.
“One of the most interesting questions is, of course, what does it take for a bright short radio pulse to be called an FRB?” Kirsten said. “How bright does it need to be? Does it need to be extragalactic?”
Scientists have even identified one FRB that flares up like clockwork over a 16-day cycle, making it the first burst to demonstrate periodicity in addition to repetition. That “implies that the source might be in a binary system with another star,” Kirsten explained.
“For FRBs to be generated, do you need that second star?” he added “If so, then how does the magnetar we discuss in our article fit into the picture” given that “it is not known to be in a binary system, i.e. it's an isolated star?”
The answers to these questions remain unknown, for now, but the discovery of a repeating FRB so close to home is likely to yield many new insights about the dazzling variety of these transient events all across the night sky.