A Milky Way magnetar surprises astronomers by burping up gamma rays right when their predictions anticipated.
A neutron star with a souped-up magnetic field is right on schedule. Just when astronomers had predicted, it started spitting out gamma rays in bursts, behaving in a way not seen before in these objects.
After a star goes supernova, it can leave behind a compressed core called a neutron star. Packing more than the mass of the Sun in a ball 20 kilometers (12 miles) wide, some of these exotic stars have a magnetic field at least a thousand trillion times stronger than Earth’s, making them magnetars – bodies with the strongest magnetic fields in the universe.
This extreme magnetism is probably behind surges of high-energy radiation called gamma rays. The crust of a magnetar may crack under the stress of intense magnetism, setting off starquakes that shoot gamma rays into space. The magnetar SGR 1935+2154 is one such known “gamma repeater” —and it’s also a source of brief, powerful radio flashes called fast radio bursts (FRBS). The question is, do these repetitive outbursts follow a pattern?
A team lead by Mikhail Denissenya (Nazarbayev University, Kazakhstan) considered the last seven years of gamma-ray data to find that this magnetar is likely exhibiting periodic episodes, bursting with gamma rays during active periods but then subsiding during quieter, inactive spans. The researchers predicted that the magnetar would end its gamma-ray slumber and come back to life by June of this year – and lo and behold, it did. The behavior could help expose the source of both gamma and radio bursts in the cosmos. Their results appear in the July Physical Review D (preprint available here).
The gamma-ray bursts from this galactic magnetar appear to occur in regular four-month periods, followed by three months of inactivity. The results of Denissenya’s analysis imply a period of 231 days from the beginning of one active period to the next, with only a 3-in-10,000 chance that these events occur randomly.
Even more convincing than the periodic probability was the detection of an anticipated gamma surge. Denissenya‘s team predicted that the magnetar should be active between June 1st and October 7th, with gamma ray bursts appearing any time between those dates. And sure enough, after a hiatus, a burst occurred right on schedule, on June 24th, just before their study was published.
Kaustubh Rajwade (University of Manchester), who was not on the study, remains skeptical despite the odds. “This is definitely some evidence of a potential periodic behavior in the bursting of SGR 1935+2154,” he says. But he adds that uneven records of events make it hard to be sure, and he agrees with Denissenya’s team that additional data is needed.
Nevertheless, the predicted gamma-ray burst “bodes well for their discovery,” Rajwade says.
Cue the Radio Bursts
To understand why SGR 1935+2154 is so punctual, astronomers must consider that it is also a source of FRBs, which happen so quickly that scientists sometimes struggle to identify where they come from, much less what causes them. But last spring, a flash of radio waves from SGR 1935+2154 – which was already being monitored – marked the first known source of FRBs within our galaxy.
Knowing that FRBs and gamma-ray bursts can both come from magnetars, astronomers have suggested that the two phenomena are connected. Since SGR 1935+2154 shows both types of outbursts, the periodic behavior that Denissenya’s team reports would be “a major boost for this hypothesis” if confirmed, says Rajwade, especially since two other FRBs have been found to repeat.
Such periodic activity in magnetars could be a function of their precession – the top-like wobble of a spinning object. In this case, we would see bursts when the neutron star’s magnetic axis crosses Earth’s line of sight, and we would stop seeing them as the axis swings away.
If our galactic magnetar truly shows periodic gamma behavior and produces an FRB during a predicted active window, then its radio bursts may also repeat following a similar pattern. Such a discovery, Rajwade states, would be a huge step towards understanding the connection between magnetars, FRBs, and gamma bursts. The next generation of surveys, such as the Vera Rubin Observatory, may be the ones to provide astronomers with this evidence.