Hubble spots a magnetar zipping through the Milky Way

Magnetars are among the rarest—and weirdest—denizens of the galactic zoo. They have powerful magnetic fields and may be the source of fast radio bursts (FRBs). A team of astronomers led by European Space Agency researcher Ashley Chrimes recently used the Hubble Space Telescope (HST) to track one of these monsters called SGR 0501+4516 (SGR0501, for short, and SGR stands for Soft Gamma Repeater). It’s whipping through the Milky Way at a rate as high as 65 kilometers per second. The big challenge was to find its birthplace and figure out its origin.

At first, astronomers thought it could be related to a supernova remnant called HB9. After a great deal of study, it turns out SGR0501 is not the product of a massive core-collapse supernova, but Chrimes and her colleagues aren’t completely sure of its origin, which makes it even more rare and strange.

“Magnetars are neutron stars—the dead remnants of stars—composed entirely of neutrons. What makes magnetars unique is their extreme magnetic fields,” said Chrimes. “Our definite conclusion is that SGR0501 did not originate in HB9. However, since there is no other clear birth site or smoking gun for a different origin, the alternatives are all plausible and we can’t yet say which is the most likely.”

Unraveling the track of the traveling magnetar

There are only about 30 known magnetars in the Milky Way galaxy. These dense balls of neutrons aren’t very big—only about 20 km (12 miles) across. Their tiny sizes belie the incredibly strong magnetic fields that they generate. As the folks at NASA like to say, those fields are so strong that if one flew by Earth at the distance of the moon, all our credit cards would be wiped out. Even worse, if we flew out to visit the magnetar on its way, our ship and astronauts would be torn apart.

Luckily, we only observe them from a distance. Chrimes estimates that it most likely lies about 2,000 parsecs (~6520 light-years) away from us. SGR0501 was originally spotted in 2008 when the Swift Observatory detected brief but bright flashes of gamma rays in its direction. It also looked like it was close to the supernova remnant HB9. Naturally, astronomers assumed the two might be related, since known magnetars are the result of core collapse supernova explosions.

The separation between the magnetar and the center of the supernova remnant on the sky is just 80 arcminutes, or slightly wider than your pinky finger when viewed at the end of your outstretched arm. However, things didn’t add up after astronomers studied the magnetar with HST. A decade-long set of Hubble observations resulted in images that helped astronomers figure out the magnetar’s path as it travels.

By tracking its position, the team charted the object’s apparent motion across the sky. Both the speed and direction of SGR 0501+4516’s movement showed that it could not be associated with the nearby supernova remnant. Tracing the magnetar’s trajectory thousands of years into the past showed that there were no other supernova remnants or massive star clusters that could have produced it.

So, what formed it?

So, if SGR0501 didn’t form in a supernova explosion, what else could form a tiny ball of neutrons with a super-strong magnetic field? That was the challenge the team faced next. It turns out that there are a couple of non-supernova ways to make magnetars. One is by merging two lower-mass neutron stars. That would create the larger, stronger SGR0501.

The other way is by something called accretion-induced collapse. For that, you need a binary star system with a white dwarf as one of the components. As it pulls in gas and material from its companion, it can get greedy and take too much. That destabilizes the white dwarf and leads to a massive explosion.

“Normally, this scenario leads to the ignition of nuclear reactions, and the white dwarf exploding, leaving nothing behind. But it has been theorized that under certain conditions, the white dwarf can instead collapse into a neutron star. We think this might be how SGR 0501 was born,” added co-investigator Andrew Levan of Radboud University in the Netherlands and the University of Warwick in the United Kingdom.

How are fast radio bursts connected to magnetars?

The birth of a magnetar is a pretty powerful event that gives off the kind of brief but strong emissions that characterize fast radio bursts. If SGR0501 formed from a merger or accretion-induced collapse, that might explain the phenomenon of FRBs. These are very short (on the order of less than a millisecond) that don’t always re-occur (in other words, they’re transient flashes in the sky). Many FRBs occur outside our Milky Way, but some are also detected within the galaxy.

“Magnetar birth rates and formation scenarios are among the most pressing questions in high-energy astrophysics, with implications for many of the universe’s most powerful transient events, such as gamma-ray bursts, super-luminous supernovae, and fast radio bursts,” said Nanda Rea of the Institute of Space Sciences in Barcelona, Spain.

Magnetars that form through accretion-induced collapse could provide the kinds of short, powerful bursts of radio waves that characterize FGBs. In particular, that could explain the FRBs seen in ancient stellar populations too old to have massive stars that could explode as supernovae. Since there are other magnetars to study, the team is planning to use HST for further observations of these weirdly magnetic stellar remnants.

The findings are published on the arXiv preprint server.