Wandering Magnetar Has Mysterious Origins

For 12 years, astronomers have tracked a lone magnetar as it roamed through the Milky Way. The wandering object is an exotic type of neutron star, a dense, city-size remnant of a massive star, only in this case with particularly powerful magnetic fields. While most magnetars form in a massive star’s collapse — accompanied by a supernova explosion — new evidence now suggests that this magnetar had some other origin.

The magnetar was discovered in 2008 in data from NASA’s Neil Gehrels Swift Observatory and dubbed soft gamma repeater (SGR) 0501+4516, for its sporadic bursts of intense gamma-ray and X-ray radiation. The Hubble Space Telescope and ground-based observatories in Hawai‘i and Spain tracked the magnetar’s motion in 2010, 2012, and 2020.

Now, a team led by Ashley Chrimes (European Space Research and Technology Center) has compared these observations with the European Gaia telescope’s precise measurements of the positions of more than 2 billion stars, calibrating the magnetar’s motion as it zips through our galaxy.

Puzzlingly, the magnetar’s motion puts it far from any supernova remnants or star clusters that could be reasonably attributed to producing SGR 0501+4516. “This is the first time that the birth site of a (thought to be) young magnetar, with good constraints on the past trajectory and minimal observational biases, has not been identified,” Chrimes says. His team’s paper on the finding, and on possible alternative scenarios, was published inAstronomy & Astrophysics.

Neutron Star with Mysterious Origins

Magnetars’ origins are not well understood, largely because of their scarcity and the challenges of observing these faraway stars. Only around 30 of these young, isolated neutron stars have been found in the Milky Way or in its galactic neighbors, the Magellanic Clouds.

Astronomers monitored SGR 0501+4516 at near-infrared wavelengths in the days, weeks, and years after the initial burst. They found that it moves across the sky at 5 milliarcseconds per year. Though that may seem small — it’s the equivalent of watching a snail inch along near the orbit of Saturn — at the magnetar’s estimated distance of around 6,500 light-years, the observed motion ends up at around 50 kilometers per second, or more than 100,000 miles per hour.

Since the magnetar is located near the supernova remnant HB9, which likely exploded around 4,000 to 7,000 years ago, scientists initially speculated that SGR 0501+4516 might have come from that explosion. However, rewinding its motion indicates that the magnetar’s past trajectory never crossed paths with the still-expanding shells of gas.

In fact, the team couldn’t link the neutron star to any nearby remnants or clusters. “It’s clear that most magnetars are ‘regular’ neutron stars, in the sense that they are products of massive star core-collapse,” Chrimes says. “But with this magnetar, SGR 0501+4516, we can’t identify a clear birth site.”

Different Origin Theories

“This discovery plays into a bigger question of how we understand how stars end their lives,” says Andrew Levan (Radboud University), a coauthor on the paper. The team presented several different possibilities to explain the magnetar’s origins.

It’s possible (although unlikely) that the magnetar is a lot older than expected — so ancient that its supernova remnant has already diffused and is impossible to observe. Or perhaps the magnetar originated from the core-collapse supernova of a less massive star, which would be smaller, fainter, and fading more rapidly. A star whose matter was stripped away from a binary companion could potentially produce a smaller explosion. However, the lack of any objects near SGR 0501+4516 places doubt on the latter explanation.

It is also possible that SGR 0510+4516 formed from a mechanism that hasn’t yet been observed to create magnetars. “Two possible non core-collapse formation channels are low-mass binary neutron star mergers and the accretion-induced collapse of a white dwarf,” Chrimes says.

The magnetar could be the stable, highly magnetized product of two neutron stars merging, as long as their merger remained under the mass limit to produce a black hole.

Alternatively, a magnetar might be able to form if two white dwarf stars merge, or if one white dwarf pulled enough matter off of its companion. White dwarfs, the remnant cores of Sun-like stars, typically go supernova if they gain too much mass, but certain composition and magnetic conditions might allow a binary white dwarf to form a neutron star instead.

“These ideas for magnetar formation have been suggested for decades, but still lack direct evidence,” Chrimes says. “SGR 0501+4516 is the best candidate for such a formation channel yet discovered in the Milky Way.”

As yet, though, it’s hard to say which scenario works better. “We have further Hubble observations planned to perform similar measurements for many more magnetars,” Chrimes adds. By determining the motions of many magnetars, the team hopes to discern their formation process.

The Roaming Magnetar and Fast Radio Bursts

In addition, the wandering magnetar may shed light on brief flashes of radio waves, known as fast radio bursts (FRBs), which have puzzled astronomers for more than a decade. FRBs have been linked to magnetars in the past, but magnetars are generally associated with younger star-forming regions, and FRBs have been found emanating from regions with older stars as well.  These signals might be attributed to the formation mechanisms hinted at by this magnetar.

“In this case it looks likely that a neutron star formed ‘in the dark,’ without a big supernova explosion,” Levan says. “If we can form neutron stars, or even potentially black holes, without launching powerful explosions, this has important implications for our understanding of stellar life and death.”