SGR 0501+4516 is the likeliest candidate in our Milky Way Galaxy for a magnetar that was not born in a supernova explosion as initially predicted; the object is so strange it might even offer clues to the mechanism behind fast radio bursts.
An artist’s impression of a magnetar. Image credit: ESA.
“Magnetars are neutron stars composed entirely of neutrons. What makes magnetars unique is their extreme magnetic field,” said Dr. Ashley Chrimes, an astronomer at the European Space Research and Technology Center.
SGR 0501+4516’s strangeness was identified with the help of sensitive instruments onboard the NASA/ESA Hubble Space Telescope as well as ESA’s Gaia spacecraft.
Initially, the magnetar was discovered in 2008 when NASA’s Swift Observatory spotted brief, intense flashes of gamma rays from the outskirts of the Milky Way.
Because magnetars are neutron stars, the natural explanation for their formation is that they are born in supernovae, when a star explodes and can collapse down to an ultra-dense neutron star.
This appeared to be the case for SGR 0501+4516, which is located close to a supernova remnant called HB9.
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.
But a decade-long study with Hubble cast doubt on the magnetar’s birthplace.
After initial observations with ground-based telescopes shortly after SGR 0501+4516’s discovery, astronomers used Hubble’s exquisite sensitivity and steady pointing to spot the magnetar’s faint infrared glow in 2010, 2012, and 2020.
Each of these images was aligned to a reference frame defined by observations from the Gaia spacecraft, which has crafted an extraordinarily precise three-dimensional map of nearly two billion stars in the Milky Way.
This method revealed the subtle motion of the magnetar as it traversed the sky.
“All of this movement we measure is smaller than a single pixel of a Hubble image,” said Dr. Joe Lyman, an astronomer at the University of Warwick.
“Being able to robustly perform such measurements really is a testament to the long-term stability of Hubble.”
By tracking the magnetar’s position, the astronomers were able to measure the object’s apparent motion across the sky.
Both the speed and direction of SGR 0501+4516’s movement showed that the magnetar 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 with which it could be associated.
If SGR 0501+4516 was not born in a supernova, the magnetar must either be older than its estimated 20,000-year age, or it may have formed in another way.
Magnetars may also be able to form through the merger of two lower-mass neutron stars or through a process called accretion-induced collapse.
Accretion-induced collapse requires a binary star system containing a white dwarf: the core of a dead Sun-like star.
If the white dwarf pulls in gas from its companion, it can grow too massive to support itself, leading to an explosion — or possibly the creation of a magnetar.
“Normally, this scenario leads to the ignition of nuclear reactions, and the white dwarf exploding, leaving nothing behind,” said Dr. Andrew Levan, an astronomer at Radboud University and the University of Warwick.
“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+4516 was born.”
SGR 0501+4516 is currently the best candidate for a magnetar in our Galaxy that may have formed through a merger or accretion-induced collapse.
Magnetars that form through accretion-induced collapse could provide an explanation for some of the mysterious fast radio bursts, which are brief but powerful flashes of radio waves.
In particular, this scenario may explain the origin of fast radio bursts that emerge from stellar populations too ancient to have recently birthed stars massive enough to explode as supernovae.
“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 Dr. Nanda Rea, an astronomer at the Institute of Space Sciences in Barcelona, Spain.
The findings appear in the journal Astronomy & Astrophysics.
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A.A. Chrime et al. 2025. The infrared counterpart and proper motion of magnetar SGR 0501+4516. A&A 696, A127; doi: 10.1051/0004-6361/202453479
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