Researchers using NASA’s Hubble Space Telescope have discovered the magnetar called SGR 0501+4516 is traversing our galaxy from an unknown place of origin. Researchers say that this runaway magnetar is the likeliest candidate in our Milky Way galaxy for a magnetar that was not born in a supernova explosion as initially predicted. It is so strange it might even offer clues to the mechanism behind events known as fast radio bursts.
“Magnetars are neutron stars — the dead remnants of stars — composed entirely of neutrons. What makes magnetars unique is their extreme magnetic fields,” said Ashley Chrimes, lead author of the discovery paper published in the April 15 journal Astronomy & Astrophysics. Chrimes is a European Space Agency Research Fellow at the European Space Research and Technology Center in the Netherlands.
Magnetars have comic-book-hero superpowers. A magnetar has a magnetic field about a trillion times more powerful than Earth’s magnetosphere. If a magnetar flew by Earth at half the Moon’s distance, its intense field would wipe out every credit card on our planet. If a human got within 600 miles, the magnetar would become a proverbial sci-fi death-ray, ripping apart every atom inside the body.
The magnetar’s strangeness was identified with the help of Hubble’s sensitive instruments as well as precise benchmarks from ESA’s (European Space Agency) Gaia spacecraft.
Initially, the mysterious magnetar was discovered in 2008 when NASA’s Swift Observatory spotted brief, intense flashes of gamma rays from the outskirts of the Milky Way. The source, which turned out to be one of only about 30 known magnetars in the Milky Way, was dubbed SGR 0501+4516.
This is an artist’s impression of a magnetar, which is a special type of neutron star with an incredibly strong magnetic field. Neutron stars are some of the most compact and extreme objects in the universe. These stars typically pack more than the mass of the Sun into a sphere of neutrons about 12 miles across. The neutron star is depicted as a white-blueish sphere. The magnetic field is shown as filaments streaming out from its polar regions.
Illustration: ESA
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, researchers 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 co-investigator Joe Lyman of the University of Warwick, United Kingdom. “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 team was 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. 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 Andrew Levan of Radboud University in the Netherlands and the University of Warwick in the United Kingdom.
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 Nanda Rea of the Institute of Space Sciences in Barcelona, Spain.
The research team has further Hubble observations planned to study the origins of other magnetars in the Milky Way, helping to understand how these extreme magnetic objects form.
The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
