Researchers and alumni from The University of Warwick have worked as part of a European Space Agency (ESA) team, to discover that the magnetar SGR 0501+4516 did not originate from a nearby supernova, as previously thought.

Magnetars are a special type of neutron star, which are some of the most extreme objects in the Universe. The magnetar in this study was first spotted in 2008 by NASA’s Swift Observatory perceiving brief, intense flashes of gamma rays from the outskirts of the Milky Way and is one of only about 30 known magnetars in the Milky Way.

Ashley Chrimes, lead author of the paper and European Space Agency Research Fellow at the European Space Research and Technology Centre (ESTEC) in the Netherlands and PhD alumnus from The University of Warwick said: “Magnetars are the dead remnants of stars, composed entirely of neutrons. They’re so heavy and dense that the electrons and protons which make up atoms have been crushed together into neutrons. What makes magnetars unique is their extreme magnetic fields, billions of times stronger than the strongest magnets we have on Earth”.

Most neutron stars are thought to be born in core-collapse supernovae. These spectacular cosmic explosions happen when stars far more massive than our Sun run out of fuel for nuclear fusion. The star’s outer layers fall inward and rebound off the collapsed core in an explosion that can briefly outshine an entire galaxy.

Because magnetars are themselves neutron stars, the natural explanation for their formation is that they too are born in supernovae. This appeared to be the case for SGR 0501+4516, which is located promisingly close to a supernova remnant called HB9.

But this decade-long study with Hubble, published in the journal Astronomy & Astrophysics, has cast doubt on the magnetar’s birthplace. Using Hubble’s exquisite sensitivity and steady pointing, researchers had previously imaged the magnetar’s faint infrared glow in 2010, 2012 and 2020.

In the study, each of these images was aligned to a reference frame defined by observations from the ESA’s 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 inched across the sky.

Joe Lyman, Associate Professor at the University of Warwick and co-investigator said: “All of this movement we measure is smaller than a single pixel of a Hubble image. 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 that it could be associated with.

If SGR 0501+4516 was not born in supernova remnant HB9, the magnetar must either be far older than its reported 20,000-year age, or it must have formed in another way, such as the merger of two lower-mass neutron stars or through accretion-induced collapse - a white dwarf ensnares gas from a companion white dwarf until it grows too massive to support itself.

Professor Andrew Levan at Radboud University in the Netherlands and the University of Warwick added: “Normally, this scenario leads to the ignition of nuclear reactions, and the white dwarf exploding, leaving nothing behind. But it has been theorised that under certain conditions, the white dwarf can instead collapse into a neutron star. We think this might be how SGR 0501 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. A magnetar formed this way could provide an explanation for some of the mysterious cosmic signals we can measure, such as the origin of fast radio bursts that emerge from stellar populations too ancient to have recently birthed stars massive enough to explode as supernovae.

Nanda Rea of the Institute of Space Sciences in Barcelona said: “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, superluminous supernovae, and fast radio bursts”.

The research team has further Hubble observations plans to study the origins of other magnetars in the Milky Way, helping to understand how these extreme objects form.

ENDS

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Notes to Editors

Science paper

Release on ESA/Hubble website

Release on ESA website

Release on NASA website

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

Image Credit: ESA

Image Caption: This is an artist’s impression of a magnetar, which is a special type of neutron star. Neutron stars are some of the most extreme objects in the Universe. These stars typically pack more than the mass of the Sun into a sphere of neutrons about 20 kilometres across. Unsurprisingly, these exotic objects can display several extreme behaviours, such as X-ray and gamma-ray outbursts, intense magnetic fields and rapid rotation. Magnetars are a specific type of neutron star that are distinguished by their exceptionally strong magnetic fields (which are significantly stronger than those of typical neutron stars).