Magnetars
Neutron stars with the most powerful magnetic fields in the universe.
A magnetar is a type of neutron star whose defining feature is an extraordinarily powerful magnetic field, typically in the range of 10⁹ to 10¹¹ tesla. The decay or dissipation of this immense magnetic field provides the energy that powers the emission of high-energy electromagnetic radiation, most notably X-rays and gamma rays. The existence of such objects was first proposed in 1992 by Robert Duncan and Christopher Thompson as a way to explain the properties of transient gamma-ray sources now known as soft gamma repeaters (SGRs). Over the following decade, the magnetar hypothesis gained widespread acceptance and was extended to also account for anomalous X-ray pulsars (AXPs). Out of the roughly 3,000 known neutron stars, 31 have been confirmed as magnetars, with five of these also exhibiting the characteristics of pulsars. More recently, findings from 2020 using the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope have suggested that magnetars may be the source of fast radio bursts (FRBs).
Like all neutron stars, magnetars are formed from the collapse of a star with a mass 10 to 25 times that of the Sun, resulting in an object about 1.4 solar masses compressed into a diameter of around 20 kilometers. Their interior density is so extreme that a tablespoon of their material would weigh over 100 million tons. Magnetars are distinguished from typical neutron stars not only by their far stronger magnetic fields but also by their slower rotation; most observed magnetars complete one rotation every two to ten seconds, whereas typical radio pulsars rotate one to ten times per second. Their magnetic fields give rise to powerful, characteristic bursts of X-rays and gamma rays. The active life of a magnetar is relatively short—their strong fields decay after about 10,000 years, after which their high-energy emissions cease. Given the number currently observable, it is estimated that the Milky Way may contain 30 million or more inactive magnetars. Starquakes on a magnetar’s surface can disturb its encompassing magnetic field, triggering extremely powerful gamma-ray flares; such events were recorded on Earth in 1979, 1998, and 2004.
- type
- Neutron star
- magnetic_field_strength
- ~10⁹ to 10¹¹ T
- typical_diameter
- ~20 km
- typical_mass
- ~1.4 solar masses
- known_for
- Extremely powerful magnetic fields; sources of soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs); possible source of fast radio bursts (FRBs)
Lore & Background
Magnetars are a rare type of neutron star, with only 31 confirmed examples among roughly 3,000 known neutron stars, five of which also exhibit pulsar qualities. Like all neutron stars, they are about 20 kilometers in diameter and possess a mass around 1.4 times that of the Sun, formed from the collapse of a star 10 to 25 times the Sun’s mass. Their defining trait is an extraordinarily powerful magnetic field, ranging from about 10⁹ to 10¹¹ teslas—a hundred million times stronger than any man-made magnet and roughly a trillion times more powerful than Earth’s geomagnetic field. This field has an energy density so immense that its mass equivalent exceeds that of lead by over 10,000 times. The magnetic field’s decay powers the emission of high-energy X-rays and gamma rays. Magnetars rotate slowly, typically once every two to ten seconds, compared to the one to ten rotations per second of typical radio pulsars. Their active life is brief, as the magnetic field decays after about 10,000 years, after which strong X-ray emission ceases; estimates suggest the Milky Way may contain 30 million or more inactive magnetars. Starquakes on their surface can trigger extremely powerful gamma-ray flares, recorded on Earth in 1979, 1998, and 2004. Their magnetic fields are so intense that at a distance of 1,000 kilometers they would distort atomic electron clouds, making life impossible, and from halfway to the Moon could erase credit card magnetic stripes. In such fields, X-ray photons split or merge, vacuum becomes birefringent, and atoms deform into long, thin cylinders. The dominant formation model involves a magnetohydrodynamic dynamo in the turbulent, dense fluid of the collapsing star, though an alternative is collapse from a star with an already unusually strong magnetic field.
Reader's Guide
Magnetars are a type of neutron star possessing magnetic fields roughly a hundred million times stronger than any man-made magnet and about a trillion times more powerful than Earth's field. Proposed in 1992 to explain certain gamma-ray sources, the magnetar hypothesis was later extended to account for anomalous X-ray pulsars. Of the approximately 3,000 known neutron stars, 31 are confirmed magnetars, with five also classified as pulsars. They form from the collapse of stars with 10–25 solar masses, resulting in a diameter of around 20 kilometers and a mass of about 1.4 Suns. Their magnetic fields, which decay after roughly 10,000 years, power high-energy X-ray and gamma-ray emissions. Starquakes on the surface can trigger extremely powerful gamma-ray flares, such as those recorded in 1979, 1998, and 2004. The magnetic field is so intense that at 1,000 kilometers it would distort atomic electron clouds, making life impossible; halfway to the Moon, it could erase credit card data. Inside a magnetar, X-ray photons can split or merge, the vacuum becomes birefringent, and atoms deform into narrow cylinders. The dominant model for these fields involves a dynamo process in the turbulent, dense fluid before the neutron star stabilizes. It is estimated that about one in ten supernovae produces a magnetar rather than a standard neutron star or pulsar.
Did You Know?
- A magnetar's magnetic field is about a trillion times more powerful than Earth's geomagnetic field.
- In a magnetic field of 10¹⁰ tesla, a hydrogen atom becomes 200 times as narrow as its normal diameter.
- It is estimated that about one in ten supernova explosions results in a magnetar rather than a standard neutron star or pulsar.
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