Magnetar

A magnetar is a neutron star whose magnetic field reaches an extreme intensity of roughly 10^9 to 10^11 tesla (10^13 to 10^15 gauss). It belongs to a special breed of neutron star rather than an ordinary one. When NASA prepared its overview of the subject only about a dozen magnetars had been found.

The magnetic field of a magnetar reaches strengths of 10^10 tesla, or 10^15 gauss, a regime in which the field itself dominates the physics of the star. At this strength the field is about a hundred million times stronger than any man-made magnet and roughly a trillion times stronger than Earth's field. The energy stored in the field is correspondingly extreme: at 10^10 T the magnetic energy density reaches 4.0 x 10^25 joules per cubic metre.

A magnetar measures roughly 20 kilometres across, equivalent to about 12 mi. Its mass is about 1.4 solar masses. The interior is so dense that a tablespoon of its material would have a mass of over 100 million tons.

A magnetar begins as a neutron star born in a supernova, where the magnetic field is amplified by conservation of magnetic flux; halving a linear dimension increases field strength fourfold. Theorists proposed in 1992 that newborn neutron stars could hold fields greater than a quadrillion gauss, the maximum efficiency of a neutron star dynamo. Duncan and Thompson first proposed the magnetar idea in 1992 to explain soft gamma repeaters.

The strong field decays after about 10,000 years, after which activity and strong X-ray emission cease. Superstrong fields make a neutron star brake and cool, so magnetars are hard to observe in radio and X-rays and may litter the Galaxy as dark relics. Because the magnetar stage is a brief phase only about a dozen magnetars are known.

Most observed magnetars rotate once every 2 to 10 seconds, whereas typical radio pulsars rotate 1 to 10 times per second. NASA scientists estimate that magnetic fields act as brakes on a neutron star, so a higher field drives a faster spin-down rate. Kouveliotou's 1998 team measured the field using RXTE and ASCA data, finding SGR 1806-20 spinning once every 7.5 seconds.

The observational record of magnetars begins with a signal that arrived without warning. Nine satellites recorded a rapid gamma-ray spike from the direction of the N49 remnant with 8-second pulsations. The giant burst of March 1979 lasted only 0.2 seconds but carried as much energy as the Sun emits in 1000 years. Nothing in the standard repertoire of stellar phenomena accounted for an event of such brevity and such yield.

A second decisive observation came at the end of 2004. For a tenth of a second the December 2004 flare released more energy than the Sun emits in 150,000 years. The extremity of the outburst reinforced the case for an object whose energy reservoir could not be gravitational or thermal alone.

• 5 March 1979
  First detection
  GRB 790305b / SGR 0526-66.
• March 1979
  Giant burst
  Lasted 0.2 seconds; energy equal to the Sun's output over 1000 years.
• March 1979
  Satellite detection
  Nine satellites recorded a rapid gamma-ray spike from the direction of the N49 remnant with 8-second pulsations.
• 1992
  Theoretical proposal
  Fields greater than a quadrillion gauss proposed.
• 1992
  Magnetar idea
  Duncan and Thompson propose the magnetar concept.
• 1998
  Measurement
  Kouveliotou team measurement.
• 27 December 2004
  Giant flare
  Giant flare from SGR 1806-20.
• January 2021
  Catalogue count
  31 confirmed magnetars.

The giant flare of 27 December 2004 from SGR 1806-20 was brighter than any explosion ever detected from beyond the Solar System. For a tenth of a second it released more energy than the Sun emits in 150,000 years. Had the flare occurred within 10 light years it would have severely damaged Earth's atmosphere.

SGR 0526-66 lies in the LMC N49 remnant and was the first soft gamma repeater ever found; for the record survey of 2015 it remained the only one known outside our galaxy. After the 1979 giant event SGR 0526-66 produced 16 weaker outbursts and has not been detected since 1983.

The magnetar stage is a brief phase in the life of a neutron star, which is why only about a dozen magnetars are known today. Based on the number of magnetars seen today, one estimate puts the number of inactive magnetars in the Milky Way at 30 million or more. Magnetars carry a magnetic field estimated at one thousand trillion times the strength of Earth's magnetic field.

The figures most often attached to magnetars concern their magnetic fields. One estimate places a magnetar's field at one thousand trillion times the strength of Earth's magnetic field ^{3Strongest Stellar Magnetic Field yet Observed Confirms Existence of Magnetars · NASA/GSFC HEASARC ↗}. Measured against human technology and against the planet itself, the same field is described as about a hundred million times stronger than any man-made magnet and roughly a trillion times stronger than Earth's field ^{1Magnetar · Wikipedia ↗}.

Population estimates extend beyond the objects actually observed. Working from the number of magnetars seen today, one estimate puts the count of inactive magnetars in the Milky Way at 30 million or more ^{1Magnetar · Wikipedia ↗}.