Magnetar

A magnetar is a type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.^[1] The theory regarding these objects was proposed by Robert Duncan and Christopher Thompson in 1992, but the first recorded burst of gamma rays thought to have been from a magnetar had been detected on March 5, 1979.^[2] During the following decade, the magnetar hypothesis has become widely accepted as a likely explanation for soft gamma repeaters (SGRs) and anomalous X-ray pulsars(AXPs).

Like other neutron stars, magnetars are around 20 kilometres (12 mi) in diameter and have a greater mass than the Sun. The density of the interior of a magnetar is such that a thimble full of its substance would have a mass of over 100 million tons.^[1] Magnetars are differentiated from other neutron stars by having even stronger magnetic fields, and rotating comparatively slowly, with most magnetars completing a rotation once every one to ten seconds,^[3] compared to less than one second for a typical neutron star. This magnetic field gives rise to very strong and characteristic bursts of X-rays and gamma rays. The active life of a magnetar is short. Their strong magnetic fields decay after about 10,000 years, after which activity and strong X-ray emission cease. Given the number of magnetars observable today, one estimate puts the number of inactive magnetars in the Milky Way at 30 million or more.^[3]

Starquakes triggered on the surface of the magnetar disturb the magnetic field which encompasses it, often leading to extremely powerful gamma ray flare emissions which have been recorded on Earth in 1979, 1998, and 2004.^[4]

Magnetic field[edit source | editbeta]

Magnetars are characterized by their extremely powerful magnetic fields of 10⁸-10¹¹ tesla.^[5] These magnetic fields are hundreds of millions of times stronger than any man-made magnet,^[6] and quadrillions of times more powerful than the field surrounding Earth.^[7] Earth has a geomagnetic field of 30–60 microteslas, and a neodymium-based, rare-earth magnet has a field of about 1 tesla, with a magnetic energy density of 4.0×10⁵ J/m³. A magnetar's 10¹⁰ tesla field, by contrast, has an energy density of 4.0×10²⁵ J/m³, with an E/c² mass density >10⁴ times that of lead. The magnetic field of a magnetar would be lethal even at a distance of 1000 km due to the strong magnetic field distorting the electron clouds of the subject's constituent atoms, rendering the chemistry of life impossible.^[8] At a distance halfway to the moon, a magnetar could strip information from the magnetic stripes of all credit cards on Earth.^[9] As of 2010, they are the most magnetic objects ever detected in the universe.^[4][10]

As described in the February 2003 Scientific American cover story, remarkable things happen within a magnetic field of magnetar strength. "X-ray photons readily split in two or merge together. The vacuum itself is polarized, becoming strongly birefringent, like a calcite crystal. Atoms are deformed into long cylinders thinner than the quantum-relativistic de Broglie wavelength of an electron."^[2] In a field of about 10⁵ teslas atomic orbitals deform into rod shapes. At 10¹⁰ teslas, a hydrogen atom becomes a spindle 200 times narrower than its normal diameter.^[2]

Origins of magnetic fields[edit source | editbeta]

The strong fields of magnetars are understood as resulting from a magnetohydrodynamic dynamo process in the turbulent, extremely dense conducting fluid that exists before the neutron star settles into its equilibrium configuration. These fields then persist due to persistent currents in a proton-superconductor phase of matter that exists at an intermediate depth within the neutron star (where neutrons predominate by mass). A similar magnetohydrodynamic dynamo process produces even more intense transient fields during coalescence of pairs of neutron stars.^[11]

Formation[edit source | editbeta]

Magnetar SGR 1900+14 is in the exact center of the image, which shows a surrounding ring of gas seven light-years across in infrared light, as seen by theSpitzer Space Telescope. The magnetar itself is not visible at this wavelength, but it has been seen in X-ray light.

When, in a supernova, a star collapses to a neutron star, its magnetic field increases dramatically in strength. Halving a linear dimension increases the magnetic field fourfold. Duncan and Thompson calculated that, when the spin, temperature and magnetic field of a newly formed neutron star falls into the right ranges, a dynamo mechanism could act, converting heat and rotational energy into magnetic energy, and increasing the magnetic field, normally an already enormous 10⁸ teslas to more than 10¹¹ teslas (or 10¹⁵ gauss). The result is a magnetar.^[12] It is estimated that about one in ten supernova explosions results in a magnetar rather than a more standard neutron star or pulsar.^[13]

1979 discovery[edit source | editbeta]

On March 5, 1979, a few months after the successful dropping of satellites into the atmosphere of Venus, the two Soviet spacecraft that were then drifting through the Solar System were hit by a blast of gamma radiation at approximately 10:51 EST. This contact raised the radiation readings on both the probes from a normal 100 counts per second to over 200,000 counts a second, in only a fraction of a millisecond.^[2]

This burst of gamma rays quickly continued to spread. Eleven seconds later, Helios 2, a NASA probe, which was in orbit around the Sun, was saturated by the blast of radiation. It soon hit Venus, and the Pioneer Venus Orbiter's detectors were overcome by the wave. Seconds later, Earth received the wave of radiation, where the powerful output of gamma rays inundated the detectors of three U.S. Department of Defense Vela satellites, the Soviet Prognoz 7 satellite, and the Einstein Observatory. Just before the wave exited the Solar System, the blast also hit the International Sun-Earth Explorer. This extremely powerful blast of gamma radiation constituted the strongest wave of extra-solar gamma rays ever detected; it was over 100 times more intense than any known previous extra-solar burst. Because gamma rays travel at the speed of light and the time of the pulse was recorded by several distant spacecraft as well as on Earth, the source of the gamma radiation could be calculated to an accuracy of about 2 arcseconds.^[14] The direction of the source corresponded with the remnants of a star that had gone supernova around 3000 B.C.E.^[4]

Recent discoveries[edit source | editbeta]

On February 21, 2008 it was announced that NASA and researchers at McGill University had discovered a neutron star with the properties of a radio pulsar which emitted some magnetically powered bursts, like a magnetar. This suggests that magnetars are not merely a rare type of pulsar but may be a (possibly reversible) phase in the lives of some pulsars.^[15] On September 24, 2008, ESO announced what it ascertained was the first optically active magnetar-candidate yet discovered, using ESO's Very Large Telescope. The newly discovered object was designated SWIFT J195509+261406.^[16]

The anti-glitch issue[edit source | editbeta]

Often magnetars speed up (and more rarely slow down) and a lot of the reasons for this behaviour has not been fully explained by astrophysics.^[17]

Astronomers have theorized that glitches occur when fluid inside the star rotates faster than the crust and suddenly transfers some extra momentum during a disturbance. They think the spectacular outbursts of x-rays occur in the 20 to 30 per cent of glitches where the disturbance is violent enough to crack the crust, Kaspi said.

Because the strange 2012 outburst was accompanied by a slowdown, Kaspi is calling it an anti-glitch.^[18]

That kind of behaviour is "not at all what you expect if that whole picture of glitches in these stars is correct. It shouldn't happen that way," Kaspi added. "It's hard to imagine how fluid interior is slower than crust."

Kaspi has since contacted theorists who have helped her come up with a possible explanation — that pockets of fluid rotating slower than the crust could be responsible.^[19]

Known magnetars[edit source | editbeta]

On 27 December 2004, a burst of gamma rays from SGR 1806-20 passed through the Solar System (artist's conception shown). The burst was so powerful that it had effects on Earth's atmosphere, at a range of about 50,000 light years.

As of October 2012, 21 magnetars are known, with three more candidates awaiting confirmation.^[5] A full listing is given in the McGill SGR/AXP Online Catalog.^[5] Examples of known magnetars include:

• SGR 1806-20, located 50,000 light-years from Earth on the far side of our Milky Way galaxy in the constellation of Sagittarius.
• SGR 1900+14, located 20,000 light-years away in the constellation Aquila. After a long period of low emissions (significant bursts only in 1979 and 1993) it became active in May–August 1998, and a burst detected on August 27, 1998 was of sufficient power to force NEAR Shoemaker to shut down to prevent damage and to saturate instruments on BeppoSAX, WIND and RXTE. On May 29, 2008, NASA's Spitzer telescope discovered a ring of matter around this magnetar. It is thought that this ring formed in the 1998 burst.^[20]
• SGR 0501+4516 was discovered on 22 August 2008.^[21]
• 1E 1048.1−5937, located 9,000 light-years away in the constellation Carina. The original star, from which the magnetar formed, had a mass 30 to 40 times that of the Sun.
• As of September 2008, ESO reports identification of an object which it has initially identified as a magnetar, SWIFT J195509+261406, originally identified by a gamma-ray burst (GRB 070610).^[16]
• CXO J164710.2-455216, located in the massive galactic cluster Westerlund 1, which formed from a star with a mass in excess of 40 solar masses.^[22]
• SWIFT J1822.3 Star-1606 discovered on 14 July 2011 by Italian and Spanish researchers of CSIC and Catalogna's space studies institute. This magnetar contrary to previsions has a low external magnetic field.