Observations from ESO’s La Silla and Paranal Observatories in Chile have for the first time demonstrated a link between a very long-lasting burst of gamma rays and an unusually bright supernova explosion. The results show that the supernova was not driven by radioactive decay, as expected, but was instead powered by the decaying super-strong magnetic fields around an exotic object called a magnetar. Pulsars make up the most common class of observable neutron stars, but magnetars are thought to develop magnetic field strengths that are 100 to 1000 times greater than those seen in pulsars.
Magnetars are characterized by their extremely powerful magnetic fields of 10^8 to 10^11 tesla. These magnetic fields are hundreds of millions of times stronger than any man-made magnet, and quadrillions of times more powerful than the field surrounding Earth. Earth has a geomagnetic field of 30–60 microteslas, and a neodymium-based, rare-earth magnet has a field of about 1.25 tesla, with a magnetic energy density of 4.0×10^5 J/m3. A magnetar’s 10^10 tesla field, by contrast, has an energy density of 4.0×10^25 J/m3, with an E/c2 mass density over 10000 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. At a distance halfway to the moon, a magnetar could strip information from the magnetic stripes of all credit cards on Earth. As of 2010, they are the most magnetic objects ever detected in the universe.
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. 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, 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. Starquakes triggered on the surface of the magnetar disturb the magnetic field which encompasses it, often leading to extremely powerful gamma ray flare emission
Gamma-ray bursts (GRBs) are one of the outcomes associated with the biggest explosions to have taken place since the Big Bang. They are detected by orbiting telescopes that are sensitive to this type of high-energy radiation, which cannot penetrate the Earth’s atmosphere, and then observed at longer wavelengths by other telescopes both in space and on the ground.
GRBs usually only last a few seconds, but in very rare cases the gamma rays continue for hours. One such ultra-long duration GRB was picked up by the Swift satellite on 9 December 2011 and named GRB 111209A. It was both one of the longest and brightest GRBs ever observed.
Long-duration gamma-ray bursts are produced only once every 10 000 to 100 000 supernovae, the star that exploded must be somehow special. Astronomers had assumed that these GRBs came from very massive stars — about 50 times the mass of the Sun — and that they signalled the formation of a black hole. But now our new observations of the supernova SN 2011kl, found after the GRB 111209A, are changing this paradigm for ultra-long duration GRB.
The only explanation that fitted the observations of the supernova following GRB 111209A was that it was being powered by a magnetar — a tiny neutron star spinning hundreds of times per second and possessing a magnetic field much stronger than normal neutron stars, which are also known as radio pulsars. Magnetars are thought to be the most strongly magnetised objects in the known Universe. This is the first time that such an unambiguous connection between a supernova and a magnetar has been possible.
Normal long-duration GRBs last between 2 and 2000 seconds. There are now four GRBs known with durations between 10 000–25 000 seconds — these are called ultra-long GRBs. There is also a distinct class of shorter-duration GRBs that are believed to be created by a different mechanism.
A new class of ultra-long-duration (more than 10,000 seconds) γ-ray bursts has recently been suggested. They may originate in the explosion of stars with much larger radii than those producing normal long-duration γ-ray bursts or in the tidal disruption of a star. No clear supernova has yet been associated with an ultra-long-duration γ-ray burst. Here we report that a supernova (SN 2011kl) was associated with the ultra-long-duration γ-ray burst GRB 111209A, at a redshift z of 0.677. This supernova is more than three times more luminous than type Ic supernovae associated with long-duration γ-ray bursts and its spectrum is distinctly different. The slope of the continuum resembles those of super-luminous supernovae but extends further down into the rest-frame ultraviolet implying a low metal content. The light curve evolves much more rapidly than those of super-luminous supernovae. This combination of high luminosity and low metal-line opacity cannot be reconciled with typical type Ic supernovae, but can be reproduced by a model where extra energy is injected by a strongly magnetized neutron star (a magnetar), which has also been proposed as the explanation for super-luminous supernovae
SOURCES – European Space Organization, Wikipedia, Nature
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