An extremely bright gamma pulsar detected in the Large Magellanic Cloud

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Fermi discovered the brightest gamma pulsar … in another galaxy! The Large Area Telescope, the principal instrument of the Fermi space mission launched in 2008, has detected a pulsed gamma emission from a pulsar located in the Large Magellanic Cloud. This about 1000 years old young pulsar was already listed as one of the most powerful known pulsars. He becomes the brightest gamma pulsar ever recorded, exceeding by a factor of 20 the previous record holder, the Crab pulsar, and is also the first extragalactic gamma pulsar. This exceptional object should help us to better understand the origin of the emission of pulsars.

artist’s impression of the pulsar located in the Large Magellanic Cloud. Credit : Astronomy: Roen Kelly

An exceptionally bright gamma ray pulsar has been detected in a galaxy neighbouring ours, the Large Magellanic Cloud. This study was conducted by two researchers from IRAP, in Toulouse (CNRS/Université Paul Sabatier), and LPC2E, in Orléans (CNRS/Université d’Orléans), in an international scientific collaboration involving IN2P3, INSU, and CEA. Long observations by the Large Area Telescope on board NASA’s Fermi satellite have revealed a periodic gamma-ray emission at the characteristic frequency of a young pulsar known for its mechanical power. This discovery confirms the uniqueness of this object and should make it possible to better understand the mechanisms of particle acceleration in pulsar magnetospheres.

Pulsars are rapidly rotating neutron stars formed during supernovae and giving rise to a periodic emission at the star’s rotational frequency. This characteristic signal has been observed from more than 2500 celestial objects, mainly from ground-based radio telescopes. Pulsars are also detected as gamma rays, the most energetic form of light. Observations from the Large Area Telescope on NASA’s Fermi satellite launched in 2008 have shown that pulsars are the largest family of gamma-ray sources in our galaxy, with more than 160 pulsars detected. However, the origin of pulsar radiation remains poorly understood: the intense magnetic field is the key, but the processes at work in the pulsar magnetosphere are complex.

Until recently, all known gamma-ray emitting pulsars were located in our galaxy or in nearby globular clusters. New observations with the Large Area Telescope have revealed, for the first time, gamma-ray signals from pulsars belonging to a galaxy other than our own: the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. A characteristic gamma emission at the 50 ms period was detected from the PSR pulsar J0540-6919, demonstrating beyond doubt that this object is the source of the signal. PSR J0540-6919 thus became the first known extragalactic gamma pulsar. The detection of an object located at such a distance, 50 kpc, is made possible by its extraordinary brightness: PSR J0540-6919 is the brightest gamma pulsar known to date. Its neighbor in the Large Magellanic Cloud, PSR J0537-6910, has a constant gamma emission, with no obvious pulses at the period of rotation of the pulsar. The origin of this gamma signal remains enigmatic and suggests that PSR J0537-6910 has a very weak pulsed or even non-pulsed emission. The two pulsars, PSR J0537-6910 and PSR J0540-6919, have the particularity of being among the three most energetic pulsars known, along with the Crab pulsar from a galactic supernova observed in 1054. Their rapid rotation releases a power greater than 10^{31} Watt, almost 100,000 times the luminosity of the Sun. Despite a comparable mechanical power, the three pulsars manifest themselves in very different ways in the gamma domain: PSR J0537-6910 is the most powerful of the trio and has no detectable gamma-ray pulses, while PSR J0540-6919 is the least powerful but has a gamma-ray luminosity 20 times higher than that of the Crab pulsar. The new results obtained with Fermi should therefore allow us to learn more about the mechanisms at play in the magnetosphere of young and powerful pulsars. In the meantime, these observations have shed more light on the gamma emission of the Large Magellanic Cloud: the identification of pulsars makes it possible to study other sources of energetic radiation more precisely, particularly cosmic radiation.

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