Researchers find highest energy gamma rays from dead star, pulsar

Researchers find highest energy gamma rays from dead star, pulsar
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Namibia, South Africa: Scientists have discovered the highest intensity gamma rays ever from a dead star known as a pulsar using the H.E.S.S. observatory in Namibia.

These gamma rays had an energy of 20 tera-electronvolts, which is roughly ten trillion times that of visible light.

As the team notes in the journal Nature Astronomy, it is difficult to square this discovery with the idea of the generation of such pulsed gamma rays.

The remains of stars that spectacularly erupted in a supernova are known as pulsars. The remnants of the explosions are a dead, small star with a diameter of only 20 km that rotates extremely quickly and has a powerful magnetic field.

"These dead stars are almost entirely made up of neutrons and are incredibly dense: a teaspoon of their material has a mass of more than five billion tonnes, or about 900 times the mass of the Great Pyramid of Giza," explained H.E.S.S. scientist Emma de Ona Wilhelmi, a co-author of the publication.

Pulsars emit rotating beams of electromagnetic radiation, somewhat like cosmic lighthouses. If their beam sweeps across our solar system, we see flashes of radiation at regular time intervals.

These flashes, also called pulses of radiation, can be searched for in different energy bands of the electromagnetic spectrum. Scientists think that the source of this radiation is fast electrons produced and accelerated in the pulsar's magnetosphere while travelling towards its periphery. The magnetosphere is made up of plasma and electromagnetic fields that surround and co-rotate with the star.

"On their outward journey, the electrons acquire energy and release it in the form of the observed radiation beams," says Bronek Rudak from the Nicolaus Copernicus Astronomical Center (CAMK PAN) in Poland, also a co-author.

The Vela pulsar, located in the Southern sky in the constellation Vela (sail of the ship), is the brightest pulsar in the radio band of the electromagnetic spectrum and the brightest persistent source of cosmic gamma rays in the giga-electronvolts (GeV) range.

It rotates about eleven times per second. However, above a few GeV, its radiation ends abruptly, presumably because the electrons reach the end of the pulsar's magnetosphere and escape from it.

But this is not the end of the story: using deep observations with H.E.S.S., a new radiation component at even higher energies has now been discovered, with energies of up to tens of tera-electronvolts (TeV). "That is about 200 times more energetic than all radiation ever detected before from this object," said co-author Christo Venter from the North-West University in South Africa.

This very high-energy component appears at the same phase intervals as the one observed in the GeV range. However, to attain these energies, the electrons might have to travel even farther than the magnetosphere, yet the rotational emission pattern needs to remain intact.

"This result challenges our previous knowledge of pulsars and requires a rethinking of how these natural accelerators work," said Arache Djannati-Atai from the Astroparticle & Cosmology (APC) laboratory in France, who led the research.

"The traditional scheme according to which particles are accelerated along magnetic field lines within or slightly outside the magnetosphere cannot sufficiently explain our observations. Perhaps we are witnessing the acceleration of particles through the so-called magnetic reconnection process beyond the light cylinder, which still somehow preserves the rotational pattern. But even this scenario faces difficulties in explaining how such extreme radiation is produced."

Whatever the explanation, next to its other superlatives, the Vela pulsar now officially holds the record as the pulsar with the highest-energy gamma rays discovered to date.

"This discovery opens a new observation window for detection of other pulsars in the tens of teraelectronvolt range with current and upcoming more sensitive gamma-ray telescopes, hence paving the way for a better understanding of the extreme acceleration processes in highly magnetised astrophysical objects," said Djannati-Atai.