Scientists using the H.E.S.S. Observatory in Namibia have detected the highest-energy gamma rays from a star called the Vela pulsar. The energy of these gamma rays is 20 teraelectronvolts, about 10 trillion times the energy of visible light. The observation is difficult to reconcile with the theory that produces such pulsed gamma rays, the international team of researchers reports in the journal Nature Astronomy.
Pulsars are the corpses left behind by stars that spectacularly exploded in supernova explosions. The explosion will leave behind a tiny dead star with a diameter of only about 20 kilometers, which rotates extremely fast and has a huge magnetic field.
Emmade Oña Wilhelmi, a scientist at H.E.S.S., explained: "These death stars are composed almost entirely of neutrons and are incredibly dense: a teaspoon of the material would have a mass of more than 5 billion tons, which is about 900 times the mass of the Great Pyramid of Giza."
Pulsars emit rotating beams of electromagnetic radiation, somewhat like cosmic beacons. If their beams swept across our solar system, we would see flashes of radiation that would appear at regular intervals. These flashes of light, also known as pulses of radiation, can be found in different energy bands of the electromagnetic spectrum.
Scientists believe the source of this radiation is fast electrons generated and accelerated in the pulsar's magnetosphere, which are moving toward the pulsar's outer reaches. The magnetosphere is composed of plasma and electromagnetic fields that surrounds and rotates with the star.
"The electrons gain energy as they move outward and release it in the form of the observed radiation beam," said Bronek Rudak of the Nicolaus Copernicus Astronomy Center (CAMKPAN) in Poland.
Located in the southern constellation Vela, the Vera pulsar is the brightest pulsar in the radio band of the electromagnetic spectrum and the brightest sustained source of cosmic gamma rays in the gigaelectronvolt (GeV) range. It spins approximately 11 times per second. Above a few GeV, however, its radiation comes to an abrupt end, probably because electrons reach the end of the pulsar's magnetosphere and escape from the magnetosphere.
But that's not the end of the story: Through in-depth observations using H.E.S.S., a new, more energetic component of radiation has now been discovered, with energies as high as tens of teraelectronvolts (TeV).
Co-author Christo Venter from North-West University in South Africa said: "This is about 200 times more energetic than all radiation previously detected from this object. This ultra-high-energy component occurs at the same phase intervals as those observed in the GeV range. However, to reach these energies, the electrons may travel further than the magnetosphere, but the rotational emission pattern needs to remain unchanged."
"This result challenges our previous understanding of pulsars and requires a rethinking of how these natural accelerators work," said Arache Djannati-Atai of France's Astroparticle and Cosmology Laboratory (APC), who led the study.
"The traditional scheme of particles accelerating along magnetic field lines inside or outside the magnetosphere cannot adequately explain our observations. Perhaps we are seeing particles accelerating outside the light column through a process called magnetic reconnection, which somehow still preserves the rotational pattern? But even if this is the case, it is difficult to explain how such extreme radiation can be produced."
Regardless of the explanation, the Vela pulsar now officially holds the record for the highest gamma-ray energy pulsar discovered to date.
"This discovery opens a new observational window, allowing the detection of other pulsars in the tens of teraelectronvolts range using current and upcoming more sensitive gamma-ray telescopes, paving the way to a better understanding of extreme acceleration processes in highly magnetized objects," said Djannati-Atai.