Astrophysics: A pulsar in a class of its own observed
The Vela pulsar emits extremely high-energy radiation. This extraordinary intensity calls into question common models for these rapidly rotating neutron stars.
Just because a star is "dead" does not mean that it can no longer radiate. The Vela pulsar proves this perfectly: according to new measurements, the energy of its gamma rays is 20 teraelectronvolts (TeV), i.e. around ten trillion times the energy of visible light. This observation is difficult to reconcile with previous theories on the generation of such pulsed gamma rays, write Arache Djannati-Ataï from the Université Paris Cité and his team in "Nature Astronomy".
Pulsars are the remains of stars that exploded in a supernova. They are extremely compact, rotate at breakneck speed and have a gigantic magnetic field. "These dead stars consist almost entirely of neutrons and are incredibly dense: a teaspoon of their material has a mass of more than five billion tonnes, which is about 900 times the mass of the Great Pyramid of Giza," explains co-author Emma de Oña Wilhelmi from DESY. Thousands of pulsars are known to date, but only four emit such powerful gamma-ray pulses that they can be detected by earthbound telescopes. And only one of them emits gamma rays consisting of photons with energies of more than one teraelectronvolt: the Vela pulsar, which rotates around its axis eleven times per second.
The working group has now observed that this pulsar radiates even more strongly than previously known. At 20 TeV, it releases 20 times more energy than any other pulsar measured to date. This does not fit with the two current models. Both are based on high-energy electrons colliding with lower-energy photons, which make up gamma rays. The models differ in how these electrons are accelerated in the first place. In one case, they are hurled away by interactions with the pulsar's magnetic field. Or they are driven to high speeds by the pulsar's rotation. However, both are difficult to reconcile with the newly discovered gamma rays, for which there is no obvious upper energy limit.
"How and where the electrons are accelerated - that is the question," says Djannati-Ataï. "Until we find out, we cannot fully understand pulsars, how they influence their environment or why the Vela pulsar produces such unusually intense radiation. Perhaps we are witnessing the acceleration of particles through so-called magnetic reconnection beyond the light cylinder, which somehow still preserves the rotation pattern? But even this scenario runs into difficulties if we want to explain how such extreme radiation is produced." In magnetic reconnection, the structure of a magnetic field changes abruptly, releasing large amounts of energy.
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Cover image: Science Communication Lab for DESY
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