They find a record of energy in the emission of a pulsar, challenging theories about stars
The astronomer Rafael Bachiller reveals to us in this series the most spectacular phenomena of the Cosmos. Topics of pulsating research, astronomical adventures and scientific news about the Universe analyzed in depth.
Gamma radiation around 20 teraelectronvolts has been detected in the Vela pulsar. This discovery challenges existing theories about neutron stars.
Star Corpses
When they run out of nuclear fuel, stars with masses greater than 10 times that of the Sun explode as supernovae.. The inner region, whose weight is no longer supported by the energy of nuclear fusion reactions, collapses to leave behind a hyperdense star (only about 20 kilometers in diameter) known as a neutron star.. A spoonful of this stellar material has as much mass as Mount Everest.
These stellar corpses rotate very quickly, generating periodic pulses of radiation, which is why they are also known as pulsars.
Pulsars are, therefore, rotating stars that behave like gigantic magnets. The electrons subjected to their very intense magnetic fields are trapped in the polar regions, forming, during this process, pulses: very narrow jets of radiation that behave like the beam of light from a lighthouse.. Every time this type of lighthouse directs its light beam towards our telescope we receive a short burst of radiation.
Artist's image of a pulsar Eleven spins per second
One of the best studied pulsars is the one located 936 light years away in the southern constellation of Vela.. This stellar corpse, which was formed in a supernova explosion that took place 11,400 years ago, rotates with devilish speed: it makes 11 revolutions every second, and is the object in the sky with the highest persistent emission of gamma rays.
An international team of researchers coordinated by Arache Djannati-Ataï (from the French CNRS) have now used the High Energy Strobe System (HESS) telescopes, installed in Namibia, to study the gamma-ray spectrum of the Vela pulsar. And what they have found is record-breaking radiation.
The HESS telescope array in Namibia
Indeed, scientists have observed emissions in the range of 20 teraelectronvolts (TeV). Let us remember that an electron volt (eV) is the energy acquired by an individual electron subjected to an electric potential difference of only one Volt.. Typically, visible light photons that reach us from the Sun are between two and three eV, approximately. Therefore, each of the photons now detected in the Vela pulsar is about 10 billion times more energetic than one of those visible light solar photons.
Furthermore, the gamma-ray spectrum of this pulsar shows that the photons do not form a decreasing continuum with energy, but rather a disruption is observed with a maximum in the TeV range.
Until now, it was known that pulsars emitted up to ranges of several gigaelectronvolts (GeV, one billion eV), and the well-known pulsar of the Crab Nebula (6000 light years away, in Taurus) was the only one that reached TeV, with a spectrum continually decreasing in energies. The behavior of the Vela pulsar is starkly different.
Extreme acceleration
There is broad consensus that gamma radiation from pulsars is created by the acceleration of electrons from the surface of the star to the edge of the magnetosphere.. From here the radiation beams that are emitted into space must arise.. But the very high energies observed in the Vela pulsar require a different acceleration mechanism that takes place beyond the magnetosphere itself.
In the recreation that heads this article, the Vela pulsar appears in the center, and the edge of its magnetosphere is marked by a bright circle. Blue lines pointing outward represent accelerated particle trajectories. These could produce gamma radiation along the arms of a rotating spiral by colliding with infrared photons emitted in the magnetosphere (in red).
Although it is only one observed case, these data offer a unique opportunity to study the extreme acceleration processes of charged particles in highly magnetized environments.. They are very peculiar conditions that cannot be studied in terrestrial laboratories and that, therefore, can reveal little-known behaviors of radiation and matter.
Of course we will have to look for more cases and we all wait impatiently for the construction of the CTA (Cherenkov Telescope Array) telescope sets in the Canary Islands and in Atacama (Chile) to be able to carry out observations of much greater sensitivity than that now provided by HESS.
The work of Arache Djannati-Ataï and collaborators has been published in a recent issue of the journal Nature Astronomy. The full article can be consulted for free at this link.
Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.