A mysterious signal from the center of the galaxy
In 2009, the Fermi Gamma-ray space telescope placed in orbit in June 2008 by NASA recorded powerful gamma radiation from the center of the galaxy. During these observations, astronomers could not determine with certainty the source of this radiation, but put forward the hypothesis of dark matter concentrated in the center of the Milky Way.
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In the Milky Way, the galaxy that houses our solar system, the vast majority of gamma radiation is emitted after collisions of cosmic rays with the gas clouds in the interstellar medium.
In some galaxies, such as the Andromeda Galaxy, astronomers aided by the Fermi Large Area Telescope (Fermi LAT) have shown that the gamma rays produced do not propagate throughout the galaxy, but remain confined to the center. The observed gamma signal for Andromeda is similar to that discovered in 2009 in the center of the Milky Way.
At that time, astronomers assume dark matter to explain this amazing source of gamma radiation in the center of the Milky Way. Dark matter is a question that is still mysterious and largely unknown to scientists. It would make up more than 80% of the total mass of the universe.
As for the galaxy, the researchers believe that dark matter would tend to accumulate in the center and that the powerful gamma radiation detected could have been produced by the dissolution of dark matter particles.
Today, astronomers at the Australian National University presented another hypothesis to explain this gamma radiation: neutron stars called millisecond pulsars.
Millisecond pulsars, neutron stars that rotate very fast
A millisecond pulsar, sometimes called a recycle pulsar, is none other than a pulsar with a very high rotational speed, that is, with a rotation period of between 1 and 10 milliseconds. One millisecond pulsar is “observable” in the microwave spectrum.
Astronomers do not know exactly the origin of millisecond pulsars, but they believe that it is initially a “classical” pulsar, that is, a neutron star that rotates rapidly on itself and produces powerful electromagnetic radiation perpendicular to its growth disk and spreading phenomenal distances across. the disk in the interstellar space.
It is precisely the phenomenon of growth that would be the cause of the progressive acceleration of the pulsars and their transformation into millisecond pulsars. Among the pulsars that emit X-rays, some would be in the process of growth and acceleration. It is very often a pair of stars where the neutron star is accelerated by transmitting the angular momentum of its companion star.
Currently, researchers have discovered about 150 millisecond pulsars. For example, the pulsar named PSR J1748-2446ad in the globular cluster Terzan 5, located 18,000 light-years from Earth, rotates about itself at a speed of 716 revolutions per second, the highest rotational speed known to date.
The majority of millisecond pulsars have been detected in globular clusters because the very high star density allows for interactions and material transfers.
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Radiation from the center of the galaxy could be a millisecond pulsar
For Professor Roland Cocker, one of the researchers behind this study, the gamma signal known as Galactic Center Excess could come from neutron stars in rapid rotation. These rapidly rotating stars can be remnants of stars that are much more massive than our sun.
The astronomers leading this discovery do not seek to cast doubt on the dark matter hypothesis to explain this gamma signal, but rather suggest a very valid one based on the existence of spinning neutron stars. very fast on itself
This is not the first time that astronomers have detected gamma emissions coming from millisecond pulsars. Astronomers’ current model of the Milky Way assumes the presence of 100,000 stars to explain the emission of this radiation from the center of the galaxy.
This discovery not only provides an opportunity to provide a different hypothesis about the presence of this powerful gamma radiation coming from the center of our galaxy, but also forces astrophysicists to reconsider their ideas about the source of gamma radiation in other galaxies.
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Source: Gautam, A., Crocker, RM, Ferrario, L. et al., “Millisecond pulsars from accretion-induced collapse as the origin of the Galactic Center’s gamma ray transmit signal.” Nature astronomy (2022), https://doi.org/10.1038/s41550-022-01658-3