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  02-05-2009: A PULSAR IN THE BACKYARD OR A SIGN OF DARK MATTER? 
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Lat detector on Fermi satellite
Lat detector on Fermi satellite

© Copyright Nasa


The Fermi mission has detected an excess of high energy electrons in cosmic rays.

An excess of high energy electrons (compared to what researchers expected) has been detected in cosmic rays by NASA’s satellite mission Fermi, in which Italy participates through the collaboration of the National Institute of Nuclear Physics (INFN, with its sections Trieste/Udine, Padova, Pisa, Perugia, Roma2 and Bari), the National Institute of Astrophysics (INAF), and the Italian Space Agency (ASI). These electrons could derive from a particularly powerful astrophysical source of particles (such as a pulsar) which is close to us but still unknown. However, it could also be another sign of dark matter, following the apparent observation made by the Earth-orbiting experiment Pamela (headed by an Italian-based collaboration) and recently published in Nature.

The research project
The Fermi mission’s discovery of the excess electrons will be published on May 4th in the prestigious American scientific journal Physical Review Letters and will be presented to the scientific community during two international conferences: one to be held today in Denver, Colorado (USA) and the other on May 4th in Paris. The result was obtained thanks to the fundamental contribution of the Italian research group of Fermi. Our researchers, through the innovative use of the detector LAT (Large Area Telescope) which is on board the satellite, have measured with extreme precision the distribution of the flow of electrons that reach us by measuring their energy, that is, their spectrum. Differently from cosmic rays, electrons lose much of their energy while traveling through the galaxy, interacting with magnetic fields and interstellar radiation. Since we are not aware of any sources of energy particles that are very close to us, the flow was expected to have been particularly rich in low energy electrons. Yet this is not the case: the spectrum of electrons observed by the researchers of Fermi has an abundant component of very high energy (reaching up to 1 TeV, that is, one trillion electron Volts).
This research, in addition to addressing traditional issues in Astrophysics (such as the origin, propagation, and interaction of cosmic rays with interstellar material, and the magnetic and radiation fields that permeate our galaxy) is above all destined to open a new frontier in terms of our knowledge and understanding of the Universe. “Measuring the spectrum of electrons” - explains Ronaldo Bellazzini, the INFN researcher who coordinates the Italian group – “has surprising and unexpected characteristics and requires modifying the conventional model of the origin and propagation of the electrons of cosmic rays". "One possible interpretation of the data recorded by Fermi” – continues Bellazzini – “is that the excess in high energy electrons is due to their copious production during the process of the annihilation of dark matter, which permeates our galaxy. However, it must be stressed that, though all of this is plausible and exciting, it is still very hypothetical. In fact, we will be able to consider this interpretation more seriously only when all of the other explanations, based on the Universe as we have understood it until now, should fail: in other words, if we don’t find a pulsar in the back yard, that is, very close to Earth, we will start to think about dark matter".
"Actually, at least a couple of new pulsars, never before detected in the radio frequencies and moreover rather close, have already been discovered”, says Patrizia Caraveo, Head INAF Representative for Fermi. "Thus we cannot exclude that they are responsible for the surplus of electrons, which is in any case an interesting result which must be looked at carefully".


The instrument
The discovery was made possible by measuring the spectrum of electrons, carried out with unprecedented precision by LAT. Designed and realised with an essential contribution provided by the Italian collaboration, LAT is a gamma ray detector, that is, a detector of high energy photons. To avoid that other more numerous particles of gamma photons contaminate the signal, LAT is equipped with a screen that is capable of rejecting particles that are not photons and that have an energy lower than 20 GeV (thus the screen does not block particles with energy that exceeds this threshold because it is believed that their study could be of interest). This measure of the spectrum of electrons was realised by using the data recorded by the instrument in an innovative manner: instead of studying gamma photons, the researchers carefully analysed the signals of all of the particles that were not blocked by the screen because of their high energy.

Dark matter
Physicists have hypothesized the existence of dark matter based on its gravitational effects on light and ordinary matter, that is, that which makes up everything we know in the Universe, such as the stars, the planets and interstellar gasses. Although studies suggest that dark matter is 5 times more abundant than ordinary matter (in fact, it is believed to constitute approximately one fourth of our Universe, whereas ordinary matter makes up only 5%), no one has ever directly observed dark matter or defined its characteristics: it is, in fact, "invisible" to our eyes and instruments.


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