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The abundance of positrons: pulsars don't seem to be the origin

The HAWC Observatory in Mexico has detected very high energy gamma rays from two pulsars. The data collected confirms the interpretation of very high energy positrons seen by the AMS spectrometer and, at lower energy levels, by PAMELA and FERMI as a possible product of the annihilation of dark matter.

It is called the High-Altitude Water Cherenkov (HAWC) and it is an observatory with a high level of gamma rays located in Mexico. The international team that built it and runs it has managed to capture, for the first time, a broad picture of very high energy gamma emissions coming from the sky, and in particular identifying the contribution coming from two pulsars close to the Earth, which are two fast-spinning neutron stars, including Geminga. The data collected provide information which is crucial on showing light on an excess of antimatter of an elementary type, so very high energy positrons, observed close to Earth.

In 2008, a surplus of positrons starting from 10 GeV of energy was actually observed by Pamela in orbit a few hundred kilometres from the earth's atmosphere. Starting from 2012, the AMS spectrometer has showed how the extent of the energy surplus reaches to over 500 GeV and then begins to fall off, showing a spectral structure. The FERMI experiment, which was planned to observe high energy gamma rays, has also confirmed the observation of a surplus of positrons. Essentially two main possible explanations have been put forward by researchers: either it is the emission caused by neutron stars, collapsed objects which spin rapidly around their own axis which are also known as pulsars, or there are other types of astrophysics processes which involve dark matter, the component of the Universe whose gravitational effects are the only thing we have observed. Thanks to the new data from HAWC, it has been possible to carry out detailed measurements to observe in detail for the first time the two pulsars and to assess their role as sources of the surplus of high energy positrons.

The scientists have discovered that it is very improbable that these two spinning stars are responsible for the surplus of antimatter observed by the AMS, since because of a series of effects analysed by HAWC, the flow of positrons observable on Earth is around 100 times lower than what had been assumed up to now. The results of the research have been published in the latest copy of Science magazine. These results apply in particular to the results of the AMS experiment which measured the spectrum of the positrons up to over 500 GeVs of energy. The AMS (Anti Matter Spectrometer) is the large, orbiting ISS laboratory which uses cosmic rays to carry out precision measurements which are an essential part of particle physics, was created with a fundamental contribution from Italy. The Italian Space Agency (ASI) and Italy’s National Institute of Nuclear Physics (INFN) are actually the principal contributors to the programme and have been collaborating with the project since 1994. The research has also shown that the surplus signal extends for hundreds of giga-electronvolts.

“This new measurement is exciting, because it really puts to the test the idea that these excess positrons area coming to the Earth from two nearby pulsars”, says Jordan Goodman, professor of physics at the University of Maryland and Principal Investigator at HAWC. “Our measurements are not decisive in favour of dark matter, but any new theory which tries to explain the surplus of antimatter using pulsars has to take account of what we have found.” To be able to measure gamma rays coming from cosmic sources, HAW uses the Cherenkov effect owing to the passage of the particles produced by gamma rays in large water tanks.

“The new HAWC data allow significant simplification of our understanding of possible sources of the impressive excess of elementary antimatter, positrons, observed by the AMS up to over 500 GeV. The theory that this signal stems from the pulsars close to the Earth can essentially be ruled out. By contrast the probability increases that it is an effect which comes from new physics, for example the annihilation of particles of dark matter, or other effects which have not been considered up to now”, declared Roberto Battiston, president of ASI and founder in 1994, together with the Nobel Prize winner Samuel Ting, of the AMS experiment:

“It is a further example of multi-messenger astrophysics, as in the case of the recent observation of the fusion of two neutron stars, where the gravitational waves and light were used at the same time to analyse this phenomenon. In this case, the messenger is the cosmic rays observed by AMS, while the high energy gamma rays mean that data from AMS can be interpreted, eliminating one of the possible astro-physics sources.”

In the case of gamma rays coming from a pulsar, a luminous emission like this shows the presence of particles accelerated by the star, and the dimensions of the region of space from which these energy rays come allow an estimate to be made of how quickly the material moves away from the pulsars, and thus how many positrons could have reached the Earth from a specific source. This diffusion speed turns out to be very much lower than what was expected, and so the flow of positrons detectable close to Earth is much less intense. The measurements taken by HAWC have thus absolved the pulsars known as Geminga and PSR B0656+14, which, even though they are of the right age, and are at the right distance to explain either the high energy effect observed by the AMS, or the lower energy tail.


This measurement would not have been possible without HAWC's wide field of view, which can observe a third of the sky. With its 300 water tanks, the HAWC observatory is optimised to gather real particle “showers” coming from gamma rays with energy levels of up to 10 million times that of an X-ray scan. If a gamma ray with so much energy crashes into the atmosphere, it produces a rain of particles that move towards the ground. When these particles reach the HAWC tanks they produce blue flashes as they go through the water, allowing researchers to reconstruct the characteristics of the original gamma ray, and specifically its energy and its position it came from in the sky.

The emission measured from the two pulsars which had been thought responsible is insufficient to explain the abundance of positrons observed by AMS around the 500 GeV range. Researchers don't rule out that more complex theories relating to the diffusion of cosmic rays might be developed to explain this effect, even though it doesn't seem very probable. Dark matter could thus provide the right explanation, but further proof will be necessary to be able to make a clear and definitive judgement.