Since 2008, astronomers have been puzzled by a mysterious feature in the cosmic-ray energy spectrum. Data from the PAMELA satellite showed a significant increase in the ratio of positrons to electrons at energies above 10 GeV. This unexpected positron excess was subsequently confirmed by both the Fermi-LAT satellite and the AMS-02 experiment onboard the ISS (CERN Courier December 2016 p26–30), sparking many explanations, ranging from dark-matter annihilation to positron emission by nearby pulsars. New measurements by the High-Altitude Water Cherenkov (HAWC) experiment now seem to rule out the second explanation, hinting at a more exotic origin of the positron excess.
Although standard cosmic-ray propagation models predict the production of positrons from interactions of high-energy protons travelling through the galaxy, the positron fraction is expected to decrease as a function of energy. One explanation for the excess is the annihilation of dark-matter particles with masses of several TeV, which would result in a bump in the electron–positron fraction, with the measured increase perhaps being the rising part of such a bump. According to other models, however, the excess is the result of positron production by astrophysical sources such as pulsars (rapidly spinning neutron stars). Since these charged particles lose energy due to interactions with interstellar magnetic and radiation fields they must be produced relatively close to Earth, making nearby pulsars a prime suspect.
HAWC, situated near the city of Puebla in Mexico, detects charged particles created in the Earth’s atmosphere from collisions between high-energy photons and atmospheric nuclei. The charged particles produced in the resulting shower produce Cherenkov radiation in HAWC’s 300 water tanks, their high altitude location making HAWC the most sensitive survey instrument to measure astrophysical photons in the TeV range. This allows the study of TeV-scale photon emission from nearby pulsars, such as Geminga and PSR B0656+14, to investigate if these objects could be responsible for the positron excess.
Pulsars are thought to emit electrons and positrons with energies up to several hundred TeV, which diffuse into the interstellar medium, but the details of the emission, acceleration and propagation of these leptons are not well understood. The TeV photons measured by HAWC are produced as the electrons and positrons emitted by the pulsars interact with low energy photons in the interstellar medium. One can, therefore, use the intensity of the TeV photon emission and the size of the emitting region to indirectly measure the high-energy positrons. The HAWC data show the large emitting regions of both the pulsars Geminga and PSR B0656+14 (see figure). The spectral and spatial features of the TeV emission were then inserted in a diffusion model for the positrons, allowing the team to calculate the positron flux from these sources reaching Earth. The results, published in Science, indicate that the positron flux from these sources reaching Earth is significantly smaller than that measured by PAMELA and AMS-02.
These indirect measurements of the positron emission appear to rule out a significant contribution of the local positron flux by these two pulsars, making it unlikely that pulsars are the origin of the positron excess. More exotic explanations such as dark matter, or other astrophysical sources such as micro-quasars and supernovae remnants, are not ruled out, however. Results from gamma-ray observations of such sources, along with more detailed measurements of the lepton flux at even higher energies by AMS-02, DAMPE or CALET, are therefore highly anticipated to fully solve the mystery of the cosmic positron excess.