September 2017
Cosmic rays are high energy particles that pervade the Galaxy. Electrons represent only a small fraction of cosmic rays, which consist primarily of protons and nuclei. However, they are able to provide us with unique information complementary to what can be learnt from protons and nuclei. Due to the important difference in mass (an electron being about 1800 times lighter than a proton or any nuclei), electrons lose energy much more rapidly while propagating from their sources to Earth. Energy losses occur when the electrons interact with magnetic fields or scatter on ambient light in the Galaxy of different wavelengths: photons from the Cosmic Microwave Background or infrared photons or also photons emitted by stars for instance. Because of the strong radiative energy losses, very-high-energy cosmic-ray electrons can only travel short distances. Therefore, they provide us with information of the Earth’s local surroundings in the Galaxy. For example, electrons with an energy of 1 TeV (*) that reach the Earth are dominated by sources closer than ~1,000 light-years away. In comparison, the distance between the Sun and the centre of the Galaxy is about 24,000 light-year. For electrons with energies beyond 1 TeV, their sources must be even closer still….on our Galactic doorstep!
Up to ∼1 TeV, cosmic-ray electrons can be measured using space based instruments such as AMS [1] or Fermi-LAT [2]. More dedicated space based instruments such as CALET or DAMPE are planning to measure the electron spectrum up to ∼10 TeV and recently CALET presented at this year’s International Cosmic Ray Conference first results up to ∼1 TeV, fully compatible with previous measurements. Above 1 TeV, the flux is very low and the use of ground-based Cherenkov telescopes, which feature very large effective areas, have proven to provide a robust probe of this flux up to high energies. Through measurements by H.E.S.S. [3], [4], MAGIC [5] and VERITAS [6], the frontier in the detected energy range of the cosmic-ray electron spectrum has been pushed up to ∼5 TeV. These experiments are designed for gamma-ray observations: they detect gamma-rays through the cascade of secondary particles resulting from the interaction between a gamma-ray and a nucleus in the atmosphere. Their ability to measure electrons comes from the fact that both electrons and gamma-rays, upon their arrival at the Earth’s atmosphere, deposit their energy by the generation of essentially identical types of cascades.
The main challenge of a cosmic-ray electron measurement is the distinction between electron and background events. This background can either be gamma-rays (which produce the same type of particle cascades) or protons and heavier nuclei (which massively outnumber the electrons). Since gamma-rays move in straight lines from their astrophysical sources, regions in the sky known for containing gamma-ray sources are excluded from the analysis. Cosmic-ray protons (and other nuclei) are the vast majority of cosmic rays, and a fraction of them can mimic atmospheric cascades induced by cosmic-ray electrons. Both protons and electrons seem to come from all directions of the sky with no preferred direction — at least to high degree of accuracy. This is due to their electric charge: whatever the sources of these charged particles, the magnetic fields in the Galaxy will affect their trajectories, leading them to a random walk through the Galaxy and eventually arriving at Earth isotropically. Therefore, protons cannot be excluded from the data in a similar fashion as for the gamma-rays. Thus, the distinction between electrons and protons is done using a specific algorithm based on the — sometimes very tiny — difference in shape of the cascades generated by electrons and protons [7].
More than 9 years after the first electron spectrum measurement with H.E.S.S., subsequent observations have increased fourfold the amount of available data. In addition, analysis techniques have improved significantly, leading to a much better suppression of the background of cosmic-ray nuclei. These improvements allow for the first time a measurement of cosmic-ray electrons up to energies of ∼ 20 TeV (see Figure 1).
This new measurement from 0.25 TeV to ∼20 TeV reveals an electron spectrum that can be described by two regimes in the high energy region. The spectrum appears quite regular with a constant slope up to an energy of about 1 TeV. Above this energy the spectrum becomes steeper. This break in the spectral slope is the sign of some different physics phenomenon at play, most probably the transition between a regime where a large number of sources contribute to the spectrum, to a regime where only a few, the closest ones from Earth, are able to contribute. The very high energies reached in this measurement allow to test models of nearby sources of cosmic-ray electrons in which one source is very prominent. These models are very popular since those nearby sources of electrons (mainly pulsars) are often invoked as a possible explanation for the excess of positrons (**) measured by some experiments such as Pamela [8] and AMS [9]. The steeply falling spectrum measured with H.E.S.S. from ∼1 TeV to ∼20 TeV allows to reject models with predictions of pronounced features in the spectrum as shown in Figure 2. The black line symbolises the individual contribution of two possible sources (the Vela and the Cygus Loop supernova remnants) for a given model presented in [10] that is obviously not reproducing the data. Therefore, this new measurement of cosmic-ray electrons reveals not only for the first time the shape of the cosmic-ray electron spectrum beyond ∼5 TeV, but also provides important information on cosmic-ray accelerators in Earth’s local neighbourhood, demanding that very local sources exist.
(*) 1 TeV = 1012 eV and one eV (abbreviation of electron-volt) is a unit of energy which, by definition, represents the amount of energy gained by an electron when accelerated by an electric potential difference of 1 volt.
(**) The positron is the antiparticle of the electron.
References:
[1] AMS Collaboration, Phys. Rev. Lett. 113, 221102 (2014)
[2] Fermi-LAT Collaboration, Physical Review D 95 (2017)
[3] F. Aharonian et al., Phys. Rev. Lett. 101, 261104 (2008)
[4] F. Aharonian et al., Astron. Astrophys. 508, 561 (2009)
[5] D. Tridon et al., Proceedings of the 32nd ICRC (2011)
[6] D. Staszak et al., proceedings of the 34th ICRC (2015)
[7] M. de Naurois and L. Rolland, Astroparticle Physics, 32, 231 (2009)
[8] PAMELA Collaboration, Nature 458, 607–609 (2009)
[9] AMS Collaboration, Phys. Rev. Lett. 110, 141102 (2013)
[10] T. Kobayashi et al., Astrophys. J. 601, 340 (2004)
