The CALorimetric Electron Telescope (CALET), a space mission led by the Japan Aerospace Exploration Agency with participation from the Italian Space Agency (ASI) and NASA, has released its first results concerning the nature of high-energy cosmic rays.

Having docked with the International Space Station (ISS) on 25 August 2015, CALET is carrying out a full science programme with long-duration observations of high-energy charged particles and photons coming from space. It is the second high-energy experiment operating on the ISS following the deployment of AMS-02 in 2011. During the summer of 2017 a third experiment, ISS-CREAM, joined these two. Unlike AMS-02, CALET and ISS-CREAM have no magnetic spectrometer and therefore measure the inclusive electron and positron spectrum. CALET’s homogeneus calorimeter is optimised to measure electrons, and one of its main science goals is to measure the detailed shape of the electron spectrum.

Due to the large radiative losses during their travel in space, high-energy cosmic electrons are expected to originate from regions relatively close to Earth (of the order of a few thousand light-years). Yet their origin is still unknown. The shape of the spectrum and the anisotropy in the arrival direction might contain crucial information as to where and how electrons are accelerated. It could also provide a clue on possible signatures of dark matter – for example, the presence of a peak in the spectrum might tell us about a possible dark-matter decay or annihilation with an electron or positron in the final state – and shed light on the intriguing electron and positron spectra reported by AMS-02 (CERN Courier December 2016 p26).

To pinpoint possible spectral features on top of the overall power-law energy dependence of the spectrum, CALET was designed to measure the energy of the incident particle with very high resolution and with a large proton rejection power, well into the TeV energy region. This is provided by a thick homogeneous calorimeter preceded by a high-granularity pre-shower with imaging capabilities with a total thickness of 30 radiation length at normal incidence. The calibration of the two instruments is the key to control the energy scale and this is why CALET – a CERN-recognised experiment – performed several calibration tests at CERN.

The first data from CALET concern a measurement of the inclusive electron and positron spectrum in the energy range from 10 GeV to 3 TeV, based on about 0.7 million candidates (1.3 million in full acceptance). Above an energy of 30 GeV the spectrum can be fitted with a single power law with a spectral index of –3.152±0.016. A possible structure observed above 100 GeV requires further investigation with increased statistics and refined data analysis. Beyond 1 TeV, where a roll-off of the spectrum is expected and low statistics is an issue, electron data are now being carefully analysed to extend the measurement. CALET has been designed to measure electrons up to around 20 TeV and hadrons up to an energy of 1 PeV.

CALET is a powerful space observatory with the ability to identify cosmic nuclei from hydrogen to elements heavier than iron. It also has a dedicated gamma-ray-burst instrument (CGBM) that so far has detected bursts at an average rate of one every 10 days in the energy range of 7 KeV–20 MeV. The search for electromagnetic counterparts of gravitational waves (GWs) detected by the LIGO and Virgo observatories proceeds around the clock thanks to a special collaboration agreement with LIGO and Virgo. Upper limits on X-ray and gamma-ray counterparts of the GW151226 event were published and further research on GW follow-ups is being carried out. Space-weather studies relative to the relativistic electron precipitation (REP) from the Van Allen belts have also been released.

With more than 500 million triggers collected so far and an expected extension of the observation time on the ISS to five years, CALET is likely to produce a wealth of interesting results in the near future.