１ The electron specctrum
The primary science goal of CALET is to perform high precision measurements of the electron spectrum from 1 GeV to 20 TeV. CALET will first make an accurate scan of the energy region already covered by previous experiments, taking advantage of an excellent energy resolution and a low background contamination. By integrating a sufficient exposure on the ISS, CALET will be able to explore the energy region above 1 TeV, where the presence of nearby sources of acceleration is expected to shape the high end of the electron spectrum and leave faint, but detectable, footprints in the anisotropy. In order to meet this experimental goal, CALET has been designed to achieve a large proton rejection capability (>100000) thanks to a full containment of electromagnetic showers in the calorimeter and a fine-grained imaging of the first 3 radiation lengths.
The TeV region. An exciting possibility is that the observation of the electron spectrum in the TeV region may result in a direct detection of nearby astrophysical sources of high energy electrons. In fact, the most energetic galactic cosmic-ray (GCR) electrons that can be observed on Earth are likely to originate from sources younger than ~100k years and located at a distance less than 1 kpc from the Solar System. This is due to the radiative energy losses that limit the propagation lifetime of high energy electrons and, consequently, the distance they can diffuse away from their source(s).
Expected energy spectra of electrons as calculated by a diffusion model [Kobayashi et al., Astrophys. J. 601 , 340-351 (2004)] with a particular choice of parameters (shown in the picture above). The predicted electron spectra are compared to a compilation of previous electron measurements. Possible contributions of Vela, Monogem and Cygnus Loop are shown as an example. Adding these three sources to the "distant component" gives the topmost curve with the expected CALET measurements (red points).
Since the number of potential sources satisfying the above constraints are very limited, the energy spectrum of electrons might have a characteristic structure (Nishimura et al. 1980), and the arrival directions are expected to show a detectable anisotropy (Ptuskin and Ormes 1995; Nishimura et al.1997). There are at least nine candidate Supernova Remnants (SNR) with ages < 100k years and distances less than 1 kpc from the solar system. Possible contributions to the observed GCR electron spectrum from both distant and nearby sources were calculated. Known candidates that may give a contribution in the TeV region include Vela, Cygnus loop and Monogem, in order of strength. Among these, Vela is quite promising as both the distance, ~ 0.25 kpc, and the age, ~ 10k years, are very suitable for the observation.
Inclusive electron (+ positron) spectrum as expected after 5 years of data taking by CALET (red points) according to a SUSY inspired model, consistent with the present data on the observed positron excess.
The TeV region might as well conceal a completely different scenario, as in the example shown in the picture above, where "nearby" acceleration sources are not detected and the spectrum rolls off at a characteristic cutoff energy. In this example, the shape of the spectrum near the cutoff is predicted by a model of dark matter (with neutralino as LSP) that takes into account the recent data on the positron excess. The measurement of the "end point" of the electron spectrum can be used to constrain the cosmic-ray diffusion coefficient.
The sub-TeV region. The electron energy spectrum from 10 GeV to 1 TeV is probably the result of the contribution of several unresolved sources. In this energy region CALET accuracy and exposure will allow to significantly improve the knowledge of the detailed spectral shape and angular distribution of the inclusive electron + positron spectrum. This will provide information on the average features of the source spectrum, the diffusion time, the density of sources and possibly their nature, either as astrophysical objects (e.g. a nearby pulsar) or the result of the annihilation/ decay of dark matter particles. Both possibilities have been proposed to explain recent measurements suggesting a hardening of the inclusive electron+positron spectrum in the range 200 GeV - 1 TeV. The presence of an additional spectral component is also required to explain the now established rise of positron fraction above ~10 GeV as measured by PAMELA and extended to the hundreds GeV range by AMS-02.
The flux below 10 GeV is strongly modulated by solar activity. Long-term observations can provide accurate data on the evolution of the electron spectrum as a function of time. This information can be used to validate models of the transport of electrons into and within the Heliosphere and improve our understanding of the modulation mechanism.
ⅡCharged Cosmic Rays
The main science objectives of the experimental study of very high energy (VHE) charged cosmic rays (CR) include:
- the understanding of the acceleration mechanism of primary cosmic rays;
- the identification of the acceleration sites (sources);
- the clarification of the interactions of primary cosmic rays with the inter-galactic medium.
Direct measurements of the composition and energy spectra of VHE cosmic rays are carried out by instruments flying on balloons (above the atmosphere at ~40 km altitude) and by space-borne experiments (on satellites and on the ISS). Their observations are known as direct because these payloads are equipped with instruments capable to identify the incoming cosmic particle, in contrast with ground-based (indirect) measurements, where the identity of the impinging nucleus is inferred only indirectly via its interaction with the atmosphere, with large systematic uncertainties. The direct observations collected so far provide the experimental ground for the contemporary standard model of galactic cosmic rays (GCR).
However, there are still many open questions, as for example:
- are high energy spectra described by a pure power-law?(as predicted by standard SNR diffusive shock acceleration models. Instead, a significant curvature of the spectrum may occur as suggested by a new class of acceleration models that take into account the dynamical interaction between the shock and the accelerated particle)
- is there a mass-dependent spectral cutoff for individual elements below the PeV scale? (as suggested by models of the all-particle spectral "knee" observed around 3-4 PeV)
- why large anisotropies are not observed? (contrary to the expectations based on the extrapolation of the propagation pathlength, as derived from measurements at a few GeV/n)
Some of these problems will be addressed in the near future by new missions that will extend to higher energies the existing measurements. A better control of the systematic errors will hopefully clarify and remove some of the present "tensions" among the measurements performed by different experiments.
Expected CALET measurement (red points) of the energy spectra of proton and He after 5 years of observations on the ISS, compared with a compilation of data from direct measurements.
Taking advantage of a long observation time on the JEM-EF, favourable duty cycle and a relatively large geometric factor, CALET can extend the existing spectral measurements and studies of cosmic ray elemental composition by nearly an order of magnitude in energy, and improve the quality of the data at lower energies reducing the systematic errors.
Energy spectra of cosmic nuclei.Equipped with a charge identifier module, placed at the top of the experimental apparatus and capable to identify the atomic number Z of the incoming cosmic ray, CALET will perform long term observations of cosmic nuclei from proton to iron and will detect trans-iron elements up to Z=40. CALET will be able to identify CR nuclei with individual element resolution and measure their energies in the range from a few tens of GeV to several hundreds of TeV. In 5 years of data taking on the JEM-EF, it is expected to extend the proton energy spectrum up to ~900 TeV, the He spectrum up to ~400 TeV/a.m.u. and to measure the energy spectra of the most abundant heavy nuclei with sufficient statistical precision up to ~20 TeV/a.m.u. for C and O and ~ 10 TeV/a.m.u. for Ne, Mg, Si and Fe.
These data will allow to determine the spectral shape of the most abundant CR elements and to investigate - with very high accuracy - the region around 200 GeV/n where a possible hardening of the spectrum has been suggested by CREAM and PAMELA. It is, in fact, still unclear to what extent the energy spectra of cosmic-ray nuclei in the 10 GeV - 1 PeV region are well described by a single power law with spectral index γ ≈ 2.7. In particular, PAMELA collaboration reported accurate measurements clearly indicating small but significant deviations from single power law spectra for both H and He in the sub-TeV region, which still need to be confirmed by independent high-accuracy measurements. PAMELA data also show a difference of ≈ 3% between the spectral indexes of H and He: it is therefore reasonable to investigate similar differences for other elements.
Secondary-to-primary flux ratios.Direct measurements of the energy dependence of the flux ratio of secondary-to-primary elements (e.g.: boron/carbon, Sub-Fe/Fe) can discriminate among different models of CR propagation in the galaxy. This observable is less prone to systematic errors than absolute flux measurements. Above 10 GeV/a.m.u., the energy dependence of the propagation pathlength is often parametrized in the form E−δ. An accurate measurement of the spectral index parameter δ is crucial to derive the spectrum at the source by correcting the observed spectral shape for the energy dependence of the propagation term. These measurements have been pushed to the highest energies with Long Duration Balloon (LDB) experiments. However, at present, they remain statistics limited to a few hundred GeV/a.m.u. and suffer from a systematic uncertainty, due to the production of secondary nuclei in the residual atmospheric grammage at balloon altitude, that may become dominant in the TeV/a.m.u. region.A partial compilation of the B/C ratio measurements as a function of the energy/nucleon and the expected statistical uncertainty of CALET measurements (red points) after 5 years.
With a long exposure and in the absence of atmosphere, CALET can provide new data to improve the accuracy of the present measurements of the B/C ratio above 100 GeV/a.m.u. and extend them beyond 1 TeV/a.m.u. A compilation of B/C data from direct measurements is shown in the picture above, where the data points expected from CALET in 5 years are marked as red filled circles in the energy range per nucleon from 15 GeV to ~ 8 TeV.
III. Dark Matter searches and gamma-ray astrophysics
Dark Matter (DM) candidates (for a concise review see, for instance Drees and Gerbier, 2004) include WIMPs (Weakly Interacting Particles) from supersymmetric theories, such as the LSP neutralino, that may annihilate and produce gamma rays and positrons as a signature. CALET will perform a sensitive search for signatures of DM candidates in both the electron (+positron) spectrum, as discussed above, and in gamma-ray spectra.
According to a class of models, the annihilation / decay of dark-matter particles in the galactic halo could produce sharp gamma-ray lines in the sub-TeV to TeV energy region, superimposed to a diffuse photon background spectrum. CALET will be capable of investigating such a distinctive signature, thanks to a gamma-ray energy resolution of 3% above 100 GeV, that can be improved to 1% with a reduced (75%) on-axis effective area (fiducial volume acceptance cuts to require a total lateral containment of the shower).
The precise determination of the line shape of any spectral feature is expected to play a crucial role in the discrimination among different models of dark matter candidates, or it might suggest an alternative astrophysical interpretation.
Expected diffuse gamma-ray flux for one year above 10GeV.
Expected CALET measurement (5-years) of a possible 1.4 TeV gamma-ray line from dark matter in the region of the galactic center, including galactic diffuse background [K. Yoshida et al., Proc. of 33rd ICRC 0735,1-4 (2013)].
Another class of DM candidates, as suggested by Cheng, Feng and Matchev (2002), are Kaluza-Klein (KK) particles, resulting from theories involving compactified extra-dimensions. They may annihilate in the galactic halo and produce an excess of positrons observable at Earth. Unlike neutralinos, however, direct annihilation of KK particles to leptons is not suppressed and, consequently, the KK electron “signal” is enhanced relative to that from neutralinos. The example in the picture below shows the predicted positron signal (a corresponding number of electrons are produced along with the positrons during the KK annihilation) for possible KK particle masses (dark shaded regions) with the estimated background flux (light shaded region) of secondary particles from interactions of cosmic rays with the interstellar material. The sharp cutoff in the excess positrons close to the KK mass, might produce a detectable “feature” in the inclusive electron/positron energy spectrum.
Predicted positron signal from annihilation of Kaluza-Klein dark matter candidate particles according to the model of Cheng, Feng and Matchev (2002).
Dark matter KK particles can also decay into gamma rays and this opens the question on how to decide between a neutralino and a KK origin for an observed gamma ray line. Bergström et al. (2006) have shown that a difference in the line shape between the two types of dark matter candidates has to be expected. Thus, CALET would have the best capability to resolve the nature of the dark matter for any high energy gamma ray "line" observed.
Gamma-ray sources. Observation of gamma-ray sources will not be a primary objective for CALET. However, its excellent energy resolution and good angular resolution (better than 0.4◦, including pointing uncertainty) will allow for accurate measurements of diffuse gamma-ray emission and detection of more than 100 bright sources at high latitude from the Fermi-LAT catalog. Given the on-axis effective area of ≈ 600 cm2 for energies above 10 GeV (reduced by ∼ 50% at 4 GeV) and field of view of 45◦ from the vertical direction, CALET is expected to detect ~25000 (~7000) photons from the galactic (extra-galactic) background with E > 4 GeV and ~300 photons from the Vela pulsar with E > 5 GeV.
VI. Energy spectrum of electron under the solar modulation
The purpose of the CALET mission pertaining to solar physics is to get new information on the cosmic ray transport in the heliosphere. We estimate the energy dependence of the diffusion of electrons through the solar magnetic field. Further we simultaneously observe electrons and protons in the 1- 10 GeV energy range and investigate the charge sign dependence of solar modulation. We also discuss some expectations in different solar polarities, using
a correlation between the cosmic-ray intensity and the neutron monitor counting rate. For short-term measurements, we expect to detect several Forbush decreases in electron flux. In such decreases, short-term variations of negative charges might differ from those of positive ones.
V. Search for Gamma ray Burst
CALET will also monitor X-ray/ gamma-ray transients in the energy region 7 keV – 20 MeV with a dedicated Gamma-ray Burst Monitor (CGBM).It will extend GRB studies being performed by other experiments (e.g. Swift and Fermi/LAT) and will provide added exposure when the other instruments will not be available or pointing to other directions. Moreover, higher energy photons associated with a burst event can be recorded over the entire CALET energy range down to 1 GeV where the CALET main telescope has still (limited) sensitivity, albeit with low resolution. Upon the detection of a GRB, an alert will be transmitted to a network of ground "antennas" (e.g.: LIGO, VIRGO) for the possible simultaneous detection of gravitational waves associated with the event.