High-energy Cosmic-ray Electrons Research Articles

Cloud-scale radio surveys of star formation and feedback in Triangulum Galaxy M 33: VLA observations

ABSTRACT Studying the interplay between massive star formation and the interstellar medium (ISM) is paramount to understand the evolution of galaxies. Radio continuum (RC) emission serves as an extinction-free tracer of both massive star formation and the energetic components of the ISM. We present a multiband RC survey of the Local Group galaxy M 33 down to ≃30 pc linear resolution observed with the Karl G. Jansky Very Large Array (VLA). We calibrate the star formation rate surface density and investigate the impact of diffuse emission on this calibration using a structural decomposition. Separating the thermal and non-thermal emission components, the correlation between different phases of the ISM, and the impact of massive star formation are being investigated. Radio sources with sizes ≲200 pc constitute about 36 per cent (46 per cent) of the total RC emission at 1.5 GHz (6.3 GHz) in the inner 18 × 18 arcmin2 (or 4 kpc × 4 kpc) disc of M 33. The non-thermal spectral index becomes flatter with increasing star formation rate surface density, indicating the escape of cosmic ray electrons from their birth places. The magnetic field strength also increases with star formation rate following a bi-modal relation, indicating that the small-scale turbulent dynamo acts more efficiently at higher luminosities and star formation rates. Although the correlations are tighter in star-forming regions, the non-thermal emission is also correlated with the more quiescent molecular gas in the ISM. An almost linear molecular star formation law exists in M 33 when excluding diffuse structures. Massive star formation amplifies the magnetic field and increases the number of high-energy cosmic ray electrons, which can help the onset of winds and outflows.

Utilizing cosmic-ray positron and electron observations to probe the averaged properties of Milky Way pulsars

Pulsars have long been studied in the electromagnetic spectrum. Their environments are rich in high-energy cosmic-ray electrons and positrons likely enriching the interstellar medium with such particles. In this work we use recent cosmic-ray observations from the AMS-02, CALET and DAMPE collaborations to study the averaged properties of the local Milky Way pulsar population. We perform simulations of the local Milky Way pulsar population, for interstellar medium assumptions in agreement with a range of cosmic-ray nuclei measurements. Each such simulation contains $\sim 10^{4}$ pulsars of unique age, location, initial spin-down power and cosmic-ray electron/positron spectra. We produce more than $7\times 10^{3}$ such Milky Way pulsar simulations. We account for and study i) the pulsars' birth rates and the stochastic nature of their birth, ii) their initial spin-down power distribution, iii) their time evolution in terms of their braking index and characteristic spin-down timescale, iv) the fraction of spin-down power going to cosmic-ray electrons and positrons and v) their propagation through the interstellar medium and the Heliosphere. We find that pulsars of ages $\sim 10^{5}-10^{7}$ yr, have a braking index that on average has to be 3 or larger. Given that electromagnetic spectrum observations of young pulsars find braking indices lower than 3, our work provides strong hints that pulsars' braking index increases on average as they age, allowing them to retain some of their rotational energy. Moreover, we find that pulsars have relatively uniform properties as sources of cosmic-ray electrons and positrons in terms of the spectra they produce and likely release O($10\%$) of their rotational energy to cosmic-rays in the ISM. Finally, we find at $\simeq$12 GeV positrons a spectral feature that suggests a new subpopulation of positron sources contributing at these energies.

Present and future status of light dark matter models from cosmic-ray electron upscattering

Non-relativistic Dark Matter (DM) can be accelerated by scattering on high-energy cosmic-ray (CR) electrons. This process leads to a sub-population of relativistic or semi-relativistic DM which extends the experimental reach for direct detection in the sub-GeV mass regime. In this paper we examine the current and future potential of this mechanism for constraining models of light dark matter. In particular, we find that Super-Kamiokande and XENON1T data can already provide leading constraints on the flux of dark matter that has been accelerated to high energies from cosmic ray electrons. We also examine future projected sensitivities for DUNE and Hyper-K, and contrary to previous findings, conclude that DUNE will be able supersede Super-K bounds on cosmic-ray upscattered DM for a variety of DM models.

Calibration and performance of the neutron detector onboard of the DAMPE mission

The DArk Matter Particle Explorer (DAMPE), one of the four space-based scientific missions within the framework of the Strategic Pioneer Program on Space Science of the Chinese Academy of Sciences, was successfully launched on 2015 Dec. 17 from Jiuquan launch center. One of the most important scientific goals of DAMPE is to search for evidence of dark matter indirectly by measuring the spectrum of high energy cosmic-ray electrons. The neutron detector, one of the four sub-payloads of DAMPE, is designed to distinguish high energy electrons from hadron background by measuring the secondary neutrons produced in the shower. In this paper, a comprehensive introduction of the neutron detector is presented, including the design, calibration and performance. The analysis with simulated data and flight data indicates a powerful proton rejection capability of the neutron detector, which plays an essential role for TeV electron identification of DAMPE.

Constraints on dark matter annihilation with electron spectrum from VERITAS

Cosmic-ray electrons (and positrons) can be used to probe self-annihilation or decay of dark matter particles. We have used spectrum of high energy cosmic-ray electrons between 300 GeV and 5 TeV measured by Very Energetic Radiation Imaging Telescope Array System (VERITAS) to constrain dark matter annihilation cross-section in the solar neighbourhood. A diffusion-loss equation with a canonical model has been used as a spectrum model function by assuming a NFW profile for the Milky Way halo. We have considered cosmic-ray electrons as products of annihilation of heavy dark matter particles (GeV -TeV). During the propagation through interstellar medium, the cosmic-ray electrons lose their energy due to synchrotron radiation, inverse-Compton scattering and ionization processes.

Search for features in the cosmic-ray electron and positron spectrum measured by the Fermi Large Area Telescope

Over its ten years of mission the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope has collected the largest ever sample of high-energy cosmic-ray electron and positron events. Possible features in their energy spectrum could be a signature of the presence of nearby astrophysical sources or of more exotic sources, such as annihilation or decay of dark matter (DM) particles in the Galaxy. We will present the results of the search for possible delta-like line features in the cosmic-ray electron and positron spectrum. We will also present the results of the search for possible features originating from DM particles annihilating into electron-positron pairs. We are able to set constraints on DM masses up to 1.7TeV/c2 and exclude the thermal value of the relic annihilation cross section for DM candidates lighter than 150 GeV/c2.

Scattering of high-energy cosmic ray electrons off the Dark Matter

High-energy cosmic ray electrons interaction with Dark Matter particles is considered. In particular, a weakening of energy spectrum of cosmic electrons is predicted resulting from inelastic electron scattering on hyperpions in the hypercolor extension of the Standard Model. Corresponding cross-section and angular distributions of secondary neutrino are calculated and studied. The number of events with these neutrinos at IceTop is also estimated.

Studying the Milky Way pulsar population with cosmic-ray leptons

Recent measurements of cosmic-ray electron and positron spectra at energies from a GeV to 5 TeV, as well as radio, X-ray and a wide range of gamma-ray observations of pulsar-wind nebulae, indicate that pulsars are significant sources of high-energy cosmic-ray electrons and positrons. Here we calculate the local cosmic-ray $e^\pm$ energy spectra from pulsars taking into account models for (a) the distribution of the pulsars spin-down properties; (b) the cosmic-ray source spectra; and (c) the physics of cosmic-ray propagation. We then use the measured cosmic-ray fluxes from AMS-02, CALET and DAMPE to constrain the space of pulsar and cosmic-ray-propagation models and in particular, local cosmic-ray diffusion and energy losses, the pulsars' energy-loss time-dependence, and the injected $e^{\pm}$ spectra. We find that the lower estimates for the local $e^{\pm}$ energy losses are inconsistent with the data. We also find that pulsar braking indexes of 2.5 or less for sources with ages more than 10 kyr are strongly disfavored. Moreover, the cosmic-ray data are consistent with a wide range of assumptions on the $e^{\pm}$ injection spectral properties and on the distribution of initial spin-down powers. Above a TeV in energy, we find that pulsars can easily explain the observed change in the $e^{+} + e^{-}$ spectral slope. These conclusions are valid as long as pulsars contribute $\gtrsim10\%$ of the observed cosmic-ray $e^\pm$ at energies $\gtrsim100$ GeV.

DAMPE excess from decaying right-handed neutrino dark matter

The flux of high-energy cosmic-ray electrons plus positrons recently measured by the DArk Matter Particle Explorer (DAMPE) exhibits a tentative peak excess at an energy of around 1.4 TeV. In this paper, we consider the minimal gauged U(1)[Formula: see text] model with a right-handed neutrino (RHN) dark matter (DM) and interpret the DAMPE peak with a late-time decay of the RHN DM into [Formula: see text]. We find that a DM lifetime [Formula: see text] can fit the DAMPE peak with a DM mass [Formula: see text]. This favored lifetime is close to the current bound on it by Fermi-LAT, our decaying RHN DM can be tested, once the measurement of cosmic gamma ray flux is improved. The RHN DM communicates with the Standard Model particles through the U(1)[Formula: see text] gauge boson ([Formula: see text] boson), and its thermal relic abundance is controlled by only three free parameters: [Formula: see text], the U(1)[Formula: see text] gauge coupling [Formula: see text], and the [Formula: see text] boson mass [Formula: see text]. For [Formula: see text], the rest of the parameters are restricted to be [Formula: see text] and [Formula: see text], in order to reproduce the observed DM relic density and to avoid the Landau pole for running [Formula: see text] below the Planck scale. This allowed region will be tested by the search for a [Formula: see text] boson resonance at the future Large Hadron Collider.

Dark Matter Particle Explorer observations of high-energy cosmic ray electrons plus positrons and their physical implications

The DArk Matter Particle Explorer (DAMPE) is a satellite-borne, high-energy particle and $\gamma$-ray detector, which is dedicated to indirectly detecting particle dark matter and studying high-energy astrophysics. The first results about precise measurement of the cosmic ray electron plus positron spectrum between 25 GeV and 4.6 TeV were published recently. The DAMPE spectrum reveals an interesting spectral softening around $0.9$ TeV and a tentative peak around $1.4$ TeV. These results have inspired extensive discussion. The detector of DAMPE, the data analysis, and the first results are introduced. In particular, the physical interpretations of the DAMPE data are reviewed.

Search for features in the cosmic-ray electron and positron spectrum measured by the Fermi Large Area Telescope

The Large Area Telescope onboard the Fermi Gamma-ray Space Telescope has collected the largest ever sample of high-energy cosmic-ray electron and positron events. Possible features in their energy spectrum could be a signature of the presence of nearby astrophysical sources, or of more exotic sources, such as annihilation or decay of dark matter (DM) particles in the Galaxy. In this paper for the first time we search for a delta-like line feature in the cosmic-ray electron and positron spectrum. We also search for a possible feature originating from DM particles annihilating into electron-positron pairs. Both searches yield negative results, but we are able to set constraints on the line intensity and on the velocity-averaged DM annihilation cross section. Our limits extend up to DM masses of 1.7 $TeV/c^2$, and exclude the thermal value of the annihilation cross-section for DM lighter than 150 $GeV/c^2$.

Bayesian analysis of the break in DAMPE lepton spectra

Recently, DAMPE has released its first results on the high-energy cosmic-ray electrons and positrons (CREs) from about $25$ GeV to $4.6$ TeV, which directly detect a break at $\sim 1$ TeV. This result gives us an excellent opportunity to study the source of the CREs excess. In this work, we used the data fo proton and helium flux (from AMS-02 and CREAM), $\bar{\mathrm{p}}/\mathrm{p}$ ratio (from AMS-02), positron flux (from AMS-02) and CREs flux (from DAMPE without the peak signal point at $\sim 1.4$ TeV) to do global fitting simultaneously, which can account the influence from the propagation model, the nuclei and electron primary source injection and the secondary lepton production precisely. For extra source to interpret the excess in lepton spectrum, we consider two separate scenarios (pulsar and dark matter annihilation via leptonic channels) to construct the bump ($\gtrsim 100$ GeV) and the break at $\sim 1$ TeV. The result shows: (i) in pulsar scenario, the spectral index of the injection should be $\nu_{\mathrm{psr}} \sim 0.65$ and the cut-off should be $R_{c} \sim 650$ GV; (ii) in dark matter scenario, the dark matter particle's mass is $m_{\chi} \sim 1208$ GeV and the cross section is $\langle \sigma v \rangle \sim 1.48 \times 10^{-23} \mathrm{cm}^{3} \mathrm{s}^{-1}$. Moreover, in the dark matter scenario, the $\tau \bar{\tau}$ annihilation channel is highly suppressed, and a DM model is built to satisfy the fitting results.

Evolved Pulsar Wind Nebulae as Sources of (Mostly Leptonic) Cosmic Rays

Pulsars, and more precisely their wind nebulae, are perhaps the most likely astrophysical sources of high-energy cosmic-ray electrons and especially positrons, and in particular of the excess positronic component above ∼20 GeV measured by PAMELA, Fermi-LAT and AMS-02. While e± pairs are created in pulsar magnetospheres, their acceleration to relevant cosmic-ray energies likely occurs at the pulsar wind termination shock, and they are subsequently confined in the pulsar wind nebula (PWN) until late stages of its evolution. We discuss the implications of radiative and adiabatic energy losses for the likely contributions of different stages of PWN evolution to the observed cosmic-ray e± spectrum, and highlight the plausibly dominant contribution of late, subsonic expansion and especially bow-shock PWN phases.The most recently opened observational window on PWNe, with implications for accelerated e± in the relevant energy range, is the domain of TeV γ-rays. I review the properties of the population of PWNe revealed in this energy domain, in particular by the H.E.S.S. array of Cherenkov telescopes and its Galactic Plane Survey, and discuss the constraints which these place on the evolution of middle-aged PWNe. Finally, I briefly review the possibility of hadron acceleration in PWNe, and discuss theoretical and observational constraints which limit a plausible major contribution of these objects to the bulk of the (hadronic) Galactic cosmic-ray spectrum.

Scalar dark matter, type II seesaw and the DAMPE cosmic ray e+ + e− excess

The DArk Matter Particle Explorer (DAMPE) has reported a measurement of the flux of high energy cosmic ray electrons plus positrons (CREs) in the energy range between 25GeV and 4.6TeV. With unprecedented high energy resolution, the DAMPE data exhibit an excess of the CREs flux at an energy of around 1.4TeV. In this letter, we discuss how the observed excess can be understood in a minimal framework where the Standard Model (SM) is supplemented by a stable SM singlet scalar as dark matter (DM) and type II seesaw for generating the neutrino mass matrix. In our framework, a pair of DM particles annihilates into a pair of the SM SU(2) triplet scalars (Δs) in type II seesaw, and the subsequent Δ decays create the primary source of the excessive CREs around 1.4TeV. The lepton flavor structure of the primary source of CREs has a direct relation with the neutrino oscillation data. We find that the DM interpretation of the DAMPE excess determines the pattern of neutrino mass spectrum to be the inverted hierarchy type, taking into account the constraints from the Fermi-LAT observations of dwarf spheroidal galaxies.

Direct detection of a break in the teraelectronvolt cosmic-ray spectrum of electrons and positrons.

High-energy cosmic-ray electrons and positrons (CREs), which lose energy quickly during their propagation, provide a probe of Galactic high-energy processes and may enable the observation of phenomena such as dark-matter particle annihilation or decay. The CRE spectrum has been measured directly up to approximately 2 teraelectronvolts in previous balloon- or space-borne experiments, and indirectly up to approximately 5 teraelectronvolts using ground-based Cherenkov γ-ray telescope arrays. Evidence for a spectral break in the teraelectronvolt energy range has been provided by indirect measurements, although the results were qualified by sizeable systematic uncertainties. Here we report a direct measurement of CREs in the energy range 25 gigaelectronvolts to 4.6 teraelectronvolts by the Dark Matter Particle Explorer (DAMPE) with unprecedentedly high energy resolution and low background. The largest part of the spectrum can be well fitted by a 'smoothly broken power-law' model rather than a single power-law model. The direct detection of a spectral break at about 0.9 teraelectronvolts confirms the evidence found by previous indirect measurements, clarifies the behaviour of the CRE spectrum at energies above 1 teraelectronvolt and sheds light on the physical origin of the sub-teraelectronvolt CREs.

High β effects on cosmic ray streaming in galaxy clusters

Diffuse, extended radio emission in galaxy clusters, commonly referred to as radio halos, indicate the presence of high energy cosmic ray (CR) electrons and cluster-wide magnetic fields. We can predict from theory the expected surface brightness of a radio halo, given magnetic field and CR density profiles. Previous studies have shown that the nature of CR transport can radically effect the expected radio halo emission from clusters (Wiener et al. 2013). Reasonable levels of magnetohydrodynamic (MHD) wave damping can lead to significant CR streaming speeds. But a careful treatment of MHD waves in a high $\beta$ plasma, as expected in cluster environments, reveals damping rates may be enhanced by a factor of $\beta^{1/2}$. This leads to faster CR streaming and lower surface brightnesses than without this effect. In this work we re-examine the simplified, 1D Coma cluster simulations (with radial magnetic fields) of Wiener et al. (2013) and discuss observable consequences of this high $\beta$ damping. Future work is required to study this effect in more realistic simulations.

Highligths from the Fermi Large Area Telescope

The Fermi mission is operating in low Earth orbit since June 2008. Thanks to its large acceptance, the Large Area Telescope (LAT), the on-board pair-conversion telescope for high-energy electromagnetic radiation, has collected more than one billion photons from the whole sky above 20 MeV. The LAT also provided the largest high-energy cosmic-ray electron sample to date, with about 10k events above 1 TeV.This unique database allows the study of thousands of gamma-ray sources of very different nature, from our own Galaxy to distant and active galactic nuclei, as well as addressing fundamental questions of particle astrophysics like the nature of dark matter and the origin of energetic gamma-ray bursts. In addition, cosmic-ray electrons provide a unique probe of the origin and propagation mechanisms of cosmic rays in local galactic environment.

Precision measurement of 0.5 GeV–3 TeV electrons and positrons using the AMS Electromagnetic Calorimeter

The Alpha Magnetic Spectrometer, AMS, is successfully collecting data on the International Space Station since its installation on May 19, 2011. One of the main objectives of AMS is the precision measurement of high energy cosmic ray electrons and positrons. The key detector for this measurement is the Electromagnetic Calorimeter. Reconstruction of electrons and positrons in the calorimeter uses a 3-dimensional shower parametrization, which accounts for the detector specifics: finite size of the calorimeter, non-uniform efficiency of the signal collection, and saturation effects due to the electronics and due to high energy density in the active calorimeter elements. This technique provides AMS with a precision energy measurement of electrons and positrons up to several TeV as well as an excellent rejection of the proton background.

Investigating the origin of high-energy cosmic-ray electrons with Monte Carlo simulation

Due to severe radiative energy losses during propagation, high-energy cosmic-ray electrons can reach Earth only from nearby sources. Although these sources clearly manifest themselves in the special features of the energy spectrum observed by recent space-borne experiments, especially the increase in the positron fraction, their exact nature is still a matter of debate. The standard method for interpreting cosmic-ray electron data consists in solving appropriate transport equations. It can be supplemented with a Monte Carlo approach taking advantage of the intrinsic random nature of cosmic-ray diffusive propagation. This analysis gives valuable information on the electron-by-electron fluctuations and hence allows to address the issue from a different angle. Here we show how to implement a fully three-dimensional time-dependent Monte Carlo simulation of the propagation of high-energy cosmic-ray electrons from nearby sources and discuss the “single-source” astrophysical scenario.

Anisotropies in the flux of cosmic ray electrons and positrons

High energy cosmic ray electrons and positrons probe the local properties of our galaxy. In fact, regardless of the production mechanism, electromagnetic energy losses limit the typical propagation scale of GeV-TeV electrons and positrons to a few kpc. In the diffusion model, the presence of nearby and dominant sources may produce an observable dipole anisotropy in the cosmic ray fluxes. We present a detailed study on the role of anisotropies from nearby sources in the interpretation of present cosmic ray electron and positrons fluxes. Predictions for the dipole anisotropy from known astrophysical sources as Supernova Remnants (SNRs) and pulsars taken from the Green and the Australia Telescope National Facility (ATNF) catalogs are shown. The results are obtained from models compatible with the most recent AMS-02 data on electrons and positrons fluxes. In particular, anisotropies for single sources as well as for a distribution of catalog sources are discussed. We compare our results with current anisotropy upper limits from the Fermi-LAT and PAMELA experiments, showing that the search of anisotropy in the electron and positron fluxes represents a complementary tool to inspect the properties of close SNRs, as for example the Vela SNR.