Cosmic Ray Electrons Research Articles

Exclusion limits on dark matter-neutrino scattering cross section

We derive new constraints on combination of dark matter-electron cross section (${\ensuremath{\sigma}}_{\ensuremath{\chi}e}$) and dark matter-neutrino cross section (${\ensuremath{\sigma}}_{\ensuremath{\chi}\ensuremath{\nu}}$) utilizing the gain in kinetic energy of the dark matter (DM) particles due to scattering with the cosmic ray electrons and the diffuse supernova neutrino background (DSNB). Since the flux of the DSNB neutrinos is comparable to the cosmic ray (CR) electron flux in the energy range $\ensuremath{\sim}1\text{ }\text{ }\mathrm{MeV}--50\text{ }\text{ }\mathrm{MeV}$, scattering with the DSNB neutrinos can also boost low-mass DM significantly in addition to the boost due to interaction with the CR electrons. We use the XENON1T as well as the Super-Kamiokande data to derive bounds on ${\ensuremath{\sigma}}_{\ensuremath{\chi}e}$ and ${\ensuremath{\sigma}}_{\ensuremath{\chi}\ensuremath{\nu}}$. While our bounds for ${\ensuremath{\sigma}}_{\ensuremath{\chi}e}$ are comparable with those in the literature, we show that the Super-Kamiokande experiment provides the strongest constraint on ${\ensuremath{\sigma}}_{\ensuremath{\chi}\ensuremath{\nu}}$ for DM masses below a few MeV.

Statistical Study of the Optimal Local Sources for Cosmic Ray Nuclei and Electrons

Local sources, such as the Geminga supernova remnant (SNR), may have played an important role in the anomaly of protons, electrons, and anisotropy in past works. In fact, there exist 12 SNRs around the solar system within 1 kpc. One question is whether other SNRs also possibly contribute to the spectra of nuclei and electrons, and explain the special structure of the anisotropy. In this work, under spatial-dependent propagation, we systematically study the contribution of all local SNRs, within 1 kpc around the solar system, to the spectra of nuclei and electrons, as well as the energy dependence of the anisotropy. As a result, only the Geminga, the Monogem, and the Vela SNRs have quantitative contributions to the nuclei and electron spectra, and the anisotropy. Here, the Geminga SNR is the sole optimal candidate and the Monogem SNR is controversial due to the tension of the anisotropy between the model calculation and the observations. The Vela SNR contributes to a new spectral structure beyond TeV energy, hinted by the HESS, the VERITAS, the DAMPE, and the CALET measurements. More interestingly, the electron anisotropy satisfies the Fermi-LAT limit below TeV energy, but rises greatly and reaches 10% at several TeV. This novel structure will shed new light on verifying our model. We hope that the new structure of the electron spectrum and anisotropy can be observed by the spaceborne DAMPE and HERD, and the ground-based HAWC and LHAASO experiments in the near future.

Millisecond pulsars from accretion-induced collapse as the origin of the Galactic Centre gamma-ray excess signal

Gamma-ray data from the Fermi-Large Area Telescope reveal an unexplained, apparently diffuse, signal from the Galactic bulge that peaks near 2 GeV with an approximately spherical intensity profile $\propto r^{-2.4}$ that extends to angular radial scales of at least 10 degrees, possibly to 20 degrees. The origin of this "Galactic Centre Excess" (GCE) has been debated with proposed sources prominently including self-annihilating dark matter and a hitherto undetected population of millisecond pulsars (MSPs). However, the conventional channel for the generation of MSPs has been found to predict too many low mass X-ray binary (LMXB) systems and, because of the expected large natal kicks, may not accommodate the close spatial correspondence between the GCE signal and stars in the bulge. Here we report a binary population synthesis forward model that demonstrates that an MSP population arising from the accretion induced collapse (AIC) of O-Ne white dwarfs in Galactic bulge binaries can naturally reproduce the morphology, spectral shape, and intensity of the GCE signal while also obeying LMXB constraints. Synchrotron emission from MSP-launched cosmic ray electrons and positrons may simultaneously explain the mysterious, microwave "haze" from the inner Galaxy

Diffuse Galactic emission spectrum between 0.5 and 8.0 MeV

The last measurement of the diffuse emission spectrum of the Milky Way in the megaelectronvolt (MeV) photon energy range was performed by CGRO/COMPTEL more than 20 yr ago. We report a new analysis with the spectrometer SPI aboard INTEGRAL in the band 0.5–8.0 MeV, finally superseding the signal-to-noise ratio (S/N) of the historic observations. This is possible thanks to an elaborate instrumental background model and careful considerations of the selected data, which are strongly affected by solar activity. We base our analysis on energy-dependent spatial template fitting in a region of Δl ×Δb = 95° ×95° around the Galactic centre. Our flux estimates are consistent with COMPTEL measurements and show no ‘MeV bump’ The spectrum follows a power-law shape with index −1.39 ± 0.09stat ± 0.10syst and an integrated flux of (5.7 ± 0.8stat ± 1.7syst) x 10−8 erg cm−2 s−1 between 0.5 and 8.0 MeV. We find that cosmic-ray electrons and propagation models consistent with the latest Fermi/LAT, Voyager 1, and AMS-02 data are broadly in agreement with the inferred inverse Compton spectral shape. However, a mismatch of a factor of 2–3 in normalisation with respect to baseline expectations may point to enhanced target photon densities and/or electron source spectra in the inner Galaxy, slightly modified diffusion properties, or the presence of an unresolved population of MeV γ-ray sources.

Constraining time dependent dark matter signals from the Sun

Dark matter (DM) particles captured by the Sun can produce high-energy electrons outside the Sun through annihilating into meta-stable mediators. The corresponding cosmic-ray electron signals observed by the space-based experiments will be time dependent due to the orbital motion of the space-based detectors. The shape of this time dependence is predictable given the orbital information of the detectors. Since the high-energy CR electron (with energy E > 100 GeV) fluxes are expected to be constant in time, non-observation of such time variation can be used to place upper limits on the DM annihilation cross section. We analyze the time dependence of dark matter cosmic-ray signals in three space-based experiments: AMS-02, DAMPE and CALET. Under the assumption that no time dependent signal is observed, we derive the 95% C.L. exclusion limits on the signal strength from the current data. We map our limits onto the parameter space of the dark photon model and find that the constraints are comparable with that derived from the supernova SN1987A.

Confronting cosmic ray electron and positron excesses with hybrid triplet Higgs portal dark matter * *Supported in part by the National Science Foundation of China (11775093, 12175082)

We perform a detailed study of scalar dark matter with triplet Higgs extensions of the Standard Model in order to explain the cosmic ray electron and positron excesses reported by AMS-02 and DAMPE. A detailed analysis of the AMS-02 positron excess reveals that for different orderings (normal, inverted, and quasi-degenerate) of neutrino mass, the hybrid triplet Higgs portal framework is more favored with respect to the single triplet Higgs portal for TeV scale dark matter. We also show that the resonant peak and continuous excess in DAMPE cosmic ray data can be well explained with the hybrid triplet Higgs portal dark matter when a dark matter sub-halo nearby is taken into account.

A Study on Monte Carlo Simulation of the Radiation Environment above GeV at the DAMPE Orbit

The Dark Matter Particle Explorer (DAMPE) has been undergoing a stable on-orbit operation for more than 6 yr and acquired observations of over 11 billion events. A better understanding of the overall radiation environment of the DAMPE orbit is crucial for both simulation data production and flight data analysis. In this work, we study the radiation environment at low Earth orbit and develop a simulation software package using the framework of ATMNC3, in which state-of-the-art full 3D models of the Earth’s atmospheric and magnetic-field configurations are integrated. We consider in our Monte Carlo procedure event-by-event propagation of cosmic rays in the geomagnetic field and their interaction with the Earth’s atmosphere, focusing on the particles above GeV that are able to trigger the DAMPE data acquisition system. We compare the simulation results with the cosmic-ray electron and positron (CRE) flux measurements made by DAMPE. The overall agreement on both the spectral and angular distribution of the CRE flux demonstrates that our simulation is well established. Our software package could be of more general usage for simulation of the radiation environment of low Earth orbit at various altitudes.

Theory of Electron Injection at Oblique Shock of Finite Thickness

Abstract A theory of electron injection into diffusive shock acceleration (DSA) for the generation of cosmic-ray electrons at collisionless shocks is presented. We consider a recently proposed particle acceleration mechanism called stochastic shock drift acceleration (SSDA). We find that SSDA may be understood as a diffusive particle acceleration mechanism at an oblique shock of finite thickness. More specifically, it is described by a solution to the diffusion–convection equation for particles with the characteristic diffusion length comparable to the shock thickness. On the other hand, the same equation yields the standard DSA if the diffusion length is much longer than the thickness. Although SSDA predicts, in general, a spectral index steeper than DSA, it is much more efficient for low-energy electron acceleration and is favorable for injection. The injection threshold energy corresponds to the transition energy between the two different regimes. It is of the order of 0.1–1 MeV in typical interstellar and interplanetary conditions if the dissipation scale of turbulence around the shock is determined by the ion inertial length. The electron injection is more efficient at high M A / cos θ Bn , where M A and θ Bn are the Alfvén Mach number and the shock obliquity, respectively. The theory suggests that efficient acceleration of electrons to ultrarelativistic energies will be more easily realized at high Mach number, young supernova remnant shocks, but not at weak or moderate shocks in the heliosphere unless the upstream magnetic field is nearly perpendicular to the shock normal.

Gamma- and Cosmic-Ray observations with the GAMMA-400 Gamma-Ray telescope

The future space-based GAMMA-400 gamma-ray telescope will operate onboard the Russian astrophysical observatory in a highly elliptic orbit during 7 years to observe Galactic plane, Galactic Center, Fermi Bubbles, Crab, Vela, Cygnus X, Geminga, Sun, and other regions and measure gamma- and cosmic-ray fluxes. Observations will be performed in the point-source mode continuously for a long time (∼100 days). GAMMA-400 will measure gamma rays in the energy range from ∼ 20 MeV to several TeV and cosmic-ray electrons + positrons up to several tens TeV. GAMMA-400 instrument will have very good angle and energy resolutions, high separation efficiency of gamma rays from cosmic-ray background, as well as electrons + positrons from protons. The main feature of GAMMA-400 is the unprecedented angular resolution for energies > 30 GeV better than the space-based and ground-based gamma-ray telescopes by a factor of 5–10. GAMMA-400 observations will permit to resolve gamma rays from annihilation or decay of dark matter particles, identify many discrete sources, clarify the structure of extended sources, specify the data on cosmic-ray electron + positron spectra.

Cosmic rays in molecular clouds probed by H2 rovibrational lines

Context. Low-energy cosmic rays (<1 TeV) play a fundamental role in the chemical and dynamical evolution of molecular clouds, as they control the ionisation, dissociation, and excitation of H2. Their characterisation is therefore important both for the interpretation of observations and for the development of theoretical models. However, the methods used so far for estimating the cosmic-ray ionisation rate in molecular clouds have several limitations due to uncertainties in the adopted chemical networks. Aims. We refine and extend a previously proposed method to estimate the cosmic-ray ionisation rate in molecular clouds by observing rovibrational transitions of H2 at near-infrared wavelengths, which are mainly excited by secondary cosmic-ray electrons. Methods. Combining models of interstellar cosmic-ray propagation and attenuation in molecular clouds with the rigorous calculation of the expected secondary electron spectrum and updated electron-H2 excitation cross sections, we derive the intensity of the four H2 rovibrational transitions observable in cold dense gas: (1−0)O(2), (1−0)Q(2), (1−0)S(0), and (1−0)O(4). Results. The proposed method allows the estimation of the cosmic-ray ionisation rate for a given observed line intensity and H2 column density. We are also able to deduce the shape of the low-energy cosmic-ray proton spectrum impinging upon the molecular cloud. In addition, we present a look-up plot and a web-based application that can be used to constrain the low-energy spectral slope of the interstellar cosmic-ray proton spectrum. We finally comment on the capability of the James Webb Space Telescope to detect these near-infrared H2 lines, which will make it possible to derive, for the first time, spatial variation in the cosmic-ray ionisation rate in dense gas. Besides the implications for the interpretation of the chemical-dynamic evolution of a molecular cloud, it will finally be possible to test competing models of cosmic-ray propagation and attenuation in the interstellar medium, as well as compare cosmic-ray spectra in different Galactic regions.

Radio halo of NGC 4631: comparing observations and simulations

ABSTRACT We present low-frequency observations at 315 and 745 MHz of the edge-on, nearby galaxy NGC 4631 with upgraded Giant Metrewave Radio Telescope. We compare the observed surface brightness profiles along the minor axis of the galaxy with those obtained from hydrodynamical simulations of galactic outflows. These are 3D simulations that replicate star formation in a Milky-Way-sized galaxy and follow magnetized outflows emerging from the disc. We detect a plateau-like feature in the observed emission at a height of 2–3 kpc from the mid-plane of the galaxy, in qualitative agreement with that expected from simulations. This feature is believed to be due to the compression of magnetic fields behind the outer shocks of galactic outflows. We model the observed surface brightness profiles by assuming an exponential as well as a Gaussian fitting model. Using χ2 statistics, we find that the exponential model fits the profiles better and we use it to determine the scale heights. We estimate the scale height for the synchrotron radio emission to be ∼1 kpc. The time-scales for advection due to outflows and diffusion of cosmic ray electrons are ≳5 and ∼160 Myr, respectively. Because advection acts on a time-scale much shorter than diffusion, we conclude that in NGC 4631 advection, rather than diffusion, plays the dominant role in the formation of radio halo. The spectral index image with regions of flatter radio spectral index in the halo appears to indicate possible effects of gas outflow from the plane of the galaxy.

Extending the Lorentz factor range and sensitivity of transition radiation with compound radiators

Transition radiation detectors (TRDs) have been used to identify high-energy particles (in particular, to separate electrons from heavier particles) in accelerator experiments. In space, they have been used to identify cosmic-ray electrons and measure the energies of cosmic-ray nuclei. To date, radiators have consisted of regular configurations of foils with fixed values of foil thickness and spacing (or foam or fiber radiators with comparable average dimensions) that have operated over a relatively restricted range of Lorentz factors. In order to extend the applicability of future TRDs (for example, to identify 0.5–3 TeV pions, kaons, and protons in the far forward region in a future accelerator experiment or to measure the energy spectrum of cosmic-ray nuclei up to 20 TeV/nucleon or higher), there is a need to increase the signal strength and extend the range of Lorentz factors that can be measured in a single detector. A possible approach is to utilize compound radiators consisting of varying radiator parameters. We discuss the case of a compound radiator and derive the yield produced in a TRD with an arbitrary configuration of foil thicknesses and spacings.

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.

Research on the structure of cosmic-ray electron and positron fluxes in GeV–TeV energy range

The positron–electron fluxes detected by AMS02, CALET, DAMPE and Fermi-LAT have shown their complex structure which deviated from a single power-law, and the results of DAMPE and Fermi-LAT are obviously higher than those of AMS02 and CALET in 100[Formula: see text]GeV–1[Formula: see text]TeV range. We point out that the complex positron–electron flux structures can be attributed to two sources with a natural background, and the difference between the two groups of data can be explained in an anomalous bremsstrahlung theory.

Role of the radiative stage for cosmic ray acceleration in SNRs

ABSTRACT We consider diffusive shock acceleration in supernova remnants throughout their evolution including a radiative stage. It is found that a more efficient acceleration and fast exit of particles at the radiative stage results in the hardening of the source cosmic ray proton and electron spectra at energies ∼100–500 GeV. The effect is stronger for cosmic ray electrons.

Cosmic-ray transport and gamma-ray emission in M31

We study the possibility that an extended cosmic-ray leptonic and/or hadronic halo is at the origin of the large-scale gamma-ray emission detected from the Andromeda Galaxy (M31). We consider a broad ensemble of non-homogeneous diffusion scenarios and of cosmic-ray injection sources. We find that cosmic-ray electrons and protons could be, and very likely are, responsible for part, or all, of the gamma-ray emission from M31, including out to more than 100 kpc from the center of the galaxy. We also simulate possible emission from pulsars in M31, and consider the effect of regions of highly inefficient diffusion around cosmic-ray acceleration sites, as suggested by recent TeV halo observations with Cherenkov telescopes.

Circular polarisation of gamma rays as a probe of dark matter interactions with cosmic ray electrons

Conventional indirect dark matter (DM) searches look for an excess in the electromagnetic emission from the sky that cannot be attributed to known astrophysical sources. Here, we argue that the photon polarisation is an important feature to understand new physics interactions and can be exploited to improve our sensitivity to DM. In particular, circular polarisation can be generated from Beyond the Standard Model interactions if they violate parity and there is an asymmetry in the number of particles which participate in the interaction. In this work, we consider a simplified model for fermionic (Majorana) DM and study the circularly polarised gamma rays below 10 GeV from the scattering of cosmic ray electrons on DM. We calculate the differential flux of positive and negative polarised photons from the Galactic Center and show that the degree of circular polarisation can reach up to 90%. Finally, once collider and DM constraints have been taken into account, we estimate the required sensitivity from future experiments to detect this signal finding that, although a distinctive peak will be present in the photon flux spectrum, a near future observation is unlikely. However, different sources or models not considered in this work could provide higher intensity fluxes, leading to a possible detection by e-ASTROGAM. In the event of a discovery, we argue that the polarisation fraction is a valuable characterisation feature of the new sector.

Constraining the annihilation of dark matter via cosmic-ray positrons and electrons

We aim to constrain the properties of dark matter particles by several measurements of positrons and electrons from cosmic-rays. We assume that collisions of dark matter particles and dark matter anti-particles can produce positrons and electrons. The electron-positron propagation is explained by a diffusion-loss equation including loss rates, diffusion, as well as source function. We use data of cosmic-ray positrons and electrons detected by PAMELA, H.E.S.S., AMS-02 and Fermi-LAT. We compare the observational data with the electron and positron spectrum from five annihilation channels in our model to derive constraining factors regarding the cross-section of the annihilation of dark matter. The tightest constraint is provided by cosmic-ray positrons of AMS-02 for the electron channel. Dark matter with mass below a few GeV gets excluded by the cosmic-ray positrons of AMS-02 for the electron, muon and tau channels.

Probing dark matter-cosmic ray electron interactions with circular polarisation of photons

Conventional indirect dark matter (DM) searches look for an excess in the electromagnetic emission from the sky that cannot be attributed to known astrophysical sources, but its polarisation has not been explored to date. In this proceeding, I argue that photon polarisation is an important feature to understand new physics interactions. Circular polarisation can be generated from Beyond the Standard Model interactions if they violate parity and there is an asymmetry in the number density of the initial state particles which participate in the interaction. I consider a simplified model for fermionic (Majorana) DM and study the circularly polarised gamma rays from DM cosmic ray electron interactions. I study the differential flux of positive and negative polarised photons from the Galactic Centre and show that the degree of circular polarization can reach up to 90%. Finally, I discuss the detection prospects of this signal in future experiments.

On the interpretation of the latest AMS-02 cosmic ray electron spectrum

The latest AMS-02 data on cosmic ray electrons show a break in the energy spectrum around 40 GeV, with a change in the slope of about 0.1. We perform a combined fit to the newest AMS-02 positron and electron flux data above 10 GeV using a semi-analytical diffusion model where sources includes production of pairs from pulsar wind nebulae (PWNe), electrons from supernova remnants (SNRs) and both species from spallation of hadronic cosmic rays with interstellar medium atoms. We demonstrate that within our setup the change of slope in the AMS-02 electron data is well explained by the interplay between the flux contributions from SNRs and from PWNe. In fact, the relative contribution to the data of these two populations changes by a factor of about 13 from 10 to 1000 GeV. The PWN contribution has a significance of at least 4σ, depending on the model used for the propagation, interstellar radiation field and energy losses. We checked the stability of this result against low-energy effects by solving numerically the transport equation, as well as adding possible breaks in the injection spectrum of SNRs. The effect of the energy losses alone, when the inverse Compton scattering is properly computed within a fully numerical treatment of the Klein-Nishina cross section, cannot explain the break in the e− flux data, as recently proposed in the literature.