Cosmic Ray Electron Spectrum Research Articles

TESTING SECONDARY MODELS FOR THE ORIGIN OF RADIO MINI-HALOS IN GALAXY CLUSTERS

We present an MHD simulation of the emergence of a radio minihalo in a galaxy cluster core in a "secondary" model, where the source of the synchrotron-emitting electrons is hadronic interactions between cosmic-ray protons with the thermal intracluster gas, an alternative to the "reacceleration model" where the cosmic ray electrons are reaccelerated by turbulence induced by core sloshing, which we discussed in an earlier work. We follow the evolution of cosmic-ray electron spectra and their radio emission using passive tracer particles, taking into account the time-dependent injection of electrons from hadronic interactions and their energy losses. We find that secondary electrons in a sloshing cluster core can generate diffuse synchrotron emission with luminosity and extent similar to observed radio minihalos. However, we also find important differences with our previous work. We find that the drop in radio emission at cold fronts is less prominent than that in our reacceleration-based simulations, indicating that in this flavor of the secondary model the emission is more spatially extended than in some observed minihalos. We also explore the effect of rapid changes in the magnetic field on the radio spectrum. While the resulting spectra in some regions are steeper than expected from stationary conditions, the change is marginal, with differences in the synchrotron spectral index of 0.15–0.25, depending on the frequency band. This is a much narrower range than claimed in the best-observed minihalos and produced in the reacceleration model. Our results provide important suggestions to constrain these models with future observations.

Measurement of the total spectrum of electrons and positrons in the energy range of 300–1500 GeV in the PAMELA experiment with the aid of a sampling calorimeter and a neutron detector

A method based on the use of a sampling calorimeter was developed for measuring the total energy spectrum of electrons and positrons from high-energy cosmic rays in the PAMELA satellite-borne experiment. This made it possible to extend the range of energies accessible to measurements by the magnetic system of the PAMELA spectrometer. Themethod involves a procedure for selecting electrons on the basis of features of a secondary-particle shower in the calorimeter. The results obtained by measuring the total spectrum of cosmic-ray electrons and positrons in the energy range of 300–1500 GeV by the method in question are presented on the basis of data accumulated over a period spanning 2006 and 2013.

NONTHERMAL RADIATION FROM RELATIVISTIC ELECTRONS ACCELERATED AT SPHERICALLY EXPANDING SHOCKS

We study the evolution of the energy spectrum of cosmic-ray electrons accelerated at spherically expanding shocks with low Mach numbers and the ensuing spectral signatures imprinted in radio synchrotron emission. Time-dependent simulations of diffusive shock acceleration (DSA) of electrons in the test-particle limit have been performed for spherical shocks with parameters relevant for typical shocks in the intracluster medium. The electron and radiation spectra at the shock location can be described properly by the test-particle DSA predictions with instantaneous shock parameters. However, the volume integrated spectra of both electrons and radiation deviate significantly from the test-particle power-laws, because the shock compression ratio and the flux of injected electrons at the shock gradually decrease as the shock slows down in time. So one needs to be cautious about interpreting observed radio spectra of evolving shocks based on simple DSA models in the test-particle regime.

The NUCLEON space experiment

The NUCLEON satellite experiment is designed to investigate directly, above the atmosphere, the energy spectra of cosmic-ray nuclei and the chemical composition (Z= 1 − 30) at energy range 100GeV- 1000 TeV. The effective geometric factor is more than 0.2m 2 sr for nuclei and 0.06 m 2 sr for electrons. The planned exposition time is more than 5 years. 1. The aims of the NUCLEON experiment The NUCLEON satellite experiment is designed to investigate directly, above the atmosphere, the energy spectra of cosmic-ray nuclei and the chemical composition from 100GeV to 1000TeV (before the knee). The additional aim is the cosmic-ray electron spectrum measurement (from 20GeV to 3TeV).

The NUCLEON space experiment for direct high energy cosmic rays investigation in TeV–PeV energy range

The NUCLEON satellite experiment is designed to investigate directly, above the atmosphere, the energy spectra of cosmic-ray nuclei and the chemical composition from 100GeV to 1000TeV as well as the cosmic-ray electron spectrum from 20GeV to 3TeV. NUCLEON is planned to be launched in 2014. This mission is aimed at clarifying the essential details of cosmic-ray origin in this energy interval: number and types of sources, identification of actual nearby sources, and the investigation of the mechanisms responsible for the knee. Specific features of the NUCLEON instrument are relatively small thickness and small weight. A special method of energy determination by the silicon tracker was developed for this case. In this paper we describe a design of the instrument and the results of accelerator beam tests in terms of charge and energy resolution. The overall evidences of the capability of the apparatus to achieve the declared aims are also presented.

THE COSMIC-RAY POPULATION OF THE GALACTIC CENTRAL MOLECULAR ZONE

The conditions in the Galactic Center are often compared with those in starburst systems, which contain higher supernova rates, stronger magnetic fields, more intense radiation fields, and larger amounts of dense molecular gas than in our own Galactic disk. Interactions between such an augmented interstellar medium and cosmic rays result in brighter radio and gamma-ray emission. Here, we test how well the comparisons between the Galactic Center and starburst galaxies hold by applying a model for cosmic ray interactions to the Galactic Center to predict the resulting gamma-ray emission. The model only partially explains the observed gamma-ray and radio emission. The model for the gamma-ray spectrum agrees with the data at TeV energies but not at GeV energies. Additionally, as the fits of the model to the radio and gamma-ray spectra require significant differences in the optimal wind speed and magnetic field strength, we find that the single-zone model alone cannot account for the observed emission from the Galactic Center. Our model is improved by including a soft, additional cosmic-ray population. We assess such a cosmic ray population and its potential sources and find that a cosmic-ray electron spectrum is energetically favored over a cosmic-ray proton spectrum.

Sensitivity of CTA to dark matter signals from the Galactic Center

The Galactic Center is one of the most promising targets for indirect detection of dark matter with gamma rays. We investigate the sensitivity of the upcoming Cherenkov Telescope Array (CTA) to dark matter annihilation and decay in the Galactic Center. As the inner density profile of the Milky Way's dark matter halo is uncertain, we study the impact of the slope of the Galactic density profile, inwards of the Sun, on the prospects for detecting a dark matter signal with CTA. Adopting the Ring Method to define the signal and background regions in an ON-OFF analysis approach, we find that the sensitivity achieved by CTA to annihilation signals is strongly dependent on the inner profile slope, whereas the dependence is more mild in the case of dark matter decay. Surprisingly, we find that the optimal choice of signal and background regions is virtually independent of the assumed density profile. For the fiducial case of a Navarro-Frenk-White profile,we find that CTA will be able to probe annihilation cross-sections well below the canonical thermal relic value for dark matter masses from a few tens of GeV up to ∼ 5 TeV for annihilation to τ+τ−, and will achieve only a slightly weaker sensitivity for annihilation to bb̄ or μ+μ−. CTA will improve significantly on current sensitivity to annihilation signals for dark matter masses above ∼ 100 GeV, covering parameter space that is complementary to that probed by searches with the Fermi Large Area Telescope. The interpretation of apparent excesses in the measured cosmic-ray electron and positron spectra as signals of dark matter decay will also be testable with CTA observations of the Galactic Center. We demonstrate that both for annihilation and for decay, including spectral information for hard channels (such as μ+μ− and τ+τ−) leads to enhanced sensitivity for dark matter masses above mDM ∼ 200 GeV.

On the origin of the particle fluxes from the thunderclouds: Energy spectra analysis

Simultaneous measurements of the gamma ray differential energy spectra, electric field disturbances, and meteorological conditions provided by experimental facilities located at Mt. Aragats in Armenia allows to establish the model of particle acceleration and propagation in thunderstorm atmosphere. We present comparisons of measured and modeled thunderstorm ground enhancements (TGEs). The origin of the majority of TGEs is the MOS process —the modification of energy spectra of cosmic ray electrons in the atmospheric electric fields. The gamma ray differential energy spectra are well described by the power law function with indexes in the range for the electric field strengths 0.8−1.5 kV/cm at altitudes of 3400−5000 m a.s.l. The good agreement of the characteristics of experimental and simulated TGEs gives hope to estimate the intracloud electric field by the observed parameters of TGE gamma ray energy spectra.

Contribution from individual nearby sources to the spectrum of high-energy cosmic-ray electrons

In the last few years, very important data on high-energy cosmic-ray electrons and positrons from high-precision space-born and ground-based experiments have attracted a great deal of interest. These particles represent a unique probe for studying local comic-ray accelerators because they lose energy very rapidly. These energy losses reduce the lifetime so drastically that high-energy cosmic-ray electrons can attain the Earth only from rather local astrophysical sources. This work aims at calculating, by means of Monte Carlo simulation, the contribution from some known nearby astrophysical sources to the cosmic-ray electron/positron spectra at high energy (≥10GeV). The background to the electron energy spectrum from distant sources is determined with the help of the GALPROP code. The obtained numerical results are compared with a set of experimental data.

Spectra of primary cosmic-ray positrons and electrons in the PAMELA experiment

The PAMELA experiment is being conducted aboard the Russian satellite Resurs-DK 1, which was launched into a near-Earth circumpolar orbit on June 15, 2006. The instrument, which includes a magnetic spectrometer and an electromagnetic calorimeter (16X0), allows us to measure the fluxes of cosmic-ray electrons and positrons over a wide range of energies from ∼100 MeV to hundreds of GeV. This work presents the measurement data collected from July 2006 through January 2010 on the spectra of primary cosmic-ray electrons and positrons. At low energies, this spectrum is in good agreement with a diffusion model that includes reacceleration and damping. At high energies, the measured spectrum is harder than the one predicted.

Cosmic ray electrons, positrons and the synchrotron emission of the Galaxy: consistent analysis and implications

A multichannel analysis of cosmic ray electron and positron spectra and of the diffuse synchrotron emission of the Galaxy is performed by using the DRAGON code. This study is aimed at probing the interstellar electron source spectrum down to E ≲ 1GeV and at constraining several propagation parameters. We find that above 4GeV the e− source spectrum is compatible with a power-law of index ∼ 2.5. Below 4GeV instead it must be significantly suppressed and the total lepton spectrum is dominated by secondary particles.The positron spectrum and fraction measured below a few GeV are consistently reproduced only within low reacceleration models. We also constrain the scale-height zt of the cosmic-ray distribution using three independent (and, in two cases, original) arguments, showing that values of zt ≲ 2kpc are excluded. This result may have strong implications for particle dark matter searches.

Low Energy Cosmic Ray Positron Fraction Explained by Charge-Sign Dependent Solar Modulation

We compute cosmic ray (CR) nuclei, proton, antiproton, electron, and positron spectra below 1 TeV at Earth by means of a detailed transport description in the galaxy and in the solar system. CR spectra below 10 GeV are strongly modified by charge-sign dependent propagation effects. These depend on the polarity of the solar magnetic field and therefore vary with the solar cycle. The puzzling discrepancy between the low-energy positron fraction measured by PAMELA and AMS-01 is then easily explained by their different data-taking epochs. We reproduce the observed spectra of CR light nuclei within the same galactic and solar-system propagation model.

Cosmic ray electron and positron spectra measured with PAMELA

The PAMELA experiment is carried out on board of the satellite Resurs DK1 launched on June 15th 2006 on polar orbit (the inclination is 70, the altitude is 350-600 km). The instrument which consists of magnetic spectrometer, silicon-tungsten imaging electromagnetic calorimeter gives a possibility to measure electron and positron fluxes over wide energy range from hundreds MeVs to hundreds GeVs. Measurements made in June 2006- January 2010 are presented and compared with other results and models. Positron spectrum appears to be harder than standard diffusive propagation models predict.

Electrons and Positrons in Cosmic Rays

This review concentrates on the results obtained, over the last ten years, on the astrophysics of high-energy cosmic ray electrons and positrons. The anomalies, observed in the data of recent experiments (possible bump in the electron spectrum and the PAMELA anomaly in the positron fraction) are discussed through the systematic use of simple analytical solutions of the transport equations for cosmic ray electrons. Three main ways of explaining the origin of the anomalies are considered: the conservative way supposing the positrons to be pure secondary particles; the nearby sources like pulsars origin; and the dark matter origin. This review discusses, also, the inability to select the pulsars model or the dark matter model to explain the electron anomalies on the basis of the electron spectra with the usual large energy binning ( 15%). It is argued that the signature of nearby pulsars origin of the anomalies against the dark matter origin could be the fine structure of the cosmic ray electron spectrum predicted in the Malyshev et al. paper (2009) and which was observed in the data from the high-resolution ATIC experiment (2009-2011). To date, the high-resolution ATIC data was the only experimental result of this type published in the literature. Therefore, they should be tested by other experiments as soon as possible. Generally, there is, also, rather controversial situations between the data of the majority of recent experiments and, consequently, there is a noted urgent need for new high-precision and high-statistical experiments.

CONSTRAINTS ON THE GALACTIC HALO DARK MATTER FROMFERMI-LAT DIFFUSE MEASUREMENTS

We have performed an analysis of the diffuse gamma-ray emission with the Fermi Large Area Telescope in the Milky Way Halo region searching for a signal from dark matter annihilation or decay. In the absence of a robust dark matter signal, constraints are presented. We consider both gamma rays produced directly in the dark matter annihilation/decay and produced by inverse Compton scattering of the e+e- produced in the annihilation/decay. Conservative limits are derived requiring that the dark matter signal does not exceed the observed diffuse gamma-ray emission. A second set of more stringent limits is derived based on modeling the foreground astrophysical diffuse emission using the GALPROP code. Uncertainties in the height of the diffusive cosmic-ray halo, the distribution of the cosmic-ray sources in the Galaxy, the index of the injection cosmic-ray electron spectrum and the column density of the interstellar gas are taken into account using a profile likelihood formalism, while the parameters governing the cosmic-ray propagation have been derived from fits to local cosmic-ray data. The resulting limits impact the range of particle masses over which dark matter thermal production in the early Universe is possible, and challenge the interpretation of the PAMELA/Fermi-LAT cosmic ray anomalies as annihilation of dark matter.

OBSERVATIONS OF HIGH-ENERGY COSMIC-RAY ELECTRONS FROM 30 GeV TO 3 TeV WITH EMULSION CHAMBERS

We have performed a series of cosmic-ray electron observations using the balloon-borne emulsion chambers since 1968. While we previously reported the results from subsets of the exposures, the final results of the total exposures up to 2001 are presented here. Our successive experiments have yielded the total exposure of 8.19 m^2 sr day at the altitudes of 4.0 - 9.4 g/cm^2. The performance of the emulsion chambers was examined by accelerator beam tests and Monte-Carlo simulations, and the on-board calibrations were carried out by using the flight data. In this work we present the cosmic-ray electron spectrum in the energy range from 30 GeV to 3 TeV at the top of the atmosphere, which is well represented by a power-law function with an index of -3.28+-0.10. The observed data can be also interpreted in terms of diffusive propagation models. The evidence of cosmic-ray electrons up to 3 TeV suggests the existence of cosmic-ray electron sources at distances within ~1 kpc and times within ~1x10^5 yr ago.

Jets and gamma-ray emission from isolated accreting black holes

The large number of isolated black holes (IBHs) in the Galaxy, estimated to be 10^8, implies a very high density of 10^-4 pc^-3 and an average distance between IBHs of 10 pc. Our study shows that the magnetic flux, accumulated on the horizon of an IBH because of accretion of interstellar matter, allows the Blandford-Znajeck mechanism to be activated. Thus, electron-positron jets can be launched. We have performed 2D numerical modelling which allowed the jet power to be estimated. Their inferred properties make such jets a feasible electron accelerator which, in molecular clouds, allows electron energy to be boosted up to 1 PeV. For the conditions expected in molecular clouds the radiative cooling time should be comparable to the escape time. Thus these sources can contribute both to the population of unidentified point-like sources and to the local cosmic ray (CR) electron spectrum. The impact of the generated electron CRs depends on the diffusion rate inside molecular clouds (MCs). If the diffusion regime in a MC is similar to Galactic diffusion, the produced electrons should rapidly escape the cloud and contribute to the Galactic CR population at very high energies >100 TeV. However, due to the modest jet luminosity (at the level of 10^35 erg s^-1) and low filling factor of MC, these sources cannot make a significant contribution to the spectrum of cosmic ray electrons at lower energies. On the other hand, if the diffusion within MCs operates at a rate close to the Bohm limit, the CR electrons escaping from the source should be confined in the cloud, significantly contributing to the local density of CRs. The IC emission of these locally-generated CRs may explain the variety of gamma ray spectra detected from nearby MCs.

Escape of cosmic-ray electrons from supernova remnants

We investigate escape of cosmic ray (CR) electrons from a supernova remnant (SNR) to interstellar space. We show that CR electrons escape in order from high energies to low energies like CR nuclei, while the escape starts later than the beginning of the Sedov phase at an SNR age of 10^3 - 7*10^3 yrs and the maximum energy of runaway CR electrons is below the knee about 0.3 - 50 TeV because unlike CR nuclei, CR electrons lose their energy due to synchrotron radiation. Highest energy CR electrons will be directly probed by AMS-02, CALET, CTA and LHAASO experiments, or have been already detected by H.E.S.S. and MAGIC as a cutoff in the CR electron spectrum. Furthermore, we also calculate the spatial distribution of runaway CR electrons and their radiation spectra around SNRs. Contrary to common belief, maximum-energy photons of synchrotron radiation around 1 keV are emitted by runaway CR electrons which have been caught up by the shock. Inverse Compton scattering by runaway CR electrons can dominate the gamma-ray emission from runaway CR nuclei via pion decay, and both are detectable by CTA and LHAASO as clues to the CR origin and the amplification of magnetic fluctuations around the SNR. We also discuss middle-aged and/or old SNRs as unidentified very-high-energy gamma-ray sources.

Study of the gamma-ray spectrum from the Galactic Center in view of multi-TeV dark matter candidates

Motivated by the complex gamma-ray spectrum of the Galactic Center source now measured over five decades in energy, we revisit the issue of the role of dark matter (DM) annihilations in this interesting region. We reassess whether the emission measured by the HESS collaboration could be a signature of dark matter annihilation, and we use the Fermi LAT spectrum to model the emission from SgrA*, using power-law spectral fits. We find that good fits are achieved by a power law with an index $\ensuremath{\sim}2.5--2.6$, in combination with a spectrum similar to the one observed from pulsar population and with a spectrum from a $\ensuremath{\gtrsim}10\text{ }\text{ }\mathrm{TeV}$ DM annihilating to a mixture of $b\overline{b}$ and harder ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ channels and with boost factors of the order of a hundred. Alternatively, we also consider the combination of a log-parabola fit with the DM contribution. Finally, as both the spectrum of gamma rays from the Galactic Center and the spectrum of cosmic ray electrons exhibit a cutoff at TeV energies, we study the dark matter fits to both data sets. Constraining the spectral shape of the purported dark matter signal provides a robust way of comparing data. We find a marginal overlap only between the 99.999% C.L. regions in parameter space.

Theoretical investigation of thunderstorm induced enhancements of cosmic ray fluxes

We conducted theoretical investigation of long lasting pulses of cosmic-ray electrons and gamma-ray radiation, which are often observed during thunderstorms by particle detectors at high altitude cosmic-ray stations. These thunderstorm ground enhancements (TGEs) last several minutes, during which the flux of electrons and gamma-quants can surpass a few hundred percent over background level. We developed theoretical model and derived energetic spectrums of electrons and gamma-quants at given value of thunderstorm electric field. The model considers two following mechanisms, which can change the flux of electrons in electric field: (i) transformation of the spectrum of cosmic-ray electrons and (ii) formation of electron avalanche. Due to (i) the number of low (few MeV) energy electrons decreases and small abundance (<5% of total flux of cosmic rays) of cosmic-ray electrons with energies >10MeV emerges. The spectral fluxes of two electron components – avalanche and cosmic-ray electrons are derived, which shows that contribution of cosmic-ray electrons in total abundance of electrons is small. Consequently, the contribution of gamma ray radiation, produced by the abundance of cosmic-ray electrons is small as well. We derived the exact equations for the spectrums of these two components of gamma ray radiation and showed that spectral curve of avalanche gamma-quants can be approximated by a simple function exp(−E/E0)/E up to ∼40MeV. At higher energies gamma-ray radiation is produced by the abundance of cosmic-ray electrons and it has approximately power-law spectrum.