Galactic Cosmic Ray Electrons Research Articles

On the 3D Transport of Low-energy Galactic Cosmic-ray and Jovian Electrons in the Inner Heliosphere in the Presence of a Fisk-type Heliospheric Magnetic Field

Modeling the transport of low-energy (1−10 MeV) cosmic-ray electrons can lead to valuable insights as to the behavior of the heliospheric magnetic field (HMF), due to the fact that the mean free path (MFP) of these particles parallel to the HMF is significantly larger than their perpendicular MFP, and that these particles experience little in the way of drift due to gradients/curvatures in the HMF and along the heliospheric current sheet. Jovian electrons are particularly suitable for such an endeavour, as they originate from a decentral source in the inner heliosphere. To this end, the transport of these electrons is studied using a 3D, ab initio particle transport code that incorporates theoretical expressions for electron diffusion coefficients, and utilizes as inputs for these transport coefficients turbulence quantities calculated using a two-component turbulence transport model. The effects of a novel Fisk-type field on the transport of these Jovian electrons are investigated and compared with the effects of a standard Parker field.

A New Component from the Quiet Sun from Radio to Gamma Rays: Synchrotron Radiation by Galactic Cosmic-Ray Electrons

The quiet Sun, i.e., in its nonflaring state or nonflaring regions, emits thermal radiation from radio to ultraviolet. The quiet Sun also produces nonthermal radiation observed in gamma rays due to interactions of Galactic cosmic rays (GCRs) with the solar atmosphere and photons. We report on a new component: the synchrotron emission by GCR electrons in the solar magnetic field. To the best of our knowledge this is the first time this emission has been theoretically claimed and modeled. We find that the measured GCR electrons with energies from tens of GeV to a few TeV produce synchrotron emission in X-rays, which is a few orders of magnitude lower than current upper limits of the quiet Sun set by RHESSI and FOXSI, with no energy losses included. For a radially decreasing solar magnetic field we find the expected synchrotron intensity to be almost constant in the solar disk, to peak in the close proximity of the Sun, and to quickly drop away from the Sun. We also estimate the synchrotron emission from radio to gamma rays, and we compare it with current observations, especially with LOFAR. While it is negligible from radio to UV compared to the solar thermal radiation, this emission can potentially be observed at high energies with NuSTAR and more promising future FOXSI observations. This could potentially allow for constraining GCR densities and magnetic-field intensities at the Sun. This study provides a more complete description and a possible new way for understanding the quiet Sun and its environment.

The Maximum Isotropic Equivalent Energy of Gamma-Ray Bursts

In the cannonball model of gamma-ray bursts (GRBs), a highly relativistic jet of plasmoids of ordinary stellar matter that is ejected during stellar collapse or shortly after by fallback matter, produces simultaneously a GRB and a cosmic-ray burst by scattering light and charged particles in its path. This association and the observed knee at ∼1 TeV in the energy spectrum of Galactic cosmic-ray electrons imply a maximum peak energy ∼2.25 MeV in the energy spectrum of GRBs in the 1 keV–10 MeV band. Such a peak energy and the Amati correlation in GRBs imply a maximum isotropic equivalent energy release of ∼3.8 × 1054 erg in GRBs, in the 1 keV–10 MeV band. Both predictions are in good agreement with up-to-date observations.

Time and Charge-sign Dependence of the Heliospheric Modulation of Cosmic Rays

Abstract Simultaneous and continuous observations of galactic cosmic-ray electrons (e −) and positrons (e +) from the PAMELA and AMS02 space experiments are most suitable for numerical modeling studies of the heliospheric modulation of these particles below 50 GeV. A well-established comprehensive three-dimensional modulation model is applied to compute full spectra for e − and e + with the purpose of reproducing the observed ratio e +/e − for a period that covers the previous long and unusual deep solar minimum activity and the recent maximum activity phase, including the polarity reversal of the solar magnetic field. For this purpose, the very local interstellar spectra for these particles were established first. Our study is focused on how the main modulation processes, including particle drifts, and other parameters, such as the three major diffusion coefficients, evolved and how the corresponding charge-sign dependent modulation subsequently occurred. The end result of our effort is the detailed reproduction of e +/e − from 2006 to 2015, displaying both qualitative and quantitative agreement with the main observed features. Particularly, we determine how much particle drift is needed to explain the time dependence exhibited by the observed e +/e − during each solar activity phase, especially during the polarity reversal phase, when no well-defined magnetic polarity was found.

Characterization of the Particle-induced Background of XMM-Newton EPIC-pn: Short- and Long-term Variability

Abstract The particle-induced background of X-ray observatories is produced by galactic cosmic ray (GCR) primary protons, electrons, and He ions. Events due to direct interaction with the detector are usually removed by onboard processing. The interactions of these primary particles with the detector environment produce secondary particles that mimic X-ray events from celestial sources, and are much more difficult to identify. The filter-wheel closed data from the XMM-Newton EPIC-pn camera in small window mode (SWM) contains both the X-ray-like background events, and the events due to direct interactions with the primary particles. From this data, we demonstrate that X-ray-like background events are spatially correlated with the primary particle interaction. This result can be used to further characterize and reduce the non-X-ray background in silicon-based X-ray detectors in current and future missions. We also show that spectrum and pattern fractions of secondary particle events are different from those produced by cosmic X-rays.

Galactic Cosmic Ray Electrons and Positrons over a Decade of Observations in the PAMELA Experiment

The PAMELA magnetic spectrometer was launched onboard the Resurs-DK1 satellite into a near-polar Earth orbit with an altitude of 350–600 km, in order to study fluxes of cosmic ray particles and antiparticles in the wide energy range of ~80 MeV to hundreds of GeV. The results from observations of electron and positron fluxes in 2006–2016 are presented.

The Implications of Simple Estimates of the 2D Outerscale Based on Measurements of Magnetic Islands for the Modulation of Galactic Cosmic-Ray Electrons

Abstract The behavior of the 2D turbulence power spectrum at the lowest wavenumbers has a significant effect on the perpendicular diffusion coefficients of charged particles in the heliosphere derived from various scattering theories, and subsequently used to model the transport of cosmic rays (CRs) and solar energetic particles. In this regard, the lengthscale at which the energy-containing range begins, as opposed to that at which the inertial range commences, is of particular interest. This 2D outerscale has, however, never before been directly observed. Recently, direct measurements of magnetic islands in the solar wind have been reported by various authors. Assuming that these may provide an estimate of the 2D ultrascale, the direct calculation of the 2D outerscale becomes possible, should an observationally motivated form for the 2D turbulence power spectrum be employed. This study presents the results of such a calculation and provides comparisons of these with previous estimates of the 2D outerscale. Furthermore, the sensitivity of galactic CR electron intensities, calculated using a 3D ab initio CR modulation model, is demonstrated, and conclusions are drawn therefrom.

The consequence of a new ISRF model of the Milky Way on predictions for diffuse gamma-ray emission

We investigate the impact of the recently published ISRF model of the Milky Way by Popescu et al. on the CR electrons and positrons in the context of cosmic-ray transport modelling. We also study predictions for diffuse Galactic gamma radiation and underline the importance of the increased ISRF intensities. We use the PICARD code for solving the CR transport equation to obtain predictions based on Galactic cosmic-ray electron fluxes. We show that the new ISRF yields gamma-ray intensity increases, most particular at the Galactic Center and in the Galactic Plane. The impact is largest at energies around 220 GeV.

Galactic synchrotron distribution derived from 152 H ii region absorption features in the full GLEAM survey

We derive the synchrotron distribution in the Milky Way disk from HII region absorption observations over -40{\deg} < l < 40{\deg} at six frequencies of 76.2, 83.8, 91.5, 99.2, 106.9, and 114.6 MHz with the GaLactic and Extragalactic All-sky Murchison widefield array survey (GLEAM). We develop a new method of emissivity calculation by taking advantage of the Haslam et al., (1981) map and known spectral indices, which enable us to simultaneously derive the emissivity and the optical depth of HII regions at each frequency. We show our derived synchrotron emissivities based on 152 absorption features of HII regions using both the method previously adopted in the literature and our improved method. We derive the synchrotron emissivity from HII regions to the Galactic edge along the line of sight and, for the first time, derive the emissivity from HII regions to the Sun. These results provide direct information on the distribution of the Galactic magnetic field and cosmic-ray electrons for future modelling.

Galactic synchrotron emissivity measurements between 250° &lt;l&lt; 355° from the GLEAM survey with the MWA

Synchrotron emission pervades the Galactic plane at low radio frequencies, originating from cosmic ray electrons interacting with the Galactic magnetic field. Using a low-frequency radio telescope, the Murchison Widefield Array (MWA), we measure the free-free absorption of this Galactic synchrotron emission by intervening HII regions along the line of sight. These absorption measurements allow us to calculate the Galactic cosmic-ray electron emissivity behind and in front of 47 detected HII regions in the region $250^\circ < l < 355^\circ$, $|b| < 2^\circ$. We find that all average emissivities between the HII regions and the Galactic edge along the line of sight ($\epsilon_b$) are in the range of 0.24$\,\,\sim\,\,$0.70$\,\,$K$\,\,$pc$^{-1}$ with a mean of 0.40$\,\,$K$\,\,$pc$^{-1}$ and a variance of 0.10$\,\,$K$\,\,$pc$^{-1}$ at 88$\,\,$MHz. Our best model, the Two-circle model, divides the Galactic disk into three regions using two circles centring on the Galactic centre. It shows a high emissivity region near the Galactic centre, a low emissivity region near the Galactic edge, and a medium emissivity region between these two regions, contrary to the trend found by previous studies.

New constraints on modelling the random magnetic field of the MW

We extend the description of the isotropic and anisotropic random component of the small-scale magnetic field within the existing magnetic field model of the Milky Way from Jansson & Farrar, by including random realizations of the small-scale component. Using a magnetic-field power spectrum with Gaussian random fields, the NE2001 model for the thermal electrons and the Galactic cosmic-ray electron distribution from the current GALPROP model we derive full-sky maps for the total and polarized synchrotron intensity as well as the Faraday rotation-measure distribution. While previous work assumed that small-scale fluctuations average out along the line-of-sight or which only computed ensemble averages of random fields, we show that these fluctuations need to be carefully taken into account. Comparing with observational data we obtain not only good agreement with 408 MHz total and WMAP7 22 GHz polarized intensity emission maps, but also an improved agreement with Galactic foreground rotation-measure maps and power spectra, whose amplitude and shape strongly depend on the parameters of the random field. We demonstrate that a correlation length of 0≈22 pc (05 pc being a 5σ lower limit) is needed to match the slope of the observed power spectrum of Galactic foreground rotation-measure maps. Using multiple realizations allows us also to infer errors on individual observables. We find that previously-used amplitudes for random and anisotropic random magnetic field components need to be rescaled by factors of ≈0.3 and 0.6 to account for the new small-scale contributions. Our model predicts a rotation measure of −2.8±7.1 rad/m2 and 04.4±11. rad/m2 for the north and south Galactic poles respectively, in good agreement with observations. Applying our model to deflections of ultra-high-energy cosmic rays we infer a mean deflection of ≈3.5±1.1 degree for 60 EeV protons arriving from CenA.

Diffuse synchrotron emission from galactic cosmic ray electrons

Abstract. Synchrotron diffuse radiation (SDR) emission is one of the major Galactic components, in the 100 MHz up to 100 GHz frequency range. Its spectrum and sky map provide valuable measure of the galactic cosmic ray electrons (GCRE) in the relevant energy range, as well as of the strength and structure of the Galactic magnetic fields (GMF), both regular and random ones. This emission is an astrophysical sky foreground for the study of the Cosmic Microwave Background (CMB), and the extragalactic microwave measurements, and it needs to be modelled as better as possible. In this regard, in order to get an accurate description of the SDR in the Galaxy, we use – for the first time in this context – 3-dimensional GCRE models obtained by running the DRAGON code. This allows us to account for a realistic spiral arm pattern of the source distribution, demanded to get a self-consistent treatment of all relevant energy losses influencing the final synchrotron spectrum.

Solar modulation of GCR electrons over the 23rd solar minimum with PAMELA

The satellite-borne PAMELA experiment has been continuously collecting data since 15th June 2006, when it was launched from the Baikonur cosmodrome to detect the charged component of cosmic rays over a wide energy range and with unprecedented statistics. The apparatus design is particularly suited for particle and antiparticle identification. The PAMELA experiment has measured the electron spectrum at Earth in great detail, extending up to about 100 GeV and down to about 200 MeV. The galactic cosmic ray electron spectra for 2007 and 2009, i.e. measured during the A<0 solar minimum of solar cycle 23, are presented. These fluxes provide important information for the study of charge dependent solar modulation effects.

AN AB INITIO MODEL FOR THE MODULATION OF GALACTIC COSMIC-RAY ELECTRONS

The modulation of galactic cosmic-ray electrons is studied using an ab initio three-dimensional steady state cosmic-ray modulation code in which the effects of turbulence on both the diffusion and drift of these cosmic-rays are treated as self-consistently as possible. A significant refinement is that a recent two-component turbulence transport model is used. This model yields results in reasonable agreement with observations of turbulence quantities throughout the heliosphere. The sensitivity of computed galactic electron intensities to choices of various turbulence parameters pertaining to the dissipation range of the slab turbulence spectrum, and to the choice of model of dynamical turbulence, is demonstrated using diffusion coefficients derived from the quasi-linear and extended nonlinear guiding center theories. Computed electron intensities and latitude gradients are also compared with spacecraft observations.

Search for Diffuse X-Rays from the Bow Shock Region of Runaway Star BD +43 3654 with Suzaku

Abstract The bow shocks of runaway stars with strong stellar winds of over 2000 km s$ ^{-1}$ can serve as particle acceleration sites. The conversion from stellar wind luminosity into particle acceleration power has an efficiency of the same order of magnitude as those in supernova remnants, based on the radio emission from the bow shock region of runaway star BD $ +$ 43 3654 (Benaglia et al. 2010, A&amp;A, 517, L10). If this object exhibits typical characteristics, then runaway star systems can contribute a non-negligible fraction of Galactic cosmic-ray electrons. To constrain the maximum energy of accelerated particles from measurements of possible non-thermal emissions in the X-ray band, Suzaku observed BD $ +$ 43 3654 in 2011 April with an exposure of 99 ks. Because the onboard instruments have a stable and low background level, Suzaku detected a possible enhancement over the background of 7.6 $ \pm$ 3.4 counts arcmin$ ^{-2}$ at the bow shock region, where the error represents the 3 $ \sigma$ statistics only. However, the excess is not significant within the systematic errors of non-X-ray and cosmic-ray backgrounds of the X-ray Imaging Spectrometer, which are $ \pm$ 6.0 and $ \pm$ 34 counts arcmin$ ^{-2}$ , respectively, and the 3 $ \sigma$ upper limit in the X-ray luminosity from the shock region, which is 1.1 $ \times$ 10$ ^{32}$ erg s$ ^{-1}$ per 41.2 arcmin$ ^2$ in the 0.5 to 10 keV band. This result leads to three conclusions: (1) a shock-heating process is inefficient on this system; (2) the maximum energy of electrons does not exceed $ \sim$ 10 TeV, corresponding to a Lorentz factor of less than $ 10^7$ ; and (3) the magnetic field in the shock acceleration site might not be as turbulent as those in pulsar wind nebulae and supernova remnants.

Exploring the extreme universe with the Fermi Gamma-Ray Space Telescope

The Fermi orbiter, mapping the entire gamma-ray sky every three hours, monitors the cosmos for high-energy phenomena both fleeting and enduring.

The interstellar cosmic-ray electron spectrum from synchrotron radiation and direct measurements

Context: The relation between Galactic cosmic-ray electrons, magnetic fields and synchrotron radiation. Aims: We exploit synchrotron radiation to constrain the low-energy interstellar electron spectrum, using various radio surveys and connecting with electron data from Fermi-LAT and other experiments. Methods: The GALPROP programme for cosmic-ray propagation, gamma-ray and synchrotron radiation is used. Secondary electrons and positrons are included. Propagation models based on cosmic-ray and gamma-ray data are tested against synchrotron data from 22 MHz to 94 GHz. Results: The synchrotron data confirm the need for a low-energy break in the cosmic-ray electron injection spectrum. The interstellar spectrum below a few GeV has to be lower than standard models predict, and this suggests less solar modulation than usually assumed. Reacceleration models are more difficult to reconcile with the synchrotron constraints. We show that secondary leptons are important for the interpretation of synchrotron emission. We also consider a cosmic-ray propagation origin for the low-energy break. Conclusions: Exploiting the complementary information on cosmic rays and synchrotron gives unique and essential constraints on electrons, and has implications for gamma rays. This connection is especially relevant now in view of the ongoing PLANCK and Fermi missions.

Cosmic ray-dark matter scattering: a new signature of (asymmetric) dark matter in the gamma ray sky

We consider the process of scattering of Galactic cosmic-ray electrons and protons off of dark matter with the radiation of a final-state photon. This process provides a novel way to search for Galactic dark matter with gamma rays. We argue that for a generic weakly interacting massive particle, barring effects such as co-annihilation or a velocity-dependent cross section, the gamma-ray emission from cosmic-ray scattering off of dark matter is typically smaller than that from dark matter pair-annihilation. However, if dark matter particles cannot pair-annihilate, as is the case for example in asymmetric dark matter scenarios, cosmic-ray scattering with final state photon emission provides a unique window to detect a signal from dark matter with gamma rays. We estimate the expected flux level and its spectral features for a generic supersymmetric setup, and we also discuss dipolar and luminous dark matter. We show that in some cases the gamma-ray emission might be large enough to be detectable with the Fermi Large Area Telescope.

Klein–Nishina steps in the energy spectrum of galactic cosmic-ray electrons

The full Klein–Nishina cross-section of the inverse Compton scattering interactions of electrons implies a significant reduction of the electron energy loss rate compared with the Thomson limit when the electron energy exceeds the critical Klein–Nishina energy EK=γKmec2=0.27m2ec4/(kBT), where T denotes the temperature of the photon graybody distribution. As a consequence, the total radiative energy loss rate of single electrons exhibits sudden drops in the overall -dependence when the electron energy reaches the critical Klein–Nishina energy. The strength of the drop is proportional to the energy density of the photon radiation field. The diffuse galactic optical photon fields from stars of spectral type B and G-K lead to critical Klein–Nishina energies of 40 and 161 GeV, respectively. Associated with the drop in the loss rate are sudden increases (Klein–Nishina steps) in the equilibrium spectrum of cosmic-ray electrons. Because the radiative loss rate of electrons is the main ingredient in any transport model of high-energy cosmic-ray electrons, Klein–Nishina steps will modify any calculated electron equilibrium spectrum irrespective of the electron sources and the spatial transport mode. To delineate most clearly the consequences of the Klein–Nishina decreases in the radiative loss rate, we chose as an illustrative example the simplest realistic model for cosmic-ray electron dynamics in the galaxy, consisting of the competition of radiative losses and secondary production by inelastic hadron–hadron collisions. We demonstrate that the spectral structure in the FERMI and HESS data is well described and even the excess measured by ATIC might be explained by Klein–Nishina steps.

Space-based observations of gamma-rays

The γ-ray space telescopes AGILE and Fermi have been successfully launched on April 2007 and June 2008. Thanks to better capabilities than their predecessor EGRET, and in particular to their considerably improved angular resolution, they are able to provide a much more detailed view of the γ-ray sky above ~20 MeV. They have already reported on a wide variety of astrophysical phenomena and sources such as diffuse galactic emission, pulsars and other galactic sources, active galactic nuclei, gamma-ray bursts, and cosmic-ray electrons.