Low Energy 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.

Detection of large-scale synchrotron radiation from the molecular envelope of the Sgr B cloud complex at the Galactic centre

ABSTRACT We present highly sensitive measurements taken with MeerKAT at 1280 MHz as well as archival Green Bank Telescope (GBT), Murchison Widefield Array, and Very Large Array (VLA) images at 333, 88, and 74 MHz. We report the detection of synchrotron radio emission from the infrared dark cloud associated with the halo of the Sgr B complex on a scale of ∼60 pc. A strong spatial correlation between low-frequency radio continuum emission and dense molecular gas, combined with spectral index measurements, indicates enhanced synchrotron emission by cosmic ray electrons. Correlation of the Fe i 6.4 keV K α line and synchrotron emission provides compelling evidence that the low energy cosmic ray electrons are responsible for producing the K α line emission. The observed synchrotron emission within the halo of the Sgr B cloud complex has a mean spectral index α ∼ −1 ± 1, which gives the magnetic field strength ∼100 µG for cloud densities nH = 104–105 cm−3, and estimated cosmic ray ionization rates between 10−13 and 10−14 s−1. Furthermore, the energy spectrum of primary cosmic ray electrons is constrained to be E−3 ± 1 for typical energies of few hundred MeV. The extrapolation of this spectrum to higher energies is consistent with X-ray and γ-ray emission detected from this cloud. These measurements have important implications on the role that high cosmic ray electron fluxes at the Galactic centre play in production of radio synchrotron emission, the Fe i K α line emission at 6.4 keV, and ∼GeV γ-ray emission throughout the Central Molecular Zone.

The Orion-Taurus ridge: A synchrotron radio loop at the edge of the Orion-Eridanus superbubble

Large-scale synchrotron loops are recognized as the main source of diffuse radio-continuum emission in the Galaxy at intermediate and high Galactic latitudes, yet their origin remain unexplained. For the first time, using a combination of multi-frequency data in the radio band of total and polarized intensities, we were able to associate one arc, hereafter, the Orion-Taurus ridge, with the wall of the most prominent stellar-feedback blown shell in the Solar neighborhood, namely, the Orion-Eridanus superbubble. We traced the Orion-Taurus ridge using 3D maps of interstellar dust extinction and column-density maps of molecular gas,NH2. We found the Orion-Taurus ridge at a distance of 400 pc, with a plane-of-the-sky extent of 180 pc. Its medianNH2value is (1.4−0.6+2.6) × 1021cm−2. Thanks to the broadband observations below 100 MHz of the Long Wavelength Array (LWA), we were also able to compute the low-frequency spectral-index map of synchrotron emissivity,β, in the Orion-Taurus ridge. We found a flat distribution ofβwith a median value of −2.24−0.02+0.03, which we interpreted in terms of depletion of low-energy (< GeV) cosmic-ray electrons in recent supernova remnants (105–106yr). Our results are consistent with plane-of-the-sky magnetic-field strengths in the Orion-Taurus ridge greater than a few tens of μG (> 30 − 40 μG). We report the first detection of diffuse synchrotron emission from cold-neutral, partly molecular gas in the surroundings of the Orion-Eridanus superbubble. This observation opens a new perspective for studies of the multiphase and magnetized interstellar medium with the advent of future high-sensitivity radio facilities, such as theC-Band All-Sky Survey (C-BASS) and the Square Kilometre Array (SKA).

On the effects of dynamical turbulence on the perpendicular diffusion of low-energy cosmic ray electrons

Diffusion perpendicular to the heliospheric magnetic field plays an integral role in the transport of charged particles in the heliosphere. In this study the perpendicular diffusion coefficient of low-energy cosmic ray electrons is calculated, using an equation derived from the random ballistic decorrelation interpretation of nonlinear guiding centre theory. An observationally motivated 2D turbulence power spectrum is assumed and the effects of various turbulence inputs on the resulting perpendicular diffusion coefficient are investigated. The perpendicular diffusion coefficients are first determined at 1 AU, for both magnetostatic and dynamical turbulence conditions. These solutions are also evaluated for radial distances of 0.1 AU to 10 AU to further investigate the values of the perpendicular diffusion coefficients in the very inner heliosphere. The results of this study show that the dissipation range of the turbulence power spectrum provides a negligible contribution towards the perpendicular diffusion coefficient, and that solutions derived using only the energy containing range serve as good approximations for solutions derived assuming the full 2D turbulence power spectrum. Finally, it is shown that the effects of dynamical turbulence, as considered in the present study, do not affect the perpendicular diffusion coefficients derived from the scattering theory considered here.

SHOCK ACCELERATION MODEL WITH POSTSHOCK TURBULENCE FOR GIANT RADIO RELICS

We explore the shock acceleration model for giant radio relics, in which relativistic electrons are accelerated via diffusive shock acceleration (DSA) by merger-driven shocks in the outskirts of galaxy clusters. In addition to DSA, turbulent acceleration by compressive MHD mode downstream of the shock is included as well as energy losses of postshock electrons by Coulomb scattering, synchrotron emission, and inverse Compton scattering off the cosmic background radiation. Considering that only a small fraction of merging clusters host radio relics, we favor the reacceleration scenario in which radio relics are generated preferentially when shocks encounter the regions containing low-energy ($\gamma_{\rm e} \lesssim 300$) cosmic ray electrons (CRe). We perform time-dependent DSA simulations of spherically expanding shocks with physical parameters relevant for the Sausage radio relic, and calculate the radio synchrotron emission from the accelerated CRe. We find that significant level of postshock turbulent acceleration is required in order to reproduce broad profiles of the observed radio flux densities of the Sausage relic. Moreover, the spectral curvature in the observed integrated radio spectrum can be explained, if the putative shock should have swept up and exited out of the preshock region of fossil CRe about 10~Myr ago.

HARD X-RAY MORPHOLOGICAL AND SPECTRAL STUDIES OF THE GALACTIC CENTER MOLECULAR CLOUD SGR B2: CONSTRAINING PAST SGR A⋆ FLARING ACTIVITY

Galactic Center (GC) molecular cloud Sgr B2 is the best manifestation of an X-ray reflection nebula (XRN) reprocessing a past giant outburst from the supermassive black hole Sgr A*. Alternatively, Sgr B2 could be illuminated by low-energy cosmic ray electrons (LECRe) or protons (LECRp). In 2013, NuSTAR for the first time resolved Sgr B2 hard X-ray emission on sub-arcminute scales. Two prominent features are detected above 10 keV - a newly emerging cloud G0.66-0.13 and the central 90" radius region containing two compact cores Sgr B2(M) and Sgr B2(N) surrounded by diffuse emission. It is inconclusive whether the remaining level of Sgr B2 emission is still decreasing or has reached a constant background level. A decreasing Fe K$\alpha$ emission can be best explained by XRN while a constant background emission can be best explained by LECRp. In the XRN scenario, the 3-79 keV Sgr B2 spectrum can well constrain the past Sgr A* outburst, resulting in an outburst spectrum with a peak luminosity of $L_{3-79\rm~keV} \sim 5\times10^{38} \rm~erg~s^{-1}$ derived from the maximum Compton-scattered continuum and the Fe K$\alpha$ emission consistently. The XRN scenario is preferred by the fast variability of G0.66-0.13, which could be a molecular clump located in the Sgr B2 envelope reflecting the same Sgr A* outburst. In the LECRp scenario, we derived the required CR ion power $dW/dt=(1-4)\times10^{39}\rm~erg~s^{-1}$ and the CR ionization rate $\zeta_{H}=(6-10)\times 10^{-15}\rm~H^{-1}~s^{-1}$. The Sgr B2 background level X-ray emission will be a powerful tool to constrain GC CR population.

The nature of the low-frequency emission of M 51

Context. Low-frequency radio continuum observations (<300 MHz) can provide valuable information on the propagation of low-energy cosmic ray electrons (CRE). Nearby spiral galaxies have hardly been studied in this frequency range because of the technical challenges of low-frequency radio interferometry. This is now changing with the start of operations of LOFAR. Aims. We aim to study the propagation of low-energy CRE in the interarm regions and the extended disk of the nearly face-on spiral galaxy Messier 51. We also search for polarisation in M 51 and other extragalactic sources in the field. Methods. The grand-design spiral galaxy M 51 was observed with the LOFAR High Frequency Antennas (HBA) and imaged in total intensity and polarisation. This observation covered the frequencies between 115 MHz and 175 MHz with 244 subbands of 8 channels each, resulting in 1952 channels. This allowed us to use RM synthesis to search for polarisation. Results. We produced an image of total emission of M 51 at the mean frequency of 151 MHz with 20′′ resolution and 0.3 mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300 MHz so far. The integrated spectrum of total radio emission is described well by a power law, while flat spectral indices in the central region indicate thermal absorption. We observe that the disk extends out to 16 kpc and see a break in the radial profile near the optical radius of the disk. The radial scale lengths in the inner and outer disks are greater at 151 MHz, and the break is smoother at 151 MHz than those observed at 1.4 GHz. The arm-interarm contrast is lower at 151 MHz than at 1400 MHz, indicating propagation of CRE from spiral arms into interarm regions. The correlations between the images of radio emission at 151 MHz and 1400 MHz and the FIR emission at 70 μm reveal breaks on scales of 1.4 and 0.7 kpc, respectively. The total (equipartition) magnetic field strength decreases from about 28 μG in the central region to about 10 μG at 10 kpc radius. No significant polarisation was detected from M 51, owing to severe Faraday depolarisation. Six extragalactic sources are detected in polarisation in the M 51 field of 4.1◦ × 4.1◦ size. Two sources show complex structures in Faraday space. Conclusions. Our main results, the scale lengths of the inner and outer disks at 151 MHz and 1.4 GHz, arm-interarm contrast, and the break scales of the radio-FIR correlations, can be explained consistently by CRE diffusion, leading to a longer propagation length of CRE of lower energy. The distribution of CRE sources drops sharply at about 10 kpc radius, where the star formation rate also decreases sharply. We find evidence that thermal absorption is primarily caused by H ii regions. The non-detection of polarisation from M 51 at 151 MHz is consistent with the estimates of Faraday depolarisation. Future searches for polarised emission in this frequency range should concentrate on regions with low star formation rates.

Diffusion of cosmic-ray electrons in the Galactic centre molecular cloud G0.13–0.13

AbstractThe Galactic center (GC) molecular cloud G0.13–0.13 exhibits a shell morphology in CS J = (1 − 0), with ∼ 105 solar masses and expansion speed ∼ 20 km s−1, yielding a total kinetic energy ∼ 1051 erg. Its morphology is also suggestive of an interaction with the nonthermal filaments of the GC arc. 74 MHz emission indicates the presence of a substantial population of low energy electrons permeating the cloud, which could either be produced by the interaction with the arc or accelerated in the shock waves responsible for the cloud's expansion. These scenarios are explored using time dependent diffusion models.With these diffusion models, we determine the penetration of low-energy cosmic-ray electrons accelerated into G0.13–0.13 and calculate the spatial distribution of the cosmic-ray ionization and heating rates. We show that the 6.4 keV Fe Kα line emission associated with the electron population provides an observational diagnostic to distinguish these two acceleration scenarios.We discuss the implications of our results for understanding the distinct character of clouds in the central molecular zone compared to clouds in the Galactic disk, and how GC nonthermal filaments interact with molecular clouds.

74 MHz Nonthermal Emission from Molecular Clouds: Evidence for a Cosmic Ray Dominated Region at the Galactic Center

We present 74 MHz radio continuum observations of the Galactic center region. These measurements show nonthermal radio emission arising from molecular clouds that is unaffected by free–free absorption along the line of sight. We focus on one cloud, G0.13-0.13, representative of the population of molecular clouds that are spatially correlated with steep spectrum (α(327MHz)(74MHz) = 1.3 ± 0.3) nonthermal emission from the Galactic center region. This cloud lies adjacent to the nonthermal radio filaments of the Arc near l 0.2° and is a strong source of 74 MHz continuum, SiO (2-1), and Fe I Kα 6.4 keV line emission. This three-way correlation provides the most compelling evidence yet that relativistic electrons, here traced by 74 MHz emission, are physically associated with the G0.13-0.13 molecular cloud and that low-energy cosmic ray electrons are responsible for the Fe I Kα line emission. The high cosmic ray ionization rate 10(–1)3 s(–1) H(–1) is responsible for heating the molecular gas to high temperatures and allows the disturbed gas to maintain a high-velocity dispersion. Large velocity gradient (LVG) modeling of multitransition SiO observations of this cloud implies H2 densities 10(4–5) cm(–3) and high temperatures. The lower limit to the temperature of G0.13-0.13 is 100 K, whereas the upper limit is as high as 1000 K. Lastly, we used a time-dependent chemical model in which cosmic rays drive the chemistry of the gas to investigate for molecular line diagnostics of cosmic ray heating. When the cloud reaches chemical equilibrium, the abundance ratios of HCN/HNC and N2H+/HCO+ are consistent with measured values. In addition, significant abundance of SiO is predicted in the cosmic ray dominated region of the Galactic center. We discuss different possibilities to account for the origin of widespread SiO emission detected from Galactic center molecular clouds.

LOW-FREQUENCY RADIO-FIR CORRELATION IN NORMAL GALAXIES AT ∼1 kpc SCALES

We study the radio--FIR correlation between the nonthermal (synchrotron) radio continuum emission at \lambda 90 cm (333 MHz) and the far infrared emission due to cool (~20 K) dust at \lambda 70\mu m in spatially resolved normal galaxies at scales of ~1 kpc. The slope of the radio--FIR correlation significantly differs between the arm and interarm regions. However, this change is not evident at a lower wavelength of \lambda 20 cm (1.4 GHz). We find the slope of the correlation in the arm to be 0.8 \pm 0.12 and we use this to determine the coupling between equipartition magnetic field (B_{eq}) and gas density (\rho_{gas}) as B_{eq} \propto \rho_{gas}^{0.51 \pm 0.12}. This is close to what is predicted by MHD simulations of turbulent ISM, provided the same region produces both the radio and far infrared emission. We argue that at 1 kpc scales this condition is satisfied for radio emission at 1.4 GHz and may not be satisfied at 333 MHz. Change of slope observed in the interarm region could be caused by propagation of low energy (~1.5 GeV) and long lived (~ 10^8 yr) cosmic ray electrons at 333 MHz.

Magnetism in galaxies – Observational overview and next generation radio telescopes

AbstractThe strength and structure of cosmic magnetic fields is best studied by observations of radio continuum emission, its polarization and its Faraday rotation. Fields with a well-ordered spiral structure exist in many types of galaxies. Total field strengths in spiral arms and bars are 20–30 μG and dynamically important. Strong fields in central regions can drive gas inflows towards an active nucleus. The strongest regular fields (10–15 μG) are found in interarm regions, sometimes forming “magnetic spiral arms” between the optical arms. The typical degree of polarization is a few % in spiral arms, but high (up to 50%) in interarm regions. The detailed field structures suggest interaction with gas flows. Faraday rotation measures of the polarization vectors reveals large-scale patterns in several spiral galaxies which are regarded as signatures of large-scale (coherent) fields generated by dynamos. – Polarization observations with the forthcoming large radio telescopes will open a new era in the observation of magnetic fields and should help to understand their origin. Low-frequency radio synchrotron emission traces low-energy cosmic ray electrons which can propagate further away from their origin. LOFAR (30–240 MHz) will allow us to map the structure of weak magnetic fields in the outer regions and halos of galaxies, in galaxy clusters and in the Milky Way. Polarization at higher frequencies (1–10 GHz), to be observed with the EVLA, MeerKAT, APERTIF and the SKA, will trace magnetic fields in the disks and central regions of galaxies in unprecedented detail. All-sky surveys of Faraday rotation measures towards a dense grid of polarized background sources with ASKAP and the SKA are dedicated to measure magnetic fields in distant intervening galaxies and clusters, and will be used to model the overall structure and strength of the magnetic field in the Milky Way.

FADING HARD X-RAY EMISSION FROM THE GALACTIC CENTER MOLECULAR CLOUD Sgr B2

The centre of our Galaxy harbours a 4 million solar mass black hole that is unusually quiet: its present X-ray luminosity is more than 10 orders of magnitude less than its Eddington luminosity. The observation of iron fluorescence and hard X-ray emission from some of the massive molecular clouds surrounding the Galactic Centre has been interpreted as an echo of a past flare. Alternatively, low-energy cosmic rays propagating inside the clouds might account for the observed emission, through inverse bremsstrahlung of low energy ions or bremsstrahlung emission of low energy electrons. Here we report the observation of a clear decay of the hard X-ray emission from the molecular cloud Sgr B2 during the past 7 years thanks to more than 20 Ms of INTEGRAL exposure. The measured decay time is compatible with the light crossing time of the molecular cloud core . Such a short timescale rules out inverse bremsstrahlung by cosmic-ray ions as the origin of the X ray emission. We also obtained 2-100 keV broadband X-ray spectra by combining INTEGRAL and XMM-Newton data and compared them with detailed models of X-ray emission due to irradiation of molecular gas by (i) low-energy cosmic-ray electrons and (ii) hard X-rays. Both models can reproduce the data equally well, but the time variability constraints and the huge cosmic ray electron luminosity required to explain the observed hard X-ray emission strongly favor the scenario in which the diffuse emission of Sgr B2 is scattered and reprocessed radiation emitted in the past by Sgr A*. Using recent parallax measurements that place Sgr B2 in front of Sgr A*, we find that the period of intense activity of Sgr A* ended between 75 and 155 years ago.

On the determination of energy spectra of MeV electrons by the Ulysses COSPIN/KET

The Ulysses mission has provided a wealth of data, particularly regarding the transport of low-energy cosmic ray electrons. These data have been used to derive significant constraints for the anisotropic spatial diffusion of these particles. Detailed model simulations allowed, in addition, to determine the relative contributions of galactic and Jovian electrons to the total flux at a given time and position in the heliosphere. Despite these insights, energy spectra have not been reliably determined as yet. This is a consequence of the uncertainty due to a background connected to proton interactions with the spacecraft. Recently, however, it was demonstrated that this uncertainty can, with some difficulty, be reduced, thus opening the opportunity to understand such spectra in the energy range 3–30 MeV, i.e., the part mostly dominated by Jovian electrons. We present results of a corresponding re-analysis of COSPIN/KET data.

Modulation of low-energy cosmic ray electrons in the outer heliosphere

A drift-modulation model with a solar wind termination shock (TS), including a jovian electron source, was used to study the effects of this heliospheric shock on electron radial intensity profiles, with emphasis on the outer heliosphere. Electron measurements between 1977 and late 2001 at 4–8 MeV and up to ∼82 AU from the Voyager spacecraft make it possible to compare computations with observations using new diffusion coefficients and a revised local interstellar electron spectrum (LIS). Compatibility between the model and the measurements are produced with the shock radius at ∼90 AU, the shock compression ratio at ∼3, and the outer boundary radius between 120 and 140 AU. The small radial gradients observed for these electrons between 15 and 82 AU can easily be reproduced with the consequence that a considerable amount of modulation must take place in the region between the TS and the modulation boundary, depending on the energies considered and the value of the LIS.

The Origin of X-Ray Emission from a Galactic Center Molecular Cloud: Low-Energy Cosmic-Ray Electrons

The Galactic center region near l ≈ 02 hosts a mixture of nonthermal linear filaments and thermal radio continuum features associated with the radio arc. Chandra observations of this region reveal an X-ray filament and diffuse emission with an extent of roughly 60'' × 2'' and 5' × 3', respectively. The X-ray filament lies at the edge of the nonthermal radio filaments and the dense molecular shell G0.13-0.13 that has an unusually high kinetic temperature ≥70 K. These observations demonstrate that the G0.13-0.13 molecular cloud and the nonthermal radio filaments of the arc are interacting. The diffuse X-ray emission is correlated with the molecular shell and is fitted either by two-temperature (1 and 10 keV) thermal emission or by power-law and 1 keV thermal gas. Fluorescent 6.4 keV line emission is also detected throughout the molecular shell. This cloud coincides within the error circle of a steady unidentified EGRET source, 3EG J1746-2851. We argue that low-energy cosmic-ray electrons produce the power-law continuum by bremsstrahlung and 6.4 keV line emission from the filament and the diffuse cloud with the implication on the origin of the Galactic ridge X-ray emission. The strong 6.4 keV Fe line emission seen from other Galactic center clouds could be produced in a similar fashion rather than via fluorescent emission induced by a transient hard X-ray source in the Galactic center. In addition, heating by ionization induced by low-energy cosmic-ray electrons are also responsible for the high temperature of G0.13-0.13. The gamma-ray source is a result of bremsstrahlung by the high-energy tail of the electron energy distribution.

Synchrotron self-Comptonized emission of low energy cosmic ray electrons in the Universe

Most of the Universe's populations of low energy cosmic ray electrons in the energy range of 1-100 MeV still manage to elude from detection by our instruments, since their synchrotron emission is at too low frequencies. We investigate a mechanism which can lead to observable emission of such electron populations: synchrotron-self Comptonization (SSC). The inverse Compton (IC) scattering can shift an otherwise unobservable low-frequency 10 kHz-10 MHz photons into observable radio, infrared (IR) or even more energetic wave-bands. The resulting emission should be polarized. We also consider IC scattering of the cosmic microwave background (CMB) and the cosmic radio background (CRB). Electron spectral aging due to synchrotron, IC and adiabatic losses or gains influences the resulting spectrum. The predicted radiation spectrum is a strong function of the history of the source, of the low energy spectrum of relativistic electrons, and of redshift. It has typically two maxima, and a decrement in between at CMB frequencies. Detection will give a sensitive probe of the environment of radio galaxies. We demonstrate that the fossil remnants of powerful radio galaxies are promising detection candidates, especially when they are embedded in a dense intra-cluster medium (ICM). GHz peaked sources (GPS) have very low SSC luminosities, which may, however, extend into the X-ray or even the gamma ray regime. Clusters of galaxies with relativistic electron populations may be another detectable sources. Fossil radio plasma released by our own Galaxy could be revealed by its large angular scale SSC flux. We discuss the expected detectability of these sources with new and upcoming instruments as LOFAR, GMRT, EVLA, ATA, ALMA, PLANCK, and HERSCHEL.

ASCA observation of X-ray emission from the Galactic ridge

The recent Chandra result confirmed the diffuse origin for the most part of the extended X-ray emission from the Galactic ridge. The ASCA spectrum of the emission shows a hard continuum (kT ∼ 10 keV if thermal) and the emission lines from highly-ionized ions of various elements up to iron. The thermal origin hypothesis contains fundamental problems concerning heating and confinement of the plasmas, and the energy source. This paper presents the results of renewed analysis of the ASCA SIS spectra of the ridge emission at three places, l ∼ 28.5°, 10° and near the Galactic Center. All the observed spectra are found to be essentially identical in shape, indicating the same emission mechanism regardless of the brightness (or the volume emissivity). Thermal emission models, either non-equilibrium or equilibrium ionization, cannot self-consistently explain the observed H-lile to He-like K-line intensity ratios of Si, S and Fe. In addition, the He-like and H-like Fe K-lines are significantly broadened, corresponding to a velocity dispersion of a few thousand km s - 1 . This is much higher than the thermal or bulk motion velocity for plasmas of kT ∼ 10 keV, and provides strong evidence against the thermal origin of the lines. An alternative possibility of line emission by charge exchange interaction of low-energy cosmic-ray heavy ions has been proposed, which gives natural explanation of the observed line broadening. The low-energy cosmic-ray origin is also consistent with the presence of a non-thermal component in the ridge emission, most probably due to low-energy cosmic-ray electrons. Heavy ions must be accelerated simultaneously with electrons.

On the Origin of the Iron K Line in the Spectrum of the Galactic X‐Ray Background

We propose a mechanism for the origin of the Galactic ridge X-ray background that naturally explains the properties of the Fe K line, specifically the detection of the centroid line energy below 6.7 keV and the apparent broadness of the line. Motivated by recent evidence of nonthermal components in the spectrum of the Galactic X-ray/γ-ray background, we consider a model that is a mixture of thermal plasma components of perhaps supernova origin and nonthermal emission from the interaction of low-energy cosmic-ray electrons (LECRe) with the interstellar medium. The LECRe may be accelerated in supernova explosions or by ambient interstellar plasma turbulence. Atomic collisions of fast electrons produce characteristic nonthermal, narrow X-ray emission lines that can explain the complex Galactic background spectrum. Using the ASCA GIS archival data from the Scutum arm region, we show that a two-temperature thermal plasma model with kT ~ 0.6 and ~2.8 keV, plus a LECRe component models the data satisfactorily. Our analysis rules out a purely nonthermal origin for the emission. It also rules out a significant contribution from low-energy cosmic-ray ions, because their nonthermal X-ray production would be accompanied by a nuclear γ-ray line diffuse emission exceeding the upper limits obtained using OSSE, as well as by an excessive Galaxy-wide Be production rate. The proposed model naturally explains the observed complex line features and removes the difficulties associated with previous interpretations of the data which evoked a very hot thermal component (kT ~ 7 keV).

Inverse Compton Scattering as the Source of Diffuse Extreme‐Ultraviolet Emission in the Coma Cluster of Galaxies

We have examined the hypothesis that the majority of the diffuse EUV flux in the Coma Cluster is due to inverse Compton scattering of low-energy cosmic-ray electrons (0.16 < < 0.31 GeV) against the 3 K blackbody background. We present data on the two-dimensional spatial distribution of the EUV flux and show that these data provide strong support for a nonthermal origin for the EUV flux. However, we show that this emission cannot be produced by an extrapolation to lower energies of the observed synchrotron radio emitting electrons and that an additional component of low-energy cosmic-ray electrons is required.

Extreme-Ultraviolet Emission from Clusters of Galaxies: Inverse Compton Radiation from a Relic Population of Cosmic-Ray Electrons?

We suggest that the luminous extreme-ultraviolet (EUV) emission that has been detected recently from clusters of galaxies is inverse Compton scattering of cosmic microwave background radiation by low-energy cosmic-ray electrons in the intracluster medium. The cosmic-ray electrons would have Lorentz factors of γ~300 and would lose energy primarily by emitting EUV radiation. These particles have lifetimes comparable to the Hubble time; thus, the electrons might represent a relic population of cosmic rays produced by nonthermal activity over the history of the cluster. The inverse Compton model naturally explains the observed increase in the ratio of EUV to X-ray emission with radius in clusters. The required energy in cosmic-ray electrons is typically 1%-10% of the thermal energy content of the intracluster gas. We suggest that the cosmic-ray electrons might have been produced by supernovae in galaxies, by radio galaxies, or by particle acceleration in intracluster shocks.