Population Of Relativistic Electrons Research Articles

Direct and indirect generation of Jovian quasiperiodic radio bursts by relativistic electron beams in the polar magnetosphere

[1] The Jovian polar magnetosphere has relativistic particle accelerations with quasiperiodicity (QP accelerations), which are accompanied by periodic auroral emissions and low-frequency radio bursts called QP bursts. Although there have been some observations, the generation process of QP bursts by relativistic electrons from QP accelerations has not been revealed yet. This paper presents calculated wave growth rates for the discussion of the QP radio burst generation processes based on wave generation theories. Linear growth rates were computed for free space mode waves and plasma waves in cold plasma dispersion relations, assuming that these waves are generated by relativistic electron beams in two kinds of polar source regions, as suggested by wave observations and by the ray-tracing results reported in our previous studies. One of the source regions is at high altitudes where emission frequency f is close to local right-handed extraordinary (RX) mode cutoff frequency fRX and the other is at low altitudes where f is close to local plasma frequency fp. We found that ordinary (O) mode free space waves are sufficiently amplified, with broad beaming at both of the sources in the duration of the relativistic electron populations when they have an unstable velocity distribution like a ring beam structure. This means that O mode free space waves can be generated directly from energetic electrons via the “cyclotron maser instability” (CMI) process. We also confirmed that extraordinary (X) mode free space waves are not sufficiently amplified at both of the sources in the beam duration but Z mode waves propagating along field lines from the sources toward the Jovian polar ionosphere are significantly excited. Z mode waves propagating toward the planet could be converted to free space O mode waves at a steep plasma density gradient via the “mode conversion” (MC) process. We conclude that both direct (CMI) and indirect (MC) process can generate O mode QP radio bursts with radiation characteristics consistent with those observed by spacecraft. This suggests that relativistic electrons with unstable velocity distributions are generated by the QP acceleration and that Z mode and O mode QP radio bursts are excited by these particles.

Relativistic plasma and ICM/radio source interaction

AbstractThe first detection of a diffuse radio source in a cluster of galaxies, dates back to the 1959 (Coma Cluster, Large et al. 1959). Since then, synchrotron radiating radio sources have been found in several clusters, and represent an important cluster component which is linked to the thermal gas. Such sources indicate the existence of large scale magnetic fields and of a population of relativistic electrons in the cluster volume. The observational results provide evidence that these phenomena are related to turbulence and shock-structures in the intergalactic medium, thus playing a major role in the evolution of the large scale structure in the Universe. The interaction between radio sources and cluster gas is well established in particular at the center of cooling core clusters, where feedback from AGN is a necessary ingredient to adequately describe the formation and evolution of galaxies and host clusters.

Modeling the broadband persistent emission of magnetars

In this paper, we discuss our first attempts to model the broadband persistent emission of magnetars within a self-consistent, physical scenario. We present the predictions of a synthetic model that we calculated with a new Monte Carlo 3D radiative code. The basic idea is that soft thermal photons (e.g. emitted by the star surface) can experience resonant cyclotron upscattering by a population of relativistic electrons treated in the twisted magnetosphere. Our code is specifically tailored to work in the ultra-magnetized regime; polarization and QED effects are consistently accounted for, as well different configurations for the magnetosphere. We discuss the predicted spectral properties in the 0.1–1000 keV range, the polarization properties, and we present the model application to a sample of magnetars soft X-ray spectra.

SPECTRAL INDEX STUDIES OF THE DIFFUSE RADIO EMISSION IN ABELL 2256: IMPLICATIONS FOR MERGER ACTIVITY

We present a multi-wavelength analysis of the merging rich cluster of galaxies Abell 2256. We have observed A2256 at 150 MHz using the Giant Metrewave Radio Telescope and successfully detected the diffuse radio halo and the relic emission over an extent $\sim1.2$ Mpc$^2$. Using this 150 MHz image and the images made using archival observations from the VLA (1369 MHz) and the WSRT (350 MHz), we have produced spectral index images of the diffuse radio emission in A2256. These spectral index images show a distribution of flat spectral index (S$\propto\nu^\alpha$, $\alpha$ in the range -0.7 to -0.9) plasma in the NW of the cluster centre. Regions showing steep spectral indices ($\alpha$ in the range -1.0 to -2.3) are toward the SE of the cluster centre. These spectral indices indicate synchrotron life times for the relativistic plasmas in the range 0.08 - 0.4 Gyr. We interpret this spectral behaviour as resulting from a merger event along the direction SE to NW within the last 0.5 Gyr or so. A shock may be responsible for the NW relic in A2256 and the Mpc scale radio halo towards the SE is likely to be generated by the turbulence injected by mergers. Furthermore, the diffuse radio emission shows spectral steepening toward lower frequencies. This low frequency spectral steepening is consistent with a combination of spectra from two populations of relativistic electrons created at two epochs (two mergers) within the last $\sim$0.5 Gyr. Earlier interpretations of the X-ray and the optical data also suggested that there were two mergers in Abell 2256 in the last 0.5 Gyr, consistent with the current findings. Also highlighted in this study is the futility of correlating the average temperatures of thermal gas and the average spectral indices of diffuse radio emission in respective clusters.

MODELING X–RAY EMISSION OF A STRAIGHT JET: PKS 0920-397

We summarize a study of PKS 0920-397 using our 42 ks Chandra observation in conjunction with our ATCA 20GHz image, and HST/ACS F814W and F475W images. We investigate the hypothesis that the jet X–ray emission is due to inverse-Compton (IC) scattering on the cosmic microwave background (CMB) from the same population of relativistic electrons that give rise to the radio emission. To calculate parameters intrinsic to the source, one must finesse the fact that we do not know the true angle of the jet to our line of sight. Typical assumptions are that the Doppler factor equals the bulk Lorentz factor, or that the Lorentz factor takes some fixed numerical value. While giving useful estimates, neither assumption can be exact in general. We try different constraints to determine the jet quantities. It is plausible that the kinetic flux is constant along the jet, prior to a terminal hotspot or lobe, and with minimal bending of the jet. Alternatively, because PKS 0920-397 appears straight in projection on the sky, we might assume the jet maintains a constant angle to our line of sight. Either approach gives bulk Lorentz factors of 6 to 8, with kinetic energy flux of order 1046 erg s-1, and with the jet at an angle 2° to 4° from our line of sight.

Searching for the inverse-Compton emission from bright cluster-centre radio galaxies

We use deep archival Chandra and XMM-Newton observations of three of the brightest cluster-centre radio galaxies in the sky, Cygnus A, Hercules A and Hydra A, to search for inverse-Compton emission from the population of electrons responsible for the low-frequency radio emission. Using simulated observations, we derive robust estimates for the uncertainties on the normalization of an inverse-Compton component in the presence of the variations in background thermal temperature actually seen in our target objects. Using these, together with the pressures external to the lobes, we are able to place interesting upper limits on the fraction of the energy density in the lobes of Hydra A and Her A that can be provided by a population of relativistic electrons with standard properties, assuming that the magnetic field is not dominant; these limits are consistent with the long-standing idea that the energy density in these lobes is dominated by a non-radiating particle population. In Cygnus A, we find evidence in the spectra for an additional hard component over and above the expected thermal emission, which is plausibly a detection of inverse-Compton emission; even in this case, though, some additional non-radiating particles and/or a departure from our standard assumptions on the electron spectrum are necessary to allow pressure balance at the mid-point of the lobes. As this is not the case in other Fanaroff-Riley class II radio galaxies, we suggest that the rich environment of Cygnus A may have some effect on its lobe particle content.

Inverse Compton X-rays from relativistic flare electrons and positrons

In solar flares, inverse Compton scattering (ICS) of photospheric photons might give rise to detectable hard X-ray photon fluxes from the corona where ambient densities are too low for significant bremsstrahlung or recombination. Gamma-ray lines and continuum in some large flares imply the presence of the necessary ~100 MeV electrons and positrons, the latter as by-products of GeV energy ions. Recent observations of coronal hard X-ray sources in particular prompt us to reconsider here the possible contribution of ICS. We aim to evaluate the ICS X-ray fluxes to be expected from prescribed populations of relativistic electrons and positrons in the solar corona. The ultimate aim is to determine if ICS coronal X-ray sources might offer a new diagnostic window on relativistic electrons and ions in flares. We use the complete formalism of ICS to calculate X-ray fluxes from possible populations of flare primary electrons and secondary positrons, paying attention to the incident photon angular distribution near the solar surface and thus improving on the assumption of isotropy made in previous solar discussions. Both primary electrons and secondary positrons produce very hard ICS X-ray spectra. The anisotropic primary radiation field results in pronounced centre-to-limb variation in predicted fluxes and spectra, with the most intense spectra, extending to the highest photon energies, expected from limb flares. Acceptable numbers of electrons or positrons could account for RHESSI coronal X/gamma-ray sources. Some coronal X-ray sources at least might be interpreted in terms of ICS by relativistic electrons or positrons, particularly when sources appear at such low ambient densities that bremsstrahlung appears implausible.

The X-ray emission from the knots in 3C 120

3C 120 is a Seyfert galaxy with a well detected X-ray jet. We investigate the X-ray emission of its five jet knots and fit their spectral energy distributions (SEDs) from the radio to the X-ray bands with a single-zone lepton model. We find that the SEDs of knots k7, s2, and s3 can be explained by synchrotron radiation, and the X-rays are the simple extension of the radio-optical emission component, but that of the inner knot k4 requires the IC/CMB model, in which the X-rays are due to the inverse Compton scattering of the cosmic microwave background photons by relativistic electrons in the jet with a beaming factor δ∼14. The outer knot k25 is resolved into a three-part sub-structure. It is shown that the fitting of the X-rays from this knot with the IC/CMB model needs an extraordinary beaming factor δ∼15–25 for a jet at the kpc scale. If the X-rays of knot k25 are produced by synchrotron radiation similar to k7, s2, and s3, they may be contributed by a relativistic electron population whose radiations in other wavelengths are not detected.

Statistical roles of storms and substorms in changing the entire outer zone relativistic electron population

Since storms/substorms can lead to flux enhancements of relativistic electrons (E > 500 keV) in one region of the outer zone (L ∼ 2–7) and simultaneously to flux decreases in another region, the final effects of storms/substorms on changing the entire outer zone relativistic electron population are indicated by the total flux variation of the entire outer zone relativistic electrons. The total flux of the relativistic electrons is the summation of their omnidirectional integral fluxes over the entire outer zone. By analyzing the total flux variations of relativistic electrons in the entire outer zone during about 18 storms from August 1990 to March 1991, this paper investigates the statistical relationships between the total flux variations of the entire outer zone relativistic electrons and the intensities of storms/substorms. The statistical results indicate that the primary impact of a storm development is the net loss of relativistic electrons from the entire outer zone via the main phase losses of relativistic electrons, whereas the continuous intense substorm activity (average AE > 200 nT) can lead to the net increases of the relativistic electrons in the entire outer zone. Furthermore, the more intense the substorm activity, the larger the increases of the relativistic electrons, indicating that the continuous intense substorm activity can effectively supply relativistic electrons for the entire outer zone. Since the net increases of the relativistic electrons usually occur during the continuous intense substorm activity, the effective acceleration of the relativistic electron is correlated with the continuous intense substorm activity, e.g., the stochastic acceleration of electron by whistler mode chorus waves.

XMM-Newton and INTEGRAL analysis of the Ophiuchus cluster of galaxies

We investigated the non-thermal hard X-ray emission in the Ophiuchus cluster of galaxies. Our aim was to characterise the physical properties of the non-thermal component and its interaction with the cosmic microwave background. We performed spatially resolved spectroscopy and imaging using XMM-Newton data to model the thermal emission. Combining this with INTEGRAL ISGRI data, we modeled the 0.6-140 keV band total emission in the central 7 arcmin region. The models that best describe both PN and ISGRI data contain a power-law component with a photon index in a range 2.2-2.5. This component produces ~10% of the total flux in the 1-10 keV band. The pressure of the non-thermal electrons is ~1% of that of the thermal electrons. Our results support the scenario whereby a relativistic electron population, which produces the recently detected radio mini-halo in Ophiuchus, also produces the hard X-rays via inverse compton scattering of the CMB photons. The best-fit models imply a differential momentum spectrum of the relativistic electrons with a slope of 3.4-4.0 and a magnetic field strength B=0.05-0.15 microG. The lack of evidence for a recent major merger in the Ophiuchus center allows the possibility that the relativistic electrons are produced by turbulence or hadronic collisions.

Clarifying the covariant formalism for the Sunyaev-Zel’dovich effect due to relativistic nonthermal electrons

We derive the covariant formalism associated with the relativistic Sunyaev-Zel'dovich effect due to a nonthermal population of high energy electrons in clusters of galaxies. More precisely, we show that the formalism proposed by Wright in 1979, based on an empirical approach to compute the inverse Compton scattering of a population of relativistic electrons on CMB photons, can actually be reinterpreted as a Boltzmann-like equation, in the single scattering approximation.

Exploring the magnetized cosmic web through low-frequency radio emission

Recent improvements in the capabilities of low frequency radio telescopes provide a unique opportunity to study thermal and non-thermal properties of the cosmic web. We argue that the diffuse, polarized emission from giant radio relics traces structure formation shock waves and illuminates the large-scale magnetic field. To show this, we model the population of shock-accelerated relativistic electrons in high-resolution cosmological simulations of galaxy clusters and calculate the resulting radio synchrotron emission. We find that individual shock waves correspond to localized peaks in the radio surface brightness map which enables us to measure Mach numbers for these shocks. We show that the luminosities and number counts of the relics strongly depend on the magnetic field properties, the cluster mass and dynamical state. By suitably combining different cluster data, including Faraday rotation measures, we are able to constrain some macroscopic parameters of the plasma at the structure formation shocks, such as models of turbulence. We also predict upper limits for the properties of the warm-hot intergalactic medium, such as its temperature and density. We predict that the current generation of radio telescopes (LOFAR, GMRT, MWA, LWA) have the potential to discover a substantially larger sample of radio relics, with multiple relics expected for each violently merging cluster. Future experiments (SKA) should enable us to further probe the macroscopic parameters of plasma physics in clusters.

Suzaku Observations of Tycho’s Supernova Remnant

Abstract Tycho’s supernova remnant was observed by the XIS and HXD instruments onboard the Suzaku satellite on 2006 June 26–29 for 92 ks. The spectrum up to 30 keV was well fitted with a two-component model, consisting of a power-law with a photon index of 2.7 and a thermal bremsstrahlung model with a temperature of 4.7 keV. The former component can alternatively be modeled as synchrotron emission from a population of relativistic electrons with an estimated roll-off energy of around 1 keV. In the XIS spectra, in addition to the prominent Fe K$\alpha$ line (6.445 keV), we observed for the first time significant K$\alpha$ line emission from trace species Cr and Mn at energies of 5.48 keV and 5.95 keV, respectively. Faint K$\beta$ lines from Ca (4.56 keV) and Fe (7.11 keV) were also seen. The ionization states of Cr and Mn, based on their line centroids, are estimated to be similar to that of Fe K$\alpha$ (FeXV or XVI).

Observational limits on inverse Compton processes in gamma-ray bursts

Inverse Compton (IC) scattering is one of two viable mechanisms that can produce prompt non-thermal soft gamma-ray emission in gamma-ray bursts. IC requires low-energy seed photons and a population of relativistic electrons that upscatter them. The same electrons will upscatter the gamma-ray photons to even higher energies in the TeV range. Using the current upper limits on the prompt optical emission, we show that under general conservative assumption the IC mechanism suffers from an ‘energy crisis’. Namely, IC will overproduce a very high energy component that would carry much more energy than the observed prompt gamma-rays, or alternatively it will require a low-energy seed that is more energetic than the prompt gamma-rays. Our analysis is general, and it makes no assumptions on the specific mechanism that produces the relativistic electron population.

Star Formation Rates in Lyman Break Galaxies: Radio Stacking of LBGs in the COSMOS Field and the Sub‐μJy Radio Source Population

We present an analysis of the radio properties of large samples of Lyman break galaxies (LBGs) at z ~ 3, 4, and 5 from the COSMOS field. The median stacking analysis yields a statistical detection of the z ~ 3 LBGs (U-band dropouts), with a 1.4 GHz flux density of 0:90 ± 0:21 µJy. The stacked emission is unresolved, with a size <1, or a physical size <8 kpc. The total star formation rate implied by this radio luminosity is 31 ± 7Mסּ yr^(-1), based on the radio-FIR correlation in low-redshift star-forming galaxies. The star formation rate derived from a similar analysis of the UV luminosities is 17 Mסּ yr^(-1), without any correction for UV dust attenuation. The simplest conclusion is that the dust attenuation factor is 1.8 at UV wavelengths. However, this factor is considerably smaller than the standard attenuation factor of ~5, normally assumed for LBGs. We discuss potential reasons for this discrepancy, including the possibility that the dust attenuation factor at z ≥ 3 is smaller than at lower redshifts. Conversely, the radio luminosity for a given star formation rate may be systematically lower at very high redshift. Two possible causes for a suppressed radio luminosity are (1) increased inverse Compton cooling of the relativistic electron population due to scattering off the increasing CMB at high redshift or (2) cosmic-ray diffusion from systematically smaller galaxies. The radio detections of individual sources are consistent with a radio-loud AGN fraction of 0.3%. One source is identified as a very dusty, extreme starburst galaxy (a ‘‘submillimeter galaxy’’)

Qualitative estimation of magnetic storm efficiency in producing relativistic electron flux in the Earth’s outer radiation belt using geomagnetic pulsations data

It is well known that during many but not all of the geomagnetic storms enhanced fluxes of high-energy electrons are observed in the outer radiation belt. Here we examine relativistic (>2 MeV) electron fluxes measured by GOES at the synchronous orbit and on-ground observations of two types of ULF pulsations during 30 magnetic storms occurred during 1996–2000. To characterize the effectiveness of the chosen magnetic storms in producing relativistic electron fluxes, following to (Reeves, G.D., McAdams, K.L., Friedel, R.H.W., O’Brien, T.R. Acceleration and loss of relativistic electrons during geomagnetic storms. Geophys. Res. Lett. 30, doi:10.1029/2002GL016513, 2003), we calculate a ratio of the maximum daily-averaged electron flux measured during the recovery phase, to the mean pre-storm electron flux. A storm is considered an effective one if its ratio exceeds 2. We compare behavior of Pi1 and Pc5 geomagnetic pulsations during effective and non-effective storms and find a tendency for a storm efficiency to be higher when the mid-latitude Pi1 pulsations are observed for a long time during the magnetic storm main phase. We note also that the prolonged powerful Pc5 pulsation activity during the recovery phase of a magnetic storm is the necessary condition for the storm effectiveness. To interpret the found dependences, we suggest that there are two prerequisites for generating relativistic electron populations during a storm: (1) the availability of seed electrons in the magnetosphere, and Pi1 emissions are indicators of the mid-energy electron interaction with the ionosphere and (2) acceleration of the seed electrons to MeV energies, and interaction of electrons with the MHD wave activity in the Pc5 range is one of the most probable mechanisms proposed in the literature for this purpose.

Simulating cosmic rays in clusters of galaxies – III. Non-thermal scaling relations and comparison to observations

Complementary views of galaxy clusters in the radio synchrotron, hard X-ray inverse Compton and high-energy γ-ray regimes are critical in calibrating them as high-precision cosmological probes. We present predictions for scaling relations between cluster mass and these non-thermal observables. To this end, we use high-resolution simulations of a sample of galaxy clusters spanning a mass range of almost two orders of magnitudes, and follow self-consistent cosmic ray physics on top of the radiative hydrodynamics. We model relativistic electrons that are accelerated at cosmological structure formation shocks and those that are produced in hadronic interactions of cosmic rays with ambient gas protons. Calibrating the magnetic fields of our model with Faraday rotation measurements, the synchrotron emission of our relativistic electron populations matches the radio synchrotron luminosities and morphologies of observed giant radio haloes and minihaloes surprisingly well. Using the complete sample of the brightest X-ray clusters observed by ROSAT in combination with our γ-ray scaling relation, we predict GLAST to detect about ten clusters allowing for Eddington bias due to the scatter in the scaling relation. The expected brightest γ-ray clusters are Ophiuchus, Fornax, Coma, A3627, Perseus and Centaurus. The high-energy γ-ray emission above 100 MeV is dominated by pion decays resulting from hadronic cosmic ray interactions. We provide an absolute lower flux limit for the γ-ray emission of Coma in the hadronic model which can be made tighter for magnetic field values derived from rotation measurements to match the GLAST sensitivity, providing thus a unique test for the possible hadronic origin of radio haloes. Our predicted hard X-ray emission, due to inverse Compton emission of shock accelerated and hadronically produced relativistic electrons, falls short of the detections in Coma and Perseus by a factor of 50. This casts doubts on inverse Compton interpretation and reinforces the known discrepancy of magnetic field estimates from Faraday rotation measurements and those obtained by combining synchrotron and inverse Compton emission.

Radiation Belt Storm Probes: A New Mission for Space Weather Forecasting

Radiation Belt Storm Probes: A New Mission for Space Weather Forecasting

Radiation belt electron precipitation into the atmosphere: Recovery from a geomagnetic storm

Large geomagnetic storms are associated with electron population changes in the outer radiation belt and the slot region, often leading to significant increases in the relativistic electron population. The increased population decays in part through the loss, that is, precipitation from the bounce loss cone, of highly energized electrons into the middle and upper atmosphere (30–90 km). However, direct satellite observations of energetic electrons in the bounce loss cone are very rare due to its small angular width. In this study we have analyzed ground‐based subionospheric radio wave observations of electrons from the bounce loss cone at L = 3.2 during and after a geomagnetic disturbance which occurred in September 2005. Relativistic electron precipitation into the atmosphere leads to large changes in observed subionospheric amplitudes. Satellite‐observed energy spectra from the CRRES and DEMETER spacecraft were used as an input to an ionospheric chemistry and subionospheric propagation model, describing the ionospheric ionization modifications caused by precipitating electrons. We find that the peak precipitated fluxes of &gt;150 keV electrons into the atmosphere were 3500 ± 300 el cm−2 s−1 at midday and 185 ± 15 el cm−2 s−1 at midnight. For 6 d following the storm onset the midday precipitated fluxes are approximately 20 times larger than observed at midnight, consistent with observed day/night patterns of plasmaspheric hiss intensities. The variation in DEMETER observed wave power at L = 3.2 in the plasmaspheric hiss frequency band shows similar time variation to that seen in the precipitating particles. Consequently, plasmaspheric hiss with frequencies below ∼500 Hz appears to be the principal loss mechanism for energetic electrons in the inner zone of the outer radiation belts during the nonstorm time periods of this study, although off‐equatorial chorus waves could contribute when the plasmapause is L &lt; 3.0.

Soft and Hard X‐Ray Excess Emission in Abell 3112 Observed withChandra

Chandra ACIS-S observations of the galaxy cluster A3112 feature the presence of an excess of X-ray emission above the contribution from the diffuse hot gas, which can be equally well modeled with an additional non-thermal power-law model or with a low-temperature thermal model of low metal abundance. We show that the excess emission cannot be due to uncertainties in the background subtraction or in the Galactic HI column density. Calibration uncertainties in the ACIS detector that may affect our results are addressed by comparing the Chandra data to XMM MOS and PN spectra. While differences between the three instruments remain, all detect the excess in similar amounts, providing evidence against an instrumental nature of the excess. Given the presence of non-thermal radio emission near the center of A3112, we argue that the excess X-ray emission is of non-thermal nature and distributed throughout the entire X-ray bandpass, from soft to hard X-rays. The excess can be explained with the presence of a population of relativistic electrons with ~7% of the cluster's gas pressure. We also discuss a possible thermal nature of the excess, and examine the problems associated with such interpretation.