Synchrotron Spectral Index Research Articles

Template fitting of WMAP 7-year data: anomalous dust or flattening synchrotron emission?

Anomalous microwave emission at 20-40 GHz has been detected across our Galactic sky. It is highly correlated with thermal dust emission and hence it is thought to be due to spinning dust grains. Alternatively, this emission could be due to synchrotron radiation with a flattening (hard) spectral index. We cross-correlate synchrotron, free-free and thermal dust templates with the WMAP 7-year maps using synchrotron templates at both 408 MHz and 2.3 GHz to assess the amount of flat synchrotron emission that is present, and the impact that this has on the correlations with the other components. We find that there is only a small amount of flattening visible in the synchrotron spectral indices by 2.3 GHz, of around \Delta \beta ~ 0.05, and that the significant level of dust-correlated emission in the lowest WMAP bands is largely unaffected by the choice of synchrotron template, particularly at high latitudes (it decreases by only ~7 per cent when using 2.3 GHz rather than 408 MHz). This agrees with expectation if the bulk of the anomalous emission is generated by spinning dust grains.

Impact on the tensor-to-scalar ratio of incorrect Galactic foreground modelling

A key goal of many Cosmic Microwave Background experiments is the detection of gravitational waves, through their B-mode polarization signal at large scales. To extract such a signal requires modelling contamination from the Galaxy. Using the Planck experiment as an example, we investigate the impact of incorrectly modelling foregrounds on estimates of the polarized CMB, quantified by the bias in tensor-to-scalar ratio r, and optical depth tau. We use a Bayesian parameter estimation method to estimate the CMB, synchrotron, and thermal dust components from simulated observations spanning 30-353 GHz, starting from a model that fits the simulated data, returning r<0.03 at 95% confidence for an r=0 model, and r=0.09+-0.03 for an r=0.1 model. We then introduce a set of mismatches between the simulated data and assumed model. Including a curvature of the synchrotron spectral index with frequency, but assuming a power-law model, can bias r high by ~1-sigma (delta r ~ 0.03). A similar bias is seen for thermal dust with a modified black-body frequency dependence, incorrectly modelled as a power-law. If too much freedom is allowed in the model, for example fitting for spectral indices in 3 degree pixels over the sky with physically reasonable priors, we find r can be biased up to ~3-sigma high by effectively setting the indices to the wrong values. Increasing the signal-to-noise ratio by reducing parameters, or adding additional foreground data, reduces the bias. We also find that neglecting a 1% polarized free-free or spinning dust component has a negligible effect on r. These tests highlight the importance of modelling the foregrounds in a way that allows for sufficient complexity, while minimizing the number of free parameters.

Relativistic jet models for two low-luminosity radio galaxies: evidence for backflow?

We show that asymmetries in total intensity and linear polarization between the radio jets and counter-jets in two lobed Fanaroff-Riley Class I (FR I) radio galaxies, B2 0206+35 (UGC 1651) and B2 0755+37 (NGC 2484), can be accounted for if these jets are intrinsically symmetrical, with decelerating relativistic outflows surrounded by mildly relativistic backflows. Our interpretation is motivated by sensitive, well-resolved Very Large Array imaging which shows that both jets in both sources have a two-component structure transverse to their axes. Close to the jet axis, a centrally-darkened counter-jet lies opposite a centrally-brightened jet, but both are surrounded by broader collimated emission that is brighter on the counter-jet side. We have adapted our previous models of FR I jets as relativistic outflows to include an added component of symmetric backflow. We find that the observed radio emission, after subtracting contributions from the extended lobes, is well described by models in which decelerating outflows with parameters similar to those derived for jets in plumed FR I sources are surrounded by backflows containing predominantly toroidal magnetic fields. These return to within a few kpc of the galaxies with velocities of roughly 0.25c and radiate with a synchrotron spectral index close to 0.55. We discuss whether such backflow is to be expected in lobed FR I sources and suggest ways in which our hypothesis can be tested by further observations.

Expected constraints on the Galactic magnetic field usingPlanckdata

We explore in this paper the ability to constrain the Galactic magnetic field intensity and spatial distribution with the incoming data from the Planck satellite experiment. We perform realistic simulations of the Planck observations at the polarized frequency bands from 30 to 353 GHz for two all-sky surveys as expected for the nominal mission. These simulations include CMB, synchrotron and thermal dust Galactic emissions and instrumental noise. (Note that systematic effects are not considered in this paper). For the synchrotron and thermal dust Galactic emissions we use a coherent 3D model of the Galaxy describing its mater density and the magnetic field direction and intensity. We first simulate the synchrotron and dust emissions at 408 MHz and 545 GHz, respectively, and then we extrapolate them to the Planck frequency bands. We perform a likelihood analysis to compare the simulated data to a set of models obtained by varying the pitch angle of the regular magnetic field spatial distribution, the relative amplitude of the turbulent magnetic field, the radial scale of the electron and dust grain distributions, and the extrapolation spectral indices for the synchrotron and thermal dust emissions. We are able to set tight constraints on all the parameters considered. We have also found that the observed spatial variations of the synchrotron and thermal dust spectral indices should not affect our ability to recover the other parameters of the model. From this, we conclude that the Planck satellite experiment can precisely measure the main properties of the Galactic magnetic field. An accurate reconstruction of the matter distribution would require on the one hand an improved modelling of the ISM and on the other hand to use extra data sets like rotation measurements of pulsars.

Galactic foreground contributions to the 5-year Wilkinson Microwave Anisotropy Probe maps

We compute the cross correlation of the intensity and polarisation from the 5-year WMAP data in different sky-regions with respect to template maps for synchrotron, dust, and free-free emission. We derive the frequency dependence and polarisation fraction for all three components in 48 different sky regions of HEALPix (Nside=2) pixelisation. The anomalous emission associated with dust is clearly detected in intensity over the entire sky at the K (23 GHz) and Ka (33 GHz) WMAP bands, and is found to be the dominant foreground at low Galactic latitude, between b=-40 and b=+10. The synchrotron spectral index obtained from the K and Ka WMAP bands from an all-sky analysis is -3.32\pm 0.12 for intensity and -3.01\pm0.03 for the polarised intensity. The polarisation fraction of the synchrotron is constant in frequency and increases with latitude from ~5% near the Galactic plane up to ~40% in some regions at high latitude; the average value for |b|<20 is 8.6\pm1.7 (stat) \pm0.5 (sys) % while for |b|>20 it is 19.3\pm0.8 (stat) \pm 0.5 (sys) %. Anomalous dust and free-free emission appear to be relatively unpolarised...[Abridged]...the average polarisation fraction of dust-correlated emission at K-band is 3.2\pm0.9 (stat) \pm 1.5 (sys) %, or less than 5% at 95% confidence. When comparing real data with simulations, 8 regions show a detected polarisation above the 99th percentile of the distribution from simulations with no input foreground polarisation, 6 of which are detected at above 2sigma and display polarisation fractions between 2.6% and 7.2%, except for one anomalous region, which has 32\pm12%. The dust polarisation values are consistent with the expectation from spinning-dust emission, but polarised dust emission from magnetic-dipole radiation cannot be ruled out. Free-free emission was found to be unpolarised with an upper limit of 3.4% at 95% confidence.

SIMPLE FOREGROUND CLEANING ALGORITHM FOR DETECTING PRIMORDIALB-MODE POLARIZATION OF THE COSMIC MICROWAVE BACKGROUND

We reconsider the pixel-based, "template" polarized foreground removal method within the context of a next-generation, low-noise, low-resolution (0.5 degree FWHM) space-borne experiment measuring the cosmological B-mode polarization signal in the cosmic microwave background (CMB). This method was put forward by the Wilkinson Microwave Anisotropy Probe (WMAP) team and further studied by Efstathiou et al. We need at least 3 frequency channels: one is used for extracting the CMB signal, whereas the other two are used to estimate the spatial distribution of the polarized dust and synchrotron emission. No external template maps are used. We extract the tensor-to-scalar ratio (r) from simulated sky maps consisting of CMB, noise (2 micro K arcmin), and a foreground model, and find that, even for the simplest 3-frequency configuration with 60, 100, and 240 GHz, the residual bias in r is as small as Delta r~0.002. This bias is dominated by the residual synchrotron emission due to spatial variations of the synchrotron spectral index. With an extended mask with fsky=0.5, the bias is reduced further down to <0.001.

FIVE-YEARWILKINSON MICROWAVE ANISOTROPY PROBE(WMAP) OBSERVATIONS: BAYESIAN ESTIMATION OF COSMIC MICROWAVE BACKGROUND POLARIZATION MAPS

We describe a sampling method to estimate the polarized CMB signal from observed maps of the sky. We use a Metropolis-within-Gibbs algorithm to estimate the polarized CMB map, containing Q and U Stokes parameters at each pixel, and its covariance matrix. These can be used as inputs for cosmological analyses. The polarized sky signal is parameterized as the sum of three components: CMB, synchrotron emission, and thermal dust emission. The polarized Galactic components are modeled with spatially varying power law spectral indices for the synchrotron, and a fixed power law for the dust, and their component maps are estimated as by-products. We apply the method to simulated low resolution maps with pixels of side 7.2 degrees, using diagonal and full noise realizations drawn from the WMAP noise matrices. The CMB maps are recovered with goodness of fit consistent with errors. Computing the likelihood of the E-mode power in the maps as a function of optical depth to reionization, tau, for fixed temperature anisotropy power, we recover tau=0.091+-0.019 for a simulation with input tau=0.1, and mean tau=0.098 averaged over 10 simulations. A `null' simulation with no polarized CMB signal has maximum likelihood consistent with tau=0. The method is applied to the five-year WMAP data, using the K, Ka, Q and V channels. We find tau=0.090+-0.019, compared to tau=0.086+-0.016 from the template-cleaned maps used in the primary WMAP analysis. The synchrotron spectral index, beta, averaged over high signal-to-noise pixels with standard deviation sigma(beta)<0.25, but excluding ~6% of the sky masked in the Galactic plane, is -3.03+-0.04. This estimate does not vary significantly with Galactic latitude, although includes an informative prior.

DIFFUSE RADIO EMISSION IN ABELL 754

We present a low-frequency study of the diffuse radio emission in the galaxy cluster A754. We present a new 150 MHz image of the galaxy cluster A754 made with the Giant Metrewave Radio Telescope and discuss the detection of four diffuse features. We compare the 150 MHz image with the images at 74, 330, and 1363 MHz; one new diffuse feature is detected. The flux density upper limits at 330 and 1363 MHz imply a synchrotron spectral index, α>2 (S ∝ ν–α), for the new feature. The west relic detected at 74 MHz is not detected at 150 MHz and is thus consistent with its nondetection at 1363 MHz and 330 MHz. Integrated spectra of all the diffuse features are presented. The fourth diffuse feature is located along the proposed merger axis in A754 and 0.7 Mpc away from the peak of X-ray emission; we refer to it as a relic. We have made use of the framework of the adiabatic compression model to obtain spectra. We show that the spectrum of the fourth diffuse feature is consistent with that of a cocoon of a radio galaxy lurking for about 9 × 107 yr; no shock compression is required. The other three diffuse emission have spectra steeper than 1.5 and could be cocoons lurking for longer time. We discuss other possibilities such as shocks and turbulent reacceleration being responsible for the diffuse emission in A754.

Model independent foreground power spectrum estimation using WMAP 5-year data

In this paper, we propose and implement on WMAP 5 yr data a model independent approach of foreground power spectrum estimation for multifrequency observations of the CMB experiments. Recently, a model independent approach of CMB power spectrum estimation was proposed by Saha et al. 2006. This methodology demonstrates that the CMB power spectrum can be reliably estimated solely from WMAP data without assuming any template models for the foreground components. In the current paper, we extend this work to estimate the galactic foreground power spectrum using the WMAP 5 yr maps following a self-contained analysis. We apply the model independent method in harmonic basis to estimate the foreground power spectrum and frequency dependence of combined foregrounds. We also study the behavior of synchrotron spectral index variation over different regions of the sky. We use the full sky Haslam map as an external template to increase the degrees of freedom, while computing the synchrotron spectral index over the frequency range from 408 MHz to 94 GHz. We compare our results with those obtained from maximum entropy method foreground maps, which are formed in pixel space. We find that relative to our model independent estimates maximum entropy method maps overestimate the foreground power close to galactic plane and underestimates it at high latitudes.

Estimating the spectral indices of correlated astrophysical foregrounds by a second-order statistical approach

We present the first tests of a new method, the Correlated Component Analysis (CCA) based on second-order statistics, to estimate the mixing matrix, a key ingredient to separate astrophysical foregrounds superimposed to the Cosmic Microwave Background (CMB). In the present application, the mixing matrix is parameterized in terms of the spectral indices of Galactic synchrotron and thermal dust emissions, while the free-free spectral index is prescribed by basic physics, and is thus assumed to be known. We consider simulated observations of the microwave sky with angular resolution and white stationary noise at the nominal levels for the PLANCK satellite, and realistic foreground emissions, with a position dependent synchrotron spectral index. We work with two sets of PLANCK frequency channels: the low frequency set, from 30 to 143 GHz, complemented with the Haslam 408 MHz map, and the high frequency set, from 217 to 545 GHz. The concentration of intense free-free emission on the Galactic plane introduces a steep dependence of the spectral index of the global Galactic emission with Galactic latitude, close to the Galactic equator. This feature makes difficult for the CCA to recover the synchrotron spectral index in this region, given the limited angular resolution of PLANCK, especially at low frequencies. A cut of a narrow strip around the Galactic equator (|b|<3 deg), however, allows us to overcome this problem. We show that, once this strip is removed, the CCA allows an effective foreground subtraction, with residual uncertainties inducing a minor contribution to errors on the recovered CMB power spectrum.

Simulated synchrotron emission from pulsar wind nebulae

Aims. A complete set of diagnostic tools aimed at producing synthetic synchrotron emissivity, polarization, and spectral index maps from relativistic MHD simulations is presented. As a first application we consider here the case of the emission from Pulsar Wind Nebulae (PWNe). Methods. The proposed method is based on the addition, on top of the basic set of MHD equations, of an extra equation describing the evolution of the maximum energy of the emitting particles. This equation takes into account adiabatic and synchrotron losses along streamlines for the distribution of emitting particles and its formulation is such that it is easily implemented in any numerical scheme for relativistic MHD. Results. Application to the axisymmetric simulations of PWNe, analogous to those described by Del Zanna et al. (2004), allows direct comparison between the numerical results and observations of the inner structure of the Crab Nebula, and similar objects, in the optical and X-ray bands. We are able to match most of the observed features typical of PWNe, like the equatorial torus and the polar jets, with velocities in the correct range, as well as finer emission details, like arcs, rings and the bright knot, that turn out to arise mainly from Doppler boosting effects. Spectral properties appear to be well reproduced too: detailed spectral index maps are produced for the first time and show softening towards the PWN outer borders, whereas spectral breaks appear in integrated spectra. The emission details are found to strongly depend on both the average wind magnetization, here σeff ≈ 0.02, and on the magnetic field shape. Conclusions. Our method, in spite of its simplicity, provides a realistic modeling of synchrotron emission properties, and twodimensional axisymmetric relativistic MHD simulations appear to be well suited to explain the main observational features of PWNe.

A polarized synchrotron template for cosmic microwave background polarization experiments based on WMAP data

We build template maps for the polarized Galactic synchrotron emission on large angular scales (7° full width at half maximum), in the 20-90 GHz microwave range, by using Wilkinson Microwave Anisotropy Probe (WMAP) data. The method, presented in recent work, requires a synchrotron total-intensity survey and the polarization horizon to model the polarized intensity and a starlight polarization map to model polarization angles. The basic template is obtained directly at 23 GHz with about 94 per cent sky coverage by using the synchrotron map released by the WMAP team. Extrapolations to 32, 60 and 90 GHz are performed by computing a synchrotron spectral-index map, which strongly reduces previous uncertainties in passing from low (1.4 GHz) to microwave frequencies. Differing from low-frequency data, none of our templates present relevant structures out of the Galactic plane. Our map at 90 GHz suggests that the synchrotron emission at high Galactic latitudes is low enough to allow robust detection of the E-mode component of the cosmological signal on large scale, even in models with low reionization (r = 0.05). Detection of the weaker B-mode on the largest scales (l 0.1. For lower levels of the gravitational-wave background the B-mode seems to be accessible only at the l ∼ 100 peak and in selected low-synchrotron emission areas.

First‐YearWilkinson Microwave Anisotropy Probe(WMAP) Observations: Foreground Emission

Full sky maps are made in five microwave frequency bands to separate the temperature anisotropy of the CMB from foreground emission. We define masks that excise regions of high foreground emission. The effectiveness of template fits to remove foreground emission from the WMAP data is examined. These efforts result in a CMB map with minimal contamination and a demonstration that the WMAP CMB power spectrum is insensitive to residual foreground emission. We construct a model of the Galactic emission components. We find that the Milky Way resembles other normal spiral galaxies between 408 MHz and 23 GHz, with a synchrotron spectral index that is flattest (beta ~ -2.5) near star-forming regions, especially in the plane, and steepest (beta ~ -3) in the halo. The significant synchrotron index steepening out of the plane suggests a diffusion process in which the halo electrons are trapped in the Galactic potential long enough to suffer synchrotron and inverse Compton energy losses and hence a spectral steepening. The synchrotron index is steeper in the WMAP bands than in lower frequency radio surveys, with a spectral break near 20 GHz to beta < -3. The modeled thermal dust spectral index is also steep in the WMAP bands, with beta ~ 2.2. Microwave and H alpha measurements of the ionized gas agree. Spinning dust emission is limited to < ~5% of the Ka-band foreground emission. A catalog of 208 point sources is presented. Derived source counts suggest a contribution to the anisotropy power from unresolved sources of (15.0 +- 1.4) 10^{-3} microK^2 sr at Q-band and negligible levels at V-band and W-band.

On the local interstellar spectrum for cosmic ray electrons

Abstract Galactic propagation models for cosmic ray electrons give a synchrotron spectral index larger than the recently determined radio index between 22 – 408 MHz in the direction of the galactic disk (Roger , 1999), and smaller than the radio index between 0.5 – 2000 MHz in the direction of the galactic poles (Peterson , 1999). Diffuse gamma-ray data appear to be ‘contaminated’ by Crab-like point sources, so that it is difficult to derive a consistent local interstellar spectrum (IS) for electrons in the 1 to 30 MeV range. Using a phenomenological approach, we show that the synchrotron spectral indices calculated from the best-fit IS of Strong (2000) - for a 3 μG, and 5 μG, interstellar magnetic field - agree well with the spectral indices calculated with their full propagation model in the frequency range of interest. This allowed us to introduce two adjusted IS, such that the model radio spectral index agrees with observations of the galactic disk- and polar approaches above and below 20 MHz. By adding the constraints expected from the heliospheric modulation of galactic electrons, we find that the IS obtained by the ‘galactic disk approach’ is marginally above the lower limit for a local IS set by Pioneer 10 electron data at ∼4 MeV and ∼16 MeV observed in the outer heliosphere. The ‘polar approach’ gives an IS which can be considered a reasonable local IS for cosmic ray electrons.

Diffuse Continuum Gamma Rays from the Galaxy

A new study of the diUuse Galactic c-ray continuum radiation is presented, using a cosmic-ray propa- gation model which includes nucleons, antiprotons, electrons, positrons, and synchrotron radiation. Our treatment of the inverse Compton scattering includes the eUect of anisotropic scattering in the Galactic interstellar radiation —eld (ISRF) and a new evaluation of the ISRF itself. Models based on locally mea- sured electron and nucleon spectra and synchrotron constraints are consistent with c-ray measurements in the 30¨500 MeV range, but outside this range excesses are apparent. A harder nucleon spectrum is considered but —tting to c-rays causes it to violate limits from positrons and antiprotons. A harder inter- stellar electron spectrum allows the c-ray spectrum to be —tted above 1 GeV as well, and this can be further improved when combined with a modi—ed nucleon spectrum which still respects the limits imposed by antiprotons and positrons. A large electron/inverse Compton halo is proposed which repro- duces well the high-latitude variation of c-ray emission; this is taken as support for the halo size for nucleons deduced from studies of cosmic-ray composition. Halo sizes in the range 4¨10 kpc are favored by both analyses. The halo contribution of Galactic emission to the high-latitude c-ray intensity is large, with implications for the study of the diUuse extragalactic component and signatures of dark matter. The constraints provided by the radio synchrotron spectral index do not allow all of the c-ray emission at less than 30 MeV to be explained in terms of a steep electron spectrum unless this takes the form of a sharp upturn below 200 MeV. This leads us to prefer a source population as the origin of the excess low-energy c-rays, which can then be seen as a continuation of the hard X-ray continuum measured by OSSE, Ginga, and RXT E. Subject headings: cosmic raysdiUuse radiationGalaxy: generalgamma rays: observations ¨ gamma rays: theoryISM: general

A Major Radio Outburst in III Z[CLC]w[/CLC] 2 with an Extremely Inverted, Millimeter-peaked Spectrum

III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio structure typical of a Seyfert galaxy, but also with an extremely variable, blazar-like radio core. We have now discovered a new radio flare in which the source has brightened more than 20-fold within less than 2 yr. A broadband radio spectrum between 1.4 and 666 GHz shows a textbook-like synchrotron spectrum peaking at 43 GHz, with a self-absorbed synchrotron spectral index +2.5 at frequencies below 43 GHz and an optically thin spectral index -0.75 at frequencies above 43 GHz. The outburst spectrum can be well fitted by two homogenous, spherical components with equipartition sizes of 0.1 and 0.2 pc at 43 and 15 GHz and with magnetic fields of 0.4 and 1 G. VLBA observations at 43 GHz confirm this double structure and these sizes. Timescale arguments suggest that the emitting regions are shocks which are continuously accelerating particles. This could be explained by a frustrated jet scenario with very compact hot spots. Similar millimeter-peaked spectrum sources could have escaped our attention because of their low flux density at typical survey frequencies and their strong variability.

Radio Emission from a Young Supernova Remnant Interacting with an Interstellar Cloud: Magnetohydrodynamic Simulation with Relativistic Electrons

We present two-dimensional MHD simulations of the evolution of a young Type Ia supernova remnant (SNR) during its interaction with an interstellar cloud of comparable size at impact. We include for the first time in such simulations explicit relativistic electron transport. This was done using a simplified treatment of the diffusion-advection equation, thus allowing us to model injection and acceleration of cosmic-ray electrons at shocks and their subsequent transport. From this information we also model radio synchrotron emission, including spectral information. The simulations were carried out in spherical coordinates with azimuthal symmetry and compare three different situations, each incorporating an initially uniform interstellar magnetic field oriented in the polar direction on the grid. In particular, we modeled the SNR-cloud interactions for a spherical cloud on the polar axis, a toroidal cloud whose axis is aligned with the polar axis, and, for comparison, a uniform medium with no cloud. We find that the evolution of the overrun cloud qualitatively resembles that seen in simulations of simpler but analogous situations: that is, the cloud is crushed and begins to be disrupted by Rayleigh-Taylor and Kelvin-Helmholtz instabilities. However, we demonstrate here that, in addition, the internal structure of the SNR is severely distorted as such clouds are engulfed. This has important dynamical and observational implications. The principal new conclusions we draw from these experiments are the following. (1) Independent of the cloud interaction, the SNR reverse shock can be an efficient site for particle acceleration in a young SNR. (2) The internal flows of the SNR become highly turbulent once it encounters a large cloud. (3) An initially uniform magnetic field is preferentially amplified along the magnetic equator of the SNR, primarily because of biased amplification in that region by Rayleigh-Taylor instabilities. A similar bias produces much greater enhancement to the magnetic energy in the SNR during an encounter with a cloud when the interstellar magnetic field is partially transverse to the expansion of the SNR. The enhanced magnetic fields have a significant radial component, independent of the field orientation external to the SNR. This leads to a strong equatorial bias in synchrotron brightness that could easily mask any enhancements to electron-acceleration efficiency near the magnetic equator of the SNR. Thus, to establish the latter effect, it will be essential to establish that the magnetic field in the brightest regions are actually tangential to the blast wave. (4) The filamentary radio structures correlate well with turbulence-enhanced magnetic structures, while the diffuse radio emission more closely follows the gas-density distribution within the SNR. (5) At these early times, the synchrotron spectral index due to electrons accelerated at the primary shocks should be close to 0.5 unless those shocks are modified by cosmic-ray proton pressures. While that result is predictable, we find that this simple result can be significantly complicated in practice by SNR interactions with clouds. Those events can produce regions with significantly steeper spectra. Especially if there are multiple cloud encounters, this interaction can lead to nonuniform spatial spectral distributions or, through turbulent mixing, produce a spectrum that is difficult to relate to the actual strength of the blast wave. (6) Interaction with the cloud enhances the nonthermal electron population in the SNR in our simulations because of additional electron injection taking place in the shocks associated with the cloud. Together with point 3, this means that SNR-cloud encounters can significantly increase the radio emission from the SNR.

A New Sample of Radio‐selected and Optically Confirmed Supernova Remnants in M33

Using radio data to identify and optical data to confirm, we have established the largest and most complete sample of extragalactic radio-bright supernova remnants (SNRs) in the nearby spiral galaxy M33. We have identified 53 radio SNRs, doubling the size of the earlier survey by Duric et al. The increased sample size provides more insight into the nature of SNRs and provides an extensive informational database about SNRs for future studies. The SNR sample was analyzed for statistical trends that relate to the evolution of their radio emission. We report no significant correlation between the radio flux densities and diameters of SNRs in our sample. The absence of a statistically meaningful Σ-D relation is consistent with the view that environmental factors govern the radio evolution of SNRs. A new radio luminosity function for SNRs is presented. Although there appears to be a break in the function, the break is the result of selection effects and not intrinsic to the sample. A comparison of the radio luminosity function with the integrated radio continuum properties of M33 suggests that SNRs may well be the primary source of the relativistic electrons in M33. A histogram of synchrotron spectral indices of the SNRs is presented. Statistical analysis of the spectral index distribution indicates that the mean spectral index of SNRs in M33 is steeper than the mean in samples of SNRs in the Galaxy or the LMC and is consistent with a particle spectrum expected from a diffusive shock acceleration process.

A Determination of the Spectral Index of Galactic Synchrotron Emission in the 1–10 GHz Range

We present an analysis of simultaneous multifrequency measurements of the Galactic emission in the 1-10 GHz range with 18° angular resolution taken from a high-altitude site. Our data yield a determination of the synchrotron spectral index between 1.4 and 7.5 GHz of βsyn = 2.81 ± 0.16. Combining our data with maps made by Haslam et al. and Reich & Reich, we find βsyn = 2.76 ± 0.11 in the 0.4-7.5 GHz range. These results are in agreement with the few previously published measurements. The variation of βsyn with frequency based on our results and compared with other data found in the literature suggests a steepening of the synchrotron spectrum toward high frequencies, as expected from theory because of the steepening of the parent cosmic-ray electron energy spectrum. Comparison between the Haslam data and the 19 GHz map of Cottingham also indicates a spatial variation of the spectral index on large angular scales. Additional high-quality data are necessary to provide a serious study of these effects.

Cosmic ray acceleration at relativistic shock waves in the presence of oblique magnetic fields with finite-amplitude perturbations

The process of first-order Fermi acceleration at relativistic shock waves is investigated by means of Monte Carlo particle simulations. The simulations are based on numerical integration of the equations of motion of the particles in a perturbed magnetic field. We consider finite-amplitude field perturbations to be a superposition of sinusoidal static waves. The downstream magnetic field structure is obtained from the upstream one by assuming simple plasma compression at the shock for the frozen magnetic field. For shock waves with oblique mean magnetic fields, we confirm our previous finding of non-monotonic change of spectral index with increasing field perturbation amplitude. It is thus possible to have steep spectra with synchrotron spectral indices below −0.5 in relativistic shocks also. In the case of quasi-perpendicular (superluminal) shocks, the acceleration is reduced to a simple compression for small-amplitude magnetic field perturbations