Shea Brown

Science with the Murchison Widefield Array

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

Extragalactic Radio Sources and the WMAP Cold Spot

We detect a dip of 20%-45% in the surface brightness and number counts of NRAO VLA Sky Survey (NVSS) sources smoothed to a few degrees at the location of the WMAP cold spot. The dip has structure on scales of ~1° to 10°. Together with independent all-sky wavelet analyses, our results suggest that the dip in extragalactic brightness and number counts and the WMAP cold spot are physically related, i.e., that the coincidence is neither a statistical anomaly nor a WMAP foreground-correction problem. If the cold spot does originate from structures at modest redshifts, as we suggest, then there is no remaining need for non-Gaussian processes at the last scattering surface of the cosmic microwave background (CMB) to explain the cold spot. The late integrated Sachs-Wolfe effect, already seen statistically for NVSS source counts, can now be seen to operate on a single region. To create the magnitude and angular size of the WMAP cold spot requires a ~140 Mpc radius completely empty void at z ≤ 1 along this line of sight. This is far outside the current expectations of the concordance cosmology, and adds to the anomalies seen in the CMB.

Diffuse radio emission in/around the Coma cluster: beyond simple accretion

We report on new 1.41-GHz Green Bank Telescope (GBT) and 352-MHz Westerbork Synthesis Radio Telescope observations of the Coma cluster and its environs. At 1.41 GHz, we tentatively detect an extension to the Coma cluster radio relic source 1253+275 which makes its total extent ∼2 Mpc. This extended relic is linearly polarized as seen in our GBT data, the NRAO VLA Sky Survey, and archival images, strengthening a shock interpretation. The extended relic borders a previously undetected ‘wall’ of galaxies in the infall region of the Coma cluster. We suggest that the radio relic is an infall shock, as opposed to the outgoing merger shocks believed responsible for other radio relics. We also find a sharp edge, or ‘front’, on the western side of the 352-MHz radio halo. This front is coincident with a similar discontinuity in the X-ray surface brightness and temperature in its southern half, suggesting a primary shock-acceleration origin for the local synchrotron emitting electrons. The northern half of the synchrotron front is less well correlated with the X-ray properties, perhaps due to projection effects. We confirm the global pixel-to-pixel power-law correlation between the 352-MHz radio brightness and X-ray brightness with a slope that is inconsistent with predictions of either primary shock acceleration or secondary production of relativistic electrons in giant radio haloes, but is allowable in the framework of the turbulent re-acceleration of relic plasma. The failure of these first-order models and the need for a more comprehensive view of the intracluster medium energization are also highlighted by the very different shapes of the diffuse radio and X-ray emission. We note the puzzling correspondence between the shape of the brighter regions of the radio halo and the surface mass density derived from weak lensing.

Diffuse radio emission in/around the Coma cluster

We report on new 1.41-GHz Green Bank Telescope (GBT) and 352-MHz Westerbork Synthesis Radio Telescope observations of the Coma cluster and its environs. At 1.41 GHz, we tentatively detect an extension to the Coma cluster radio relic source 1253+275 which makes its total extent ∼2 Mpc. This extended relic is linearly polarized as seen in our GBT data, the NRAO VLA Sky Survey, and archival images, strengthening a shock interpretation. The extended relic borders a previously undetected ‘wall’ of galaxies in the infall region of the Coma cluster. We suggest that the radio relic is an infall shock, as opposed to the outgoing merger shocks believed responsible for other radio relics. We also find a sharp edge, or ‘front’, on the western side of the 352-MHz radio halo. This front is coincident with a similar discontinuity in the X-ray surface brightness and temperature in its southern half, suggesting a primary shock-acceleration origin for the local synchrotron emitting electrons. The northern half of the synchrotron front is less well correlated with the X-ray properties, perhaps due to projection effects. We confirm the global pixel-to-pixel power-law correlation between the 352-MHz radio brightness and X-ray brightness with a slope that is inconsistent with predictions of either primary shock acceleration or secondary production of relativistic electrons in giant radio haloes, but is allowable in the framework of the turbulent re-acceleration of relic plasma. The failure of these first-order models and the need for a more comprehensive view of the intracluster medium energization are also highlighted by the very different shapes of the diffuse radio and X-ray emission. We note the puzzling correspondence between the shape of the brighter regions of the radio halo and the surface mass density derived from weak lensing.

Complex Faraday depth structure of active galactic nuclei as revealed by broad‐band radio polarimetry

We present a detailed study of the Faraday depth structure of four bright (>1 Jy), strongly polarized, unresolved radio-loud quasars. The Australia Telescope Compact Array (ATCA) was used to observe these sources with 2 GHz of instantaneous bandwidth from 1.1 to 3.1 GHz. This allowed us to spectrally resolve the polarization structure of spatially unresolved radio sources, and by fitting various Faraday rotation models to the data, we conclusively demonstrate that two of the sources cannot be described by a simple rotation measure (RM) component modified by depolarization from a foreground Faraday screen. Our results have important implications for using background extragalactic radio sources as probes of the Galactic and intergalactic magneto-ionic media as we show how RM estimations from narrow-bandwidth observations can give erroneous results in the presence of multiple interfering Faraday components. We postulate that the additional RM components arise from polarized structure in the compact inner regions of the radio source itself and not from polarized emission from galactic or intergalactic foreground regions. We further suggest that this may contribute significantly to any RM time variability seen in RM studies on these angular scales. Follow-up, high-sensitivity very long baseline interferometry (VLBI) observations of these sources will directly test our predictions.

Probing the origin of giant radio haloes through radio and γ-ray data: the case of the Coma cluster

We combine for the first time all available information about the spectral shape and morphology of the radio halo of the Coma cluster with the recent γ-ray upper limits obtained by the Fermi-Large Area Telescope (LAT) and with the magnetic field strength derived from Faraday rotation measures. We explore the possibility that the radio emission is due to synchrotron emission of secondary electrons. First, we investigate the case of pure secondary models that are merely based on the mechanism of continuous injection of secondary electrons via proton–proton collisions in the intracluster medium. We use the observed spatial distribution of the halos radio brightness to constrain the amount of cosmic ray protons and their spatial distribution in the cluster that are required by the model. Under the canonical assumption that the spectrum of cosmic rays is a power law in momentum and that the spectrum of secondaries is stationary, we find that the combination of the steep spectrum of cosmic ray protons necessary to explain the spectrum of the halo and the very broad spatial distribution (and large energy density) of cosmic rays result in a γ-ray emission in excess of present limits, unless the cluster magnetic field is relatively large. However, this large magnetic field required to not violate present γ-ray limits appears inconsistent with that derived from recent Faraday rotation measures. Secondly, we investigate more complex models in which the cosmic rays confined diffusively in the Coma cluster and their secondary electrons are all reaccelerated by magnetohydrodynamics turbulence. We show that under these conditions it is possible to explain the radio spectrum and morphology of the radio halo and to predict γ-ray fluxes in agreement with the Fermi-LAT upper limits without tension with present constraints on the cluster magnetic field. Reacceleration of secondary particles also requires a very broad cosmic ray spatial profile, much flatter than that of the intracluster medium, at least provided that both the turbulent and magnetic field energy densities scale with that of the intracluster medium. However, this requirement can be easily alleviated if we assume that a small amount of (additional) seed primary electrons is reaccelerated in the clusters external regions, or if we adopt flatter scalings of the turbulent and magnetic field energy densities with distance from the cluster centre.

Another shock for the Bullet cluster, and the source of seed electrons for radio relics

ABSTRACT With Australia Telescope Compact Array observations, we detect a highly elongatedMpc-scale diffuse radio source on the eastern periphery of the Bullet cluster 1E0657-55.8,which we argue has the positional, spectral and polarimetric characteristics of a radio relic.This powerful relic (2:3 0:1 10 25 WHz 1 ) consists of a bright northern bulb and a faintlinear tail. The bulb emits 94% of the observed radio flux and has the highest surface bright-ness of any known relic. Exactly coincident with the linear tail we find a sharp X-ray surfacebrightness edge in the deep Chandra image of the cluster – a signature of a shock front inthe hot intracluster medium (ICM), located on the opposite side of the cluster to the famousbow shock. This new example of an X-ray shock coincident with a relic further supports thehypothesis that shocks in the outer regions of clusters can form relics via diffusive shock (re-)acceleration. Intriguingly, our new relic suggests that seed electrons for reacceleration arecoming from a local remnant of a radio galaxy, which we are lucky to catch before its com-plete disruption. If this scenario, in which a relic forms when a shock crosses a well-definedregion of the ICM polluted with aged relativistic plasma – as opposed to the usual assumptionthat seeds are uniformly mixed in the ICM – is also the case for other relics, this may explaina number of peculiar properties of peripheral relics.Key words: radiation mechanisms: non-thermal – acceleration of particles – shock waves –galaxies: clusters: individual: 1E 0657-55.8 – galaxies: clusters: intracluster medium – radiocontinuum general

MULTIPLE SHOCK STRUCTURES IN A RADIO-SELECTED CLUSTER OF GALAXIES

FITS is the standard astronomical data format that stores the image, metadata, and data tables for easy transport and can be manipulated. See http://fits.gsfc.nasa.gov/ for more information and software downloads. The FITS header for this image file is included as a separate text file.

Deep radio observations of the radio halo of the bullet cluster 1E 0657−55.8

We present deep 1.1-3.1 GHz Australia Telescope Compact Array observations of the radio halo of the bullet cluster, 1E 0657-55.8. In comparison to existing images of this radio halo the detection in our images is at higher significance. The radio halo is as extended as the X-ray emission in the direction of cluster merger but is significantly less extended than the X-ray emission in the perpendicular direction. At low significance we detect a faint second peak in the radio halo close to the X-ray centroid of the smaller sub-cluster (the bullet) suggesting that, similarly to the X-ray emission, the radio halo may consist of two components. Finally, we find that the distinctive shape of the western edge of the radio halo traces out the X-ray detected bow shock. The radio halo morphology and the lack of strong point-to-point correlations between radio, X-ray and weak-lensing properties suggests that the radio halo is still being formed. The colocation of the X-ray shock with a distinctive radio brightness edge illustrates that the shock is influencing the structure of the radio halo. These observations support the theory that shocks and turbulence influence the formation and evolution of radio halo synchrotron emission.

The application of compressive sampling to radio astronomy - II. Faraday rotation measure synthesis

Faraday rotation measure (RM) synthesis is an important tool to study and analyze galactic and extra-galactic magnetic fields. Since there is a Fourier relation between the Faraday dispersion function and the polarized radio emission, full reconstruction of the dispersion function requires knowledge of the polarized radio emission at both positive and negative square wavelengths λ 2 .H owever, one can only make observations for λ 2 > 0. Furthermore observations are possible only for a limited range of wavelengths. Thus reconstructing the Faraday dispersion function from these limited measurements is ill-conditioned. In this paper, we propose three new reconstruction algorithms for RM synthesis based upon compressive sensing/sampling (CS). These algorithms are designed to be appropriate for Faraday thin sources only, thick sources only, and mixed sources respectively. Both visual and numerical results show that the new RM synthesis methods provide superior reconstructions of both magnitude and phase information than RM-CLEAN.