arXiv subjects

Lawrence Rudnick

Publications and source records attributed to Lawrence Rudnick.

86 records · Page 5Linked to original sources

arXiv2007

Extragalactic Radio Sources and the WMAP Cold Spot

We detect a dip of 20-45% in the surface brightness and number counts of NVSS sources smoothed to a few degrees at the location of the WMAP cold spot. The dip has structure on scales of approximately 1-10 degrees. 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 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.

astro-ph↗

arXiv2006

Unbiased Studies of Diffuse Extragalactic Radio Sources

We present an overview of unbiased studies of diffuse extragalactic radio sources. We use a previously developed filtering technique to remove compact sources from large surveys such as WENSS, WISH, and NVSS and examine the residual diffuse emission. A search of these residuals, unbiased by optical or X-ray selection, has uncovered a wide variety of diffuse sources, including those associated with groups of galaxies, ``blank'' fields, and previously unrecognized diffuse radio galaxy emission, as well as halos and relics of rich clusters. A second, targeted survey of the brightest X-ray clusters results in a number of new sources, and illuminates some important selection effects. When the well-established relationship between X-ray and radio halo luminosities is extrapolated to lower levels, the apparent correlation holds quite well for any type of diffuse source, even background noise. The observed correlations must therefore be scrutinized for possible selection effects before physical interpretations are made. We briefly mention a wide-field mapping project to search for large-scale structures, and goals for the next generation of studies in magnetic field - angular size parameter space.

astro-ph↗

arXiv2005

Diffuse Radio Sources in Groups and Poor Clusters

We have discovered new diffuse radio sources likely associated with groups of galaxies at low redshift (0.01-0.04) and without apparent AGN by using the WENSS and WISH catalogs to perform an unbiased survey. These sources resemble the radio halos, mini-halos, and 'relics' of rich clusters, which are thought to be powered by shocks and turbulence from infall into their deep potential wells. Our detection of similar sources within the shallow potential wells of groups of galaxies challenges this model. Their radio luminosities are approximately two orders of magnitude higher than expected from the extrapolation of the apparent rich cluster radio/X-ray luminosity relation. Even if these sources are misidentified distant clusters, they would lie above the apparent rich cluster radio/X-ray luminosity relation in the literature, suggesting that detections of radio halos and relics thus far may be more biased than previously thought.

astro-ph↗

arXiv2005

Canonical Particle Acceleration in FRI Radio Galaxies

Matched resolution multi-frequency VLA observations of four radio galaxies are used to derive the asymptotic low energy slope of the relativistic electron distribution. Where available, low energy slopes are also determined for other sources in the literature. They provide information on the acceleration physics independent of radiative and other losses, which confuse measurements of the synchrotron spectra in most radio, optical and X-ray studies. We find a narrow range of inferred low energy electron energy slopes, n(E)=const*E^-2.1 for the currently small sample of lower luminosity sources classified as FRI (not classical doubles). This distribution is close to, but apparently inconsistent with, the test particle limit of n(E)=const*E^-2.0 expected from strong diffusive shock acceleration in the non-relativistic limit. Relativistic shocks or those modified by the back-pressure of efficiently accelerated cosmic rays are two alternatives to produce somewhat steeper spectra. We note for further study the possiblity of acceleration through shocks, turbulence or shear in the flaring/brightening regions in FRI jets as they move away from the nucleus. Jets on pc scales and the collimated jets and hot spots of FRII (classical double) sources would be governed by different acceleration sites and mechanisms; they appear to show a much wider range of spectra than for FRI sources.

astro-ph↗

arXiv2004

Observing Magnetic Fields on Large Scales

Observations of magnetic fields on scales up to several Mpc are important for understanding cluster and large-scale structure evolution. Our current census of such structures is heavily biased -- towards fields of several microG, towards fields in deep potential wells, and towards high inferred field strengths in cooling flow and other clusters from improper analysis of rotation measure data. After reviewing these biases, I show some recent results on two relics that are powered in very different ways. I describe new investigations that are now uncovering weak diffuse fields in the outskirts of clusters and other low density environments, and the good prospects for further progress.

astro-ph↗

arXiv2004

A Million-Second Chandra View of Cassiopeia A

We introduce a million-second observation of the supernova remnant Cassiopeia A with the Chandra X-ray Observatory. The bipolar structure of the Si-rich ejecta (NE jet and SW counterpart) is clearly evident in the new images, and their chemical similarity is confirmed by their spectra. These are most likely due to jets of ejecta as opposed to cavities in the circumstellar medium, since we can reject simple models for the latter. The properties of these jets and the Fe-rich ejecta will provide clues to the explosion of Cas A.

astro-ph↗

arXiv2003

A Taxonomy of Extended Radio Sources in Clusters of Galaxies

At the request of the conference attendees, we have compiled a classification of extended radio sources in clusters. These range from scales of tens of parsecs to over a megaparsec in scale, and include both sources associated with AGN and sources thought to derive from the electron population in the ionized ICM. We pay special attention to distinguishing between the types of AGN in the cores of cooling flow clusters and between the multiple classes of objects referred to over the years as ``radio relics.'' We suggest new names based on physical arguments for some of these classes of objects where their commonly used names are inappropriate or confusing.

astro-ph↗

arXiv2002

Radio Spectral Index Variations and Physical conditions in Kepler's Supernova Remnant

A new epoch of VLA measurements of Kepler's supernova remnant was obtained to make accurate measurements of the radio spectral index variations and polarization. We have compared these new radio images with Halpha, infrared, and X-ray data to better understand the three dimensional structure and dynamics of Kepler, and to better understand the physical relationships between the various nonthermal and thermal plasmas. Spatial variations in the radio spectral index from -0.85 to -0.6 are observed between 6 cm and 20 cm. There is a strong correspondence between the radial and azimuthal profiles of the radio, X-ray, Halpha, and infrared emission in different locations around the remnant although there is no single, global pattern. Spectral tomography shows that the flat- and steep-spectrum radio emissions have distinct structures. The flat-spectrum radio emission is found either at a larger radius than or coincident with the steep-spectrum emission. We interpret these spectral components as tracing forward- and reverse-shocked material, respectively. The Halpha and IR images are very similar. Their leading edges are coincident and are either in front of or coincident with the leading edges of the X-ray and radio emission. The X-ray emission matches the Halpha and IR emission in places, and in other places traces the steep-spectrum radio emission. In the north there is also an anticorrelation in the azimuthal profiles around the remnant of the flat-spectrum radio emission and the thermal X-ray, Halpha, and IR emissions. We suggest that this could be due to a relative weakening of the particle acceleration at the forward shock due to Alfven wave damping in regions of high density.

astro-ph↗

arXiv1999

The Problems of Ageing

I present a brief overview of observational, modeling and theoretical issues related to ageing calculations based on spectral steepening. These include problems such as inhomogeneous magnetic fields, diffusion of relativistic particles, confusion from multiple particle populations and particle acceleration. Although some of the effects are only of order unity, others call into question the entire ageing paradigm. I refer to and show some data illustrating these problems and make a few recommendations about how we should proceed given these uncertainties.

astro-ph↗

arXiv1999

Multi-Resolution Imaging and Spectra of Extended Sources

I introduce a straightforward technique for the filtering of extended astronomical images into components of different spatial scales. For a positive original image, each component is positive definite, and the sum of all components equals the original image. In this way, the components are each individually suitable for flux measurements and broadband spectra calculations. I present an illustration of this technique on the radio galaxy Cygnus~A.

astro-ph↗

arXiv1999

Electron Population Aging Models for Wide-Angle Tails

Color-color diagrams have been useful in studying the spectral shapes in radio galaxies. At the workshop we presented color-color diagrams for two wide-angle tails, 1231+674 and 1433+553, and found that the standard aging models do not adequately represent the observed data. Although the JP and KP models can explain some of the observed points in the color-color diagram, they do not account for those found near the power-law line. This difficulty may be attributable to several causes. Spectral tomography has been previously used to discern two separate electron populations in these sources. The combination spectra from two such overlying components can easily resemble a range of power-laws. In addition, any non-uniformity in the magnetic field strength can also create a power-law-like spectrum. We will also discuss the effects that angular resolution has on the shape of the spectrum.

astro-ph↗

arXiv1999

The Supernova Remnant Cas A at Millimeter Wavelengths

We used the BIMA array to map the supernova remnant Cas A from 28 to 87 GHz with 2'' to 7'' angular resolution. Data from 75 to 87 GHz, with 19 pointing centers were combined with single dish data to form a completely sampled image. These new BIMA images were compared with VLA images at 1.5 and 5 GHz to look for spectral index variations across Cas A. The images were spatially filtered and convolved to a common sampled uv-range corresponding to angular scales from 7'' to 95'' . The images give direct evidence for a steeper spectral index in the knots outside the bright ring. Analysis of the 26 brightest peaks gives statistical evidence for spectral index variations throughout the remnant. The high frequency spectra confirm and extend the spectral variations seen previously at lower frequencies. The spectra are largely consistent with different power laws, and not curved spectra. This provides strong support for local variations in the acceleration of relativistic particles in Cas A.

astro-ph↗

arXiv1997

$10^{51}$ Ergs: The Evolution of Shell Supernova Remnants

This paper reports on a workshop hosted by the University of Minnesota, March 23-26, 1997. It addressed fundamental dynamical issues associated with the evolution of shell supernova remnants and the relationships between supernova remnants and their environments. The workshop considered, in addition to classical shell SNRs, dynamical issues involving X-ray filled composite remnants and pulsar driven shells, such as that in the Crab Nebula. Approximately 75 participants with wide ranging interests attended the workshop. An even larger community helped through extensive on-line debates prior to the meeting. Each of the several sessions, organized mostly around chronological labels, also addressed some underlying, general physical themes: How are SNR dynamics and structures modified by the character of the CSM and the ISM and vice versa? How are magnetic fields generated in SNRs and how do magnetic fields influence SNRs? Where and how are cosmic-rays (electrons and ions) produced in SNRs and how does their presence influence or reveal SNR dynamics? How does SNR blast energy partition into various components over time and what controls conversion between components? In lieu of a proceedings volume, we present here a synopsis of the workshop in the form of brief summaries of the workshop sessions. The sharpest impressions from the workshop were the crucial and under-appreciated roles that environments have on SNR appearance and dynamics and the critical need for broad-based studies to understand these beautiful, but enigmatic objects. \\

astro-ph↗

arXiv1996

A Comparison of X-ray and Radio Emission from the Supernova Remnant Cassiopeia A

We compare the radio and soft X-ray brightness as a function of position within the young supernova remnant Cassiopeia A. A moderately strong correlation (r = 0.7) was found between the X-ray emission (corrected for interstellar absorption) and radio emission, showing that the thermal and relativistic plasmas occupy the same volumes and are regulated by common underlying parameters. The logarithmic slope of the relationship, ln(Sx-ray) = 1.2 x Sradio + ln(k) implies that the variations in brightness are primarily due to path length differences. The X-ray and radio emissivities are both high in the same general locations, but their more detailed relationship is poorly constrained and probably shows significant scatter. The strongest radio and X-ray absorption is found at the western boundary of Cas A. Based on the properties of Cas A and the absorbing molecular cloud, we argue that they are physically interacting. We also compare ASCA derived column densities with 21 cm H I and 18 cm OH optical depths in the direction of Cas A, in order to provide an independent estimate of ISM properties. We derive an average value for the H I spin temperature of about 40 K and measure the ratio of OH to molecular hydrogen to be nominally larger than previous estimates. Keywords: Cas A, Cassiopeia A, interstellar medium, molecular clouds, radio astronomy, supernova remnants, X-ray astronomy

astro-ph↗