A Radio Survey of Seven Southern X-ray Luminous Clusters of Galaxies

The Australia Telescope Compact Array (ATCA) has been used at 1.38 and 2.38 GHz to survey seven southern Abell clusters of galaxies with high X-ray luminosities: A2746, A2837, A3126, A3216, A3230, A3827 and A3836. The clusters have also been surveyed at 0.843 GHz with the Molonglo Observatory Synthesis Telescope (MOST). We have listed a complete 1.38-GHz sample of 149 radio sources within the Abell circles centred on their X-ray centroids. We compare their identification fractions, emitted 1.38-GHz and optical powers, radio spectral indices and radial variation in projected source density with those of the radio-selected samples of Slee et al. (1998). We compare our fractional radio luminosity function with that of the radio-selected samples of Ledlow and Owen (1996) and Slee et al. (1998). Three significant differences are noted between X-ray and radio-selected samples of clusters; (1) the X-ray sample has an excess of flat-spectrum radio sources; (2) the fractional radio luminosity function for the FR I sources in the X-ray selected sample is much steeper, implying that fewer of their cluster galaxies become hosts for the stronger FR I radio galaxies; (3) a complete absence of FR II radio galaxies in the X-ray selected sample. The average excess projected density of radio sources near our cluster centres is approx. 5 times the background source density.Comment: 24 pages, 8 figures, plus 6 figures to be published online only; accepted to appear in MNRA

Elucidating the Anti-Inflammatory Roles of Heparin and Shear Stress in Atherosclerosis

Atherosclerosis, an inflammatory disease characterized by the hardening of the arteries and often attributed to poor life style choices, is the leading cause of death in the United States, Europe, and most of Asia. This disease is caused by injury to the arterial wall, causing an inflammatory response which can become misregulated, and over the lifetime of an individual can lead to plaque formation. Hallmarks of atherosclerotic plaque formation include the proliferation and migration of vascular smooth muscle cells (VSMCs) to the injured site in the arterial wall and endothelial cell dysfunction, both of which contribute to plaque formation. In an attempt to control this unwarranted inflammation and cellular proliferation, heparin has been studied because of its anti-inflammatory and anti-proliferative effects. Unfortunately, the mechanisms by which heparin induces these effects are not well understood. In this study, experiments aimed at identification of a receptor for heparin furthered the understanding of the signaling mechanisms underlying heparin\u27s anti-inflammatory and anti-proliferative effects. To gain a better understanding of the underlying signaling cascade induced by heparin, various gene expression analyses were performed in heparin-treated VSMCs. While heparin is one of the major signals opposing vascular disease progression, other signals including laminar shear stress also provide similar opposing actions. Along with investigation of heparin signaling, experiments demonstrating a role for cofilin in actin remodeling during laminar shear stress have been completed. Cofilin, a member of the Actin Depolymerizing Factor family of proteins, is an actin severing protein which promotes actin depolymerization from the actin minus end when cofilin is unphosphorylated. This work also underscored the importance of cofilin and actin realignment in shear stress-induced endothelial barrier integrity. The culmination of this work has unveiled a deeper understanding of the molecular mechanisms underlying atherosclerosis and the anti-inflammatory effects of heparin and laminar shear stress

A search for electron cyclotron maser emission from compact binaries

Unipolar induction (UI) is a fundamental physical process, which occurs when a conducting body transverses a magnetic field. It has been suggested that UI is operating in RX J0806+15 and RX J1914+24, which are believed to be ultra-compact binaries with orbital periods of 5.4 min and 9.6 min respectively. The UI model predicts that those two sources may be electron cyclotron maser sources at radio wavelengths. Other systems in which UI has been predicted to occur are short period extra-solar terrestrial planets with conducting cores. If UI is present, circularly polarised radio emission is predicted to be emitted. We have searched for this predicted radio emission from short period binaries using the VLA and ATCA. In one epoch we find evidence for a radio source, coincident in position with the optical position of RX J0806+15. Although we cannot completely exclude that this is a chance alignment between the position of RX J0806+15 and an artifact in the data reduction process, the fact that it was detected at a significance level of 5.8 sigma and found to be transient, suggests that it is more likely that RX J0806+15 is a transient radio source. We find an upper limit on the degree of circular polarisation to be ~50%. The inferred brightness temperature exceeds 10^18 K, which is too high for any known incoherent process, but is consistent with maser emission and UI being the driving mechanism. We did not detect radio emission from ES Cet, RX J1914+24 or Gliese 876.Comment: Accepted for publication MNRA

The VLA Low-frequency Sky Survey

The Very Large Array (VLA) Low-frequency Sky Survey (VLSS) has imaged 95% of the 3*pi sr of sky north of declination = -30 degrees at a frequency of 74 MHz (4 meter wavelength). The resolution is 80" (FWHM) throughout, and the typical RMS noise level is ~0.1 Jy/beam. The typical point-source detection limit is 0.7 Jy/beam and so far nearly 70,000 sources have been catalogued. This survey used the 74 MHz system added to the VLA in 1998. It required new imaging algorithms to remove the large ionospheric distortions at this very low frequency throughout the entire ~11.9 degree field of view. This paper describes the observation and data reduction methods used for the VLSS and presents the survey images and source catalog. All of the calibrated images and the source catalog are available online (http://lwa.nrl.navy.mil/VLSS) for use by the astronomical community.Comment: 53 pages, including 3 tables and 15 figures. Has been accepted for publication in the Astronomical Journa

Diffuse radio sources in the cluster of galaxies Abell 548b

We report extensive VLA and ATCA observations of the two diffuse radio sources in the cluster of galaxies Abell 548b, which confirm their classification as relics. The two relics (named A and B) show similar flux density, extent, shape, polarization and spectral index and are located at projected distances of about 430 and 500 kpc from the cluster center, on the same side of the cluster's X-ray peak. On the basis of spectral indices of discrete radio sources embedded within the diffuse features, we have attempted to distinguish emission peaks of the diffuse sources from unrelated sources. We have found that both relics, in particular the B-relic, show possible fine structure, when observed at high resolution. Another diffuse source (named C) is detected close in projection to the cluster center. High-resolution images show that it contains two discrete radio sources and a diffuse component, which might be a candidate for a small relic source. The nature and properties of the diffuse radio sources are discussed. We conclude that they are likely related to the merger activity in the cluster.Comment: Accepted for publication in MNRAS. 11 pages, 7 figures. Some figures are degraded to reduce their size. A version with high resolution images is available at http://www.ira.inaf.it/~lferetti/OUTGOING/papA548b.ps.g

The Origin of Radio Emission in Low-Luminosity Active Galactic Nuclei: Jets, Accretion Flows, or Both?

The low-luminosity active galactic nuclei in NGC 3147, NGC 4203, and NGC 4579 have been imaged at four frequencies with the Very Long Baseline Array. The galaxies are unresolved at all frequencies, with size upper limits of $10^3-10^4$ times the Schwarzschild radii of their central massive black holes. The spectral indices between 1.7 and 5.0 GHz range from 0.2 to 0.4; one and possibly two of the galaxies show spectral turnovers between 5.0 and 8.4 GHz. The high brightness temperatures ($> 10^9$ K) and relatively straight spectra imply that free-free emission and/or absorption cannot account for the slightly inverted spectra. Although the radio properties of the cores superficially resemble predictions for advection-dominated accretion flows, the radio luminosities are too high compared to the X-ray luminosities. We suggest that the bulk of the radio emission is generated by a compact radio jet, which may coexist with a low radiative efficiency accretion flow.Comment: To appear in ApJ (Letters). 4 page

Non-thermal X-rays from the Ophiuchus galaxy cluster and dark matter annihilation

We investigate a scenario where the recently discovered non-thermal hard X-ray emission from the Ophiuchus cluster originates from inverse Compton scattering of energetic electrons and positrons produced in weakly interacting dark matter pair annihilations. We show that this scenario can account for both the X-ray and the radio emission, provided the average magnetic field is of the order of 0.1 microGauss. We demonstrate that GLAST will conclusively test the dark matter annihilation hypothesis. Depending on the particle dark matter model, GLAST might even detect the monochromatic line produced by dark matter pair annihilation into two photons.Comment: 6 pages, 4 figures, matches published versio

Four Extreme Relic Radio Sources in Clusters of Galaxies

(Abridged) We describe the results of the highest-resolution radio observations yet made of four relic radio sources in the Abell clusters A13, A85, A133 and A4038. Our VLA images at 1.4 GHz with 4" resolution show a remarkable variety of fine structure in the form of spectacular arcs, wisps, plumes and loops. Their integrated radio flux densities fall very rapidly with frequency, with power-law slopes between 2.1 and 4.4 near 1.4 GHz The relics possess linear polarization levels ranging between 2.3 % (A133) and 35 % (A85); the higher polarization fractions imply a highly ordered magnetic field in the fine structure and low differential Faraday rotation in the intervening cluster gas. The optical identification of host galaxies remains problematic. In A85, A133 and A4038 the travel times for the brightest cluster galaxies are significantly longer than the modeled ages of the relics and nearby bright ellipticals provide a better match. Excess X-ray emission in the 0.5 keV-to-2 keV band was found near the relics in A85 and A133. The surface brightness was too high to be attributed to the inverse-Compton mechanism alone. We found excellent fits to the broad-band radio spectra using the anisotropic (KGKP) model of spectral ageing, and we have extended the model to include diffusion of particles between regions of different field strength (the Murgia-JP, or MJP, model). The steep radio spectra imply ages for the relics of ~ 10^8 yr, at the start of which period their radio luminosities would have been ~ 10^25 W/Hz at 1.4 GHz.Comment: 43 pages, 13 figures, AJ, Sep 2001 (accepted