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New Abstracts of Interest
A cumulative list of titles for the current year can be found here (new window). Previous years' titles: 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 (new window). Abstract: 0911.2489 Full Text: [ PostScript, PDF] Title: A new light boson from MAGIC observations?(Submitted on 12 Nov 2009)
Abstract: Recent detection of blazar 3C279 by MAGIC has confirmed previous indications by H.E.S.S. that the Universe is more transparent to very-high-energy gamma rays than currently thought. This circumstance can be reconciled with observations of nearby blazars provided that photon oscillations into a very light Axion-Like Particle occur in extragalactic magnetic fields. The emerging "DARMA scenario" can be tested in the near future by the satellite-borne Fermi LAT detector as well as by the ground-based Imaging Atmospheric Cherenkov Telescopes H.E.S.S., MAGIC, CANGAROO III, VERITAS and by the Extensive Air Shower arrays ARGO-YBJ and MILAGRO.
Abstract: 0911.2535 Full Text: [ PostScript, PDF] Title: Discarded low energy particles in extensive air shower simulations: Effect on the shower Energy Deposit(Submitted on 13 Nov 2009)
Abstract: The simulation of particle cascades initiated in the atmosphere by ultra high energy cosmic ray particles involves the generation and propagation of a huge amount of particles. As it is unpractical to follow every particle to its end, particles below a certain energy ($E_{Cut}$) are discarded from the simulation. In this article we study in detail the influence that this cut has on the total energy deposited in the atmosphere by the particle cascade in AIRES simulations. The energy deposit is directly related to the emission of fluorescence light and is critical for the accurate simulation of shower signals in fluorescence detectors. Not correcting for the discarded particles introduces a bias on several shower observables related to the energy deposit that can range from 3 to 30% or more depending on the $E_{Cut}$ value used. A prescription for the correct treatment of these particles is proposed, and the resulting corrections to the total energy deposit are addressed, including a new universal parametrization of the mean energy deposit per particle. The low energy cut is introduced in the simulations to reduce the required CPU time per shower at the expense of simulation accuracy. We find that a 0.4 MeV cut for electrons and 0.9 MeV cut for gammas is an adequate compromise, and that the proposed prescription is capablable of removing the bias introduced by this cut. The prescription is independent of the energy cut value and can be used to correct and compare simulations made with different energy cuts.
Abstract: 0911.2537 Full Text: [ PostScript, PDF] Title: Discarded low energy particles in extensive air shower simulations: Effect on the shower Missing and Invisible Energy(Submitted on 13 Nov 2009)
Abstract: The energy carried away by neutral particles in ultra high energy cosmic ray showers can not be detected by fluorescence detectors. This energy is usually referred to as the "invisible energy". Since every shower has a fraction of invisible energy, the energy determined using the fluorescence technique is always less than the primary energy and a correction needs to be applied. This correction, usually referred to as the "missing energy", can only be estimated using Monte Carlo simulations.
Abstract: 0911.2709 Full Text: [ PostScript, PDF] Title: PKS 2005-489 at VHE: Four Years of Monitoring with HESS and Simultaneous Multi-wavelength Observations(Submitted on 13 Nov 2009)
Abstract: VHE observations of PKS 2005-489 were made with HESS from 2004 through 2007, together with three simultaneous multi-wavelength campaigns performed with XMM-Newton and RXTE in 2004 and 2005. A strong VHE signal, ~17sigma total, is detected during the four years of HESS observations (90.3 hrs live time). The integral flux above the average analysis threshold of 400 GeV is ~3% of the Crab and varies weakly on time-scales from days to years. At X-ray energies the flux is observed to vary by more than an order of magnitude between 2004 and 2005. Strong changes in the X-ray spectrum (DeltaGamma~0.7) are also observed, which appear to be mirrored in the VHE band. The SED of PKS 2005-489, constructed for the first time with contemporaneous data on both humps, shows significant evolution. The large flux variations in the X-ray band, coupled with weak or no variations in the VHE band and a similar spectral behavior, suggest the emergence of a new, separate, harder emission component in September 2005.
Abstract: 0911.3009 Full Text: [ PostScript, PDF] Title: LUNASKA experiments using the Australia Telescope Compact Array to search for ultra-high energy neutrinos and develop technology for the lunar Cherenkov technique(Submitted on 16 Nov 2009)
Abstract: We describe the design, performance, sensitivity and results of our recent experiments using the Australia Telescope Compact Array (ATCA) for lunar Cherenkov observations with a very wide (600 MHz) bandwidth and nanosecond timing, including a limit on an isotropic neutrino flux. We also make a first estimate of the effects of small-scale surface roughness on the effective experimental aperture, finding that contrary to expectations, such roughness will act to increase the detectability of near-surface events over the neutrino energy-range at which our experiment is most sensitive. The aim of our "Lunar UHE Neutrino Astrophysics using the Square Kilometer Array" (LUNASKA) project is to develop the lunar Cherenkov technique of using terrestrial radio telescope arrays for ultra-high energy (UHE) cosmic ray (CR) and neutrino detection, and in particular to prepare for using the Square Kilometer Array (SKA) and its path-finders such as the Australian SKA Pathfinder (ASKAP) and the Low Frequency Array (LOFAR) for lunar Cherenkov experiments.
Abstract: 0911.3809 Full Text: [ PostScript, PDF] Title: The discovery of a massive white dwarf in the peculiar binary system HD 49798/RX J0648.0-4418(Submitted on 19 Nov 2009)
Abstract: An XMM-Mewton observation performed in May 2008 has confirmed that the 13 seconds pulsations in the X-ray binary HD 49798/RX J0648.0-4418 are due to a rapidly rotating white dwarf. From the pulse time delays induced by the 1.55 days orbital motion, and the system's inclination, constrained by the duration of the X-ray eclipse discovered in this observation, we could derive a mass of 1.28+/-0.05 M_sun for the white dwarf. The future evolution of this post common envelope binary system will likely involve a new phase of mass accretion through Roche-lobe overflow that could drive the already massive white dwarf above the Chandrasekhar limit and produce a Type Ia supernova.
Abstract: 0911.3063 Full Text: [ PostScript, PDF] Title: Gamma-ray and Radio Properties of Six Pulsars Detected by the Fermi Large Area Telescope(Submitted on 16 Nov 2009)
Abstract: We report the detection of pulsed gamma-rays for PSRs J0631+1036, J0659+1414, J0742-2822, J1420-6048, J1509-5850 and J1718-3825 using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (formerly known as GLAST). Although these six pulsars are diverse in terms of their spin parameters, they share an important feature: their gamma-ray light curves are (at least given the current count statistics) single peaked. For two pulsars there are hints for a double-peaked structure in the light curves. The shapes of the observed light curves of this group of pulsars are discussed in the light of models for which the emission originates from high up in the magnetosphere. The observed phases of the gamma-ray light curves are, in general, consistent with those predicted by high-altitude models, although we speculate that the gamma-ray emission of PSR J0659+1414, possibly featuring the softest spectrum of all Fermi pulsars coupled with a very low efficiency, arises from relatively low down in the magnetosphere. High-quality radio polarization data are available showing that all but one have a high degree of linear polarization. This allows us to place some constraints on the viewing geometry and aids the comparison of the gamma-ray light curves with high-energy beam models.
Abstract: 0911.3485 Full Text: [ PostScript, PDF] Title: Radio-Loud Narrow-Line Seyfert 1 as a New Class of Gamma-Ray AGN(Submitted on 18 Nov 2009)
Abstract: We report the discovery with Fermi/LAT of gamma-ray emission from three radio-loud narrow-line Seyfert 1 galaxies: PKS 1502+036 (z=0.409), 1H 0323+342 (z=0.061) and PKS 2004-447 (z=0.24). In addition to PMN J0948+0022 (z=0.585), the first source of this type to be detected in gamma rays, they may form an emerging new class of gamma-ray active galactic nuclei (AGN). These findings can have strong implications on our knowledge about relativistic jets and the unified model of AGN.
Abstract: 0911.3774 Full Text: [ PostScript, PDF] Title: Characteristic velocities of stripped-envelope core-collapse supernova cores(Submitted on 19 Nov 2009)
Abstract: The velocity of the inner ejecta of stripped-envelope core-collapse supernovae (CC-SNe) is studied by means of an analysis of their nebular spectra. Stripped-envelope CC-SNe are the result of the explosion of bare cores of massive stars ($\geq 8$ M$_{\odot}$), and their late-time spectra are typically dominated by a strong [O {\sc i}] $\lambda\lambda$6300, 6363 emission line produced by the innermost, slow-moving ejecta which are not visible at earlier times as they are located below the photosphere. A characteristic velocity of the inner ejecta is obtained for a sample of 56 stripped-envelope CC-SNe of different spectral types (IIb, Ib, Ic) using direct measurements of the line width as well as spectral fitting. For most SNe, this value shows a small scatter around 4500 km s$^{-1}$. Observations ($< 100$ days) of stripped-envelope CC-SNe have revealed a subclass of very energetic SNe, termed broad-lined SNe (BL-SNe) or hypernovae, which are characterised by broad absorption lines in the early-time spectra, indicative of outer ejecta moving at very high velocity ($v \geq 0.1 c$). SNe identified as BL in the early phase show large variations of core velocities at late phases, with some having much higher and some having similar velocities with respect to regular CC-SNe. This might indicate asphericity of the inner ejecta of BL-SNe, a possibility we investigate using synthetic three-dimensional nebular spectra.
Abstract: 0911.2476 Full Text: [ PostScript, PDF] Title: Detecting the orientation of magnetic fields in galaxy clusters(Submitted on 13 Nov 2009)
Abstract: Clusters of galaxies, filled with hot, magnetised plasma, are the largest bound objects in existence and an important touchstone in understanding the formation of structures in our Universe. Because in clusters, thermal conduction follows field lines, magnetic fields strongly shape the cluster's thermal history, which remains mysterious; some should have long since cooled and collapsed. In a seemingly unrelated puzzle, recent observations of Virgo cluster spiral galaxies imply ridges of strong, coherent magnetic fields offset from their centre. Here we demonstrate, using 3D magneto-hydrodynamical simulations, that such ridges are easily explained by galaxies sweeping up field lines as they orbit inside the cluster. This magnetic drape is then literally lit up with cosmic rays from the galaxies' stars, generating coherent polarised emission at the galaxies' leading edges. This immediately presents a first technique for probing local orientations and characteristic length scales of cluster magnetic fields. The first application of this technique, mapping the field of the Virgo cluster, gives a startling result - outside a central region, the magnetic field is preferentially oriented radially. Our results strongly suggest a mechanism for maintaining some clusters in a 'non-cooling-core' state.
Links to: arXiv, astro-ph, find, abs
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