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At least 253 records · Page 14Linked to original sources
arXiv2007

Studies of Direct Cherenkov Emission with VERITAS

Ground-based composition measurements of high-energy cosmic rays can be significantly improved by using the direct Cherenkov method. This technique targets the Cherenkov light produced by the primary particle prior to its production of an extensive air shower. With the appropriate time and angular resolution, the direct Cherenkov photons can be separated from those produced in the extensiveair shower. By utilizing the 0.15 degree angular and 2 nanosecond timing resolution of the very high energy gamma-ray telescope system, VERITAS, the charge and energy of cosmic rays at TeV energies can be identified on an event-by-event basis. Results from a preliminary search for direct Cherenkov events are discussed.

astro-ph↗
arXiv2007

Calibration Techniques for VERITAS

VERITAS is an array of four identical telescopes designed for detecting and measuring astrophysical gamma rays with energies in excess of 100 GeV. Each telescope uses a 12 m diameter reflector to collect Cherenkov light from air showers initiated by incident gamma rays and direct it onto a `camera' comprising 499 photomultiplier tubes read out by flash ADCs. We describe here calibration methods used for determining the values of the parameters which are necessary for converting the digitized PMT pulses to gamma-ray energies and directions. Use of laser pulses to determine and monitor PMT gains is discussed, as are measurements of the absolute throughput of the telescopes using muon rings.

astro-ph↗
arXiv2007

Gamma-Ray Emission from PWNe Interacting with Molecular Clouds

We consider a situation in which a pulsar (and its nebula) is formed inside or close to a high density regions of a molecular cloud. We apply a recent model for the gamma radiation of pulsar wind nebulae (PWN), which includes not only radiation processes due to injected leptons but also processes due to injection of relativistic hadrons, in order to calculate the expected gamma-ray emission from such interacting PWNe. The example calculations have been performed for two objects of this type from which directions TeV gamma-ray sources have recently been observed (IC443 and W41). We show that the gamma-ray emission below a few TeV can be produced by leptons accelerated in the past in the vicinity of the pulsars. gamma-rays with energies above ~10 TeV can be produced by hadrons interacting with the matter inside the supernova remnant and surrounding dense clouds. In contrary to the low energy TeV emission, this high energy TeV emission should be correlated with the location of dense clouds able to capture hadrons due to their strong magnetic fields.

astro-ph↗
arXiv2007

H.E.S.S. observations of the supernova remnant RX J0852.0-4622

The shell-type supernova remnant RX J0852.0-4622 was detected in 2004 and re-observed between December 2004 and May 2005 with the High Energy Stereoscopic System (H.E.S.S.), an array of four Imaging Cherenkov Telescopes located in Namibia and dedicated to the observations of gamma-rays above 100 GeV. The angular resolution of <0.1 degree and the large field of view of H.E.S.S. (5 degrees diameter) are well adapted to studying the morphology of the object in very high energy gamma-rays, which exhibits a remarkably thin shell very similar to the features observed in the radio range and in X-rays. The spectral analysis of the source from 300 GeV to 20 TeV will be presented. Finally, the possible origins of the very high energy gamma-ray emission (Inverse Compton scattering by electrons or the decay of neutral pions produced by proton interactions) will be discussed, on the basis of morphological and spectral features obtained at different wavelengths.

astro-ph↗
arXiv2007

Search for Ultra-High Energy Photons with the Pierre Auger Observatory

Data taken at the Pierre Auger Observatory are used to search for air showers initiated by ultra-high energy (UHE) photons. Results of searches are reported from hybrid observations where events are measured with both fluorescence and array detectors. Additionally, a more stringent test of the photon fluxes predicted with energies above 10^19 eV is made using a larger data set measured using only the surface detectors of the observatory.

astro-ph↗
arXiv2007

The Lorentz force effect on the On-Off dynamo intermittency

An investigation of the dynamo instability close to the threshold produced by an ABC forced flow is presented. We focus on the on-off intermittency behavior of the dynamo and the counter-effect of the Lorentz force in the non-linear stage of the dynamo. The Lorentz force drastically alters the statistics of the turbulent fluctuations of the flow and reduces their amplitude. As a result much longer burst (on-phases) are observed than what is expected based on the amplitude of the fluctuations in the kinematic regime of the dynamo. For large Reynolds numbers, the duration time of the ``On'' phase follows a power law distribution, while for smaller Reynolds numbers the Lorentz force completely kills the noise and the system transits from a chaotic state into a ``laminar'' time periodic flow. The behavior of the On-Off intermittency as the Reynolds number is increased is also examined. The connections with dynamo experiments and theoretical modeling are discussed.

physics.flu-dyn↗
arXiv2007

Blazar Observations with VERITAS

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an array of four 12m diameter Imaging Atmospheric Cherenkov Technique (IACT) telescopes operated at the base of Mt. Hopkins in southern Arizona. The four-telescope experiment started operation in April, 2007. GeV and TeV gamma-ray observations of blazars can be used to probe the structure and composition of their jets, and to contribute to our understanding of how supermassive black holes accrete matter. In this contribution, we present first VERITAS blazar results obtained with three and four telescopes.

astro-ph↗
arXiv2007

Expected discovery potential and sensitivity of the ANTARES neutrino telescope to neutrino point-like sources

The ANTARES telescope is being built in the Mediterranean Sea. The detector consists of a 3D array of photomultipliers (PMTs) that detects the Cherenkov light induced by the muons produced in neutrino interactions. Other signatures can also be detected. Since the neutrino fluxes from point-like sources are expected to be small, it is of the utmost importance to take advantage of the ANTARES pointing accuracy (angular resolution better than 0.3 degrees for muon events above 10 TeV) to disentangle a possible signal from the unavoidable atmospheric neutrino background. In order to distinguish an excess of neutrino events from the background, several searching algorithms have been developed within the ANTARES collaboration. In this contribution, the discovery potential and sensitivity to point-like sources of the ANTARES neutrino telescope are presented.

astro-ph↗
arXiv2007

The ANTARES neutrino telescope: a status report

ANTARES is a large volume neutrino telescope currently under construction off La Seyne-sur-mer, France, at 2475m depth. Neutrino telescopes aim at detecting neutrinos as a new probe for a sky study at energies greater than 1 TeV. The detection principle relies on the observation, using photomultipliers, of the Cherenkov light emitted by charged leptons induced by neutrino interactions in the surrounding detector medium. Since late January 2007, the ANTARES detector consists of 5 lines, comprising 75 optical detectors each, connected to the shore via a 40 km long undersea cable. The data from these lines not only allow an extensive study of the detector properties but also the reconstruction of downward going cosmic ray muons and the search for the first upward going neutrino induced muons.The operation of these lines follows on from that of the ANTARES instrumentation line, which has provided data for more than a year on the detector stability and the environmental conditions. The full 12 line detector is planned to be fully operational early 2008.

astro-ph↗
arXiv2012

Modified Gravity: Cosmology without dark matter or Einstein's cosmological constant

We explore the cosmological consequences of Modified Gravity (MOG), and find that it provides, using a minimal number of parameters, good fits to data, including CMB temperature anisotropy, galaxy power spectrum, and supernova luminosity-distance observations without exotic dark matter. MOG predicts a bouncing cosmology with a vacuum energy term that yields accelerating expansion and an age of ~13 billion years.

astro-ph↗
arXiv2007

Primordial magnetic fields at preheating

Using lattice techniques we investigate the generation of long range cosmological magnetic fields during a cold electroweak transition. We will show how magnetic fields arise, during bubble collisions, in the form of magnetic strings. We conjecture that these magnetic strings originate from the alignment of magnetic dipoles associated with EW sphaleron-like configurations. We also discuss the early thermalisation of photons and the turbulent behaviour of the scalar fields after tachyonic preheating.

hep-lat↗
arXiv2007

Extended Einstein-Maxwell model

A self-consistent extended Einstein-Maxwell model for relativistic non-stationary polarizable-magnetizable anisotropic media is presented. Based on the analogy with relativistic extended irreversible (transient) thermodynamics, the extended constitutive equations for the electrodynamics of continua are formulated phenomenologically, the convective derivatives of the first, second, etc. orders being taken into account. The master equations for the gravity field contain a modified effective (symmetric) stress-energy tensor of the electromagnetic field in a material medium, the use of this tensor being motivated both by historical analogies and direct variational procedure. By way of example we consider the exact solution of the extended Einstein-Maxwell model, describing the isotropic cosmological model with hidden non-vanishing electromagnetic field, electric polarization and magnetization.

gr-qc↗
arXiv2012

The Cosmos in Your Pocket: How Cosmological Science Became Earth Technology. I

Astronomy provides a laboratory for extreme physics, a window into environments at extremes of distance, temperature and density that often can't be reproduced in Earth laboratories, or at least not right away. A surprising amount of the science we understand today started out as solutions to problems in astronomy. Some of this science was key in the development of many technologies which we enjoy today. This paper describes some of these connections between astronomy and technology and their history.

physics.pop-ph↗
arXiv2007

Habitability of Planets in Binaries

A survey of currently known extrasolar planets indicates that close to 20% of their hosting stars are members of binary systems. While the majority of these binaries are wide (i.e., with separations between 250 and 6500 AU), the detection of Jovian-type planets in the three binaries of Gamma Cephei (separation of 18.5 AU), GL 86 (separation of 21 AU), and HD 41004 (separation of 23 AU) have brought to the forefront questions on the formation of giant planets and the possibility of the existence of smaller bodies in moderately close binary star systems. This paper discusses the late stage of the formation of habitable planets in binary systems that host Jovian-type bodies, and reviews the effects of the binary companion on the formation of Earth-like planets in the system's habitable zone. The results of a large survey of the parameter-space of binary-planetary systems in search of regions where habitable planets can form and have long-term stable orbits are also presented.

astro-ph↗
arXiv2007

The afterglow onset for GRB060418 and GRB060607A

Gamma-ray burst are thought to be produced by highly relativistic outflows. Although upper and lower limits for the outflow initial Lorentz factor $Γ_0$ are available, observational efforts to derive a direct determination of $Γ_0$ have so far failed or provided ambiguous results. As a matter of fact, the shape of the early-time afterglow light curve is strongly sensitive on $Γ_0$ which determines the time of the afterglow peak, i.e. when the outflow and the shocked circumburst material share a comparable amount of energy. We now comment early-time observations of the near-infrared afterglows of GRB 060418 and GRB 060607A performed by the REM robotic telescope. For both events, the afterglow peak was singled out and allowed us to determine the initial fireball Lorentz, $Γ_0\sim 400$.

astro-ph↗
arXiv2007

Photometric Observations of Supernova 2002hh

CCD VRI photometry is presented for SN 2002hh from 14 days after the outburst till day 347. SN 2002hh appears to be normal type IIP supernova regarding both luminosity and the shape of the light curve, which is similar to SN 1999gi.

astro-ph↗
arXiv2007

Hard X-rays from Galaxy Clusters and SIMBOL-X

Non thermal emission from galaxy clusters demonstrates the existence of relativistic particles and magnetic fields in the Intra Cluster Medium (ICM). Present instruments do not allow to firmly establish the energy associated to these components. In a few years gamma ray observations will put important constraints on the energy content of non thermal hadrons in clusters, while the combination of radio and hard X-ray data will be crucial to measure the energy content in the form of relativistic electrons and magnetic field. SIMBOL-X is expected to drive an important breakthrough in the field also because it is expected to operate in combination with the forthcoming low frequency radio telescopes (LOFAR, LWA). In this contribution we report first estimates of statistical properties of the hard X-ray emission in the framework of the re-acceleration model. This model allows to reproduce present radio data for Radio Halos and to derive expectations for future low frequency radio observations, and thus our calculations provide hints for observational strategies for future radio and hard--X-ray combined observations.

astro-ph↗