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arXiv2007

Observations of 1ES 0647+250 and 1ES 0806+524 with VERITAS

Observations of the blazars 1ES 0647+250 and 1ES 0806+524 with VERITAS are reported here. These objects are among the favoured candidate extragalactic sources in the very high-energy regime due to the presence of high-energy electrons and adequate seed photons. The presence of high-energy electrons is established from the location of the synrchrotron peak in the spectral energy distribution of the blazars. The presence of adequate seed photons is determined by the flux in the radio-through-optical wavebands. These are the key ingredients for very high-energy gamma-ray emission in the context of the synchrotron self-Compton model. The redshift of 1ES 0647+250 has been tentatively reported as 0.203 and the redshift of 1ES 0806+524 is 0.138, thus the detection of very high-energy gamma-ray emission from these objects could make significant contributions to the understanding of the extragalactic infrared background light. The analysis of these data relies on standard techniques in very high-energy gamma-ray astronomy, and the results are compared to previously reported upper limits and to theoretical predictions.

astro-ph↗

arXiv2007

Transdet: a matched-filter based algorithm for transit detection - application to simulated COROT light curves

We present a matched-filter based algorithm for transit detection and its application to simulated COROT light curves. This algorithm stems from the work by Bordé, Rouan & Léger (2003). We describe the different steps we intend to take to discriminate between planets and stellar companions using the three photometric bands provided by COROT. These steps include the search for secondary transits, the search for ellipsoidal variability, and the study of transit chromaticity. We also discuss the performance of this approach in the context of blind tests organized inside the COROT exoplanet consortium.

astro-ph↗

arXiv2007

Observations of Gamma-ray Bursts with VERITAS and Whipple

Many authors have predicted very-high-energy (VHE; E > 100 GeV) emission from gamma-ray bursts (GRBs) both during the prompt phase and during the multi-component afterglow. To date, however, there has been no definitive detection of such emission. Recently, the Swift Satellite made the exciting discovery that almost 50% of GRBs are accompanied by one or more X-ray flares, which are found to occur from several seconds to many hours after the prompt emission. The discovery of this phenomenon and the many predictions that VHE emission should accompany these flares increases the already strong motivation for making immediate follow-up VHE observations of GRBs. Observations of GRBs have high priority at VERITAS, preempting any observations that may be in progress. GRB alerts are received from the GCN via a socket connection. This is interfaced to the VERITAS Tracking Software to minimize the time between a notification arriving and the telescope being slewed to the GRB. We report here on GRB observations with VERITAS and with the Whipple Telescope from 2005 through 2007.

astro-ph↗

arXiv2007

Inverse-Compton emission from halos around stars

Inverse Compton scattering by relativistic electrons produces a major component of the diffuse emission from the Galaxy. The photon fields involved are the cosmic microwave background and the interstellar radiation field from stars and dust. Calculations of the inverse Compton distribution have usually assumed a smooth ISRF, but in fact a large part of the Galactic luminosity comes from the most luminous stars which are rare. Therefore we expect the ISRF, and hence the inverse Compton emission, to be clumpy. We also show that some of the most luminous stars may be visible to GLAST. In this paper we give an update on our previous work including examples of the intensity distribution around stars, and the predicted spectrum of Cygnus OB2.

astro-ph↗

arXiv2007

Observations of the Crab Nebula and Pulsar with VERITAS

Observations of the Crab Nebula have proven to be the best tool to calibrate and to characterize the performance of a Cherenkov telescope. Scientifically, it is interesting to measure the energy spectrum of the Crab Nebula close to the inverse-Compton peak where a deviation is expected from the power law seen at energies above 300 GeV. Additionally, it is important to search for pulsed emission from the Crab Pulsar at energies beyond the 10 GeV upper limit of the EGRET pulsar detection. Since current models predict a cut-off in pulsed emission between 10 and 100 GeV, measurements at energies close to this range may help to discriminate between them. We observed the Crab extensively in the 2006-2007 season during the VERITAS 2- and 3-telescope commissioning phases. Using this data set we reconstructed a preliminary energy spectrum of the signal from the Crab Nebula. We also measured the optical pulsed signal to validate our GPS time-stamping and barycentering techniques and obtained an upper limit for the pulsed emission at gamma-ray energies.

astro-ph↗

arXiv2007

Results from the Blazar Monitoring Campaign at the Whipple 10m Gamma-ray Telescope

In September 2005, the observing program of the Whipple 10 m gamma-ray telescope was redefined to be dedicated almost exclusively to AGN monitoring. Since then the five Northern Hemisphere blazars that had already been detected at Whipple are monitored routinely each night that they are visible. Thanks to the efforts of a large number of multiwavelength collaborators, the first year of this program has been very successful. We report here on the analysis of Markarian 421 observations taken from November, 2005 to May, 2006 in the gamma-ray, X-ray, optical and radio bands.

astro-ph↗

arXiv2008

Features of the Acoustic Mechanism of Core-Collapse Supernova Explosions : Revisited using the Kinetic Approach

The discrete kinetic model is used to study the propagation of sound waves in system of hard-disk-like rotating stars (or vortex gases). The anomalous (negative) attenuation or amplification which is possibly due to the binary collision of a dilute-enough rotating disk (or vortex-gas) system (each with opposite-sign rotating direction or angular momenta but the total (net) angular momenta or vorticity is zero) or microreversibility might arise from the implicit balance of the angular momentum during encounter and give clues to the understanding of possible acceleration of cosmic rays passing through this kind of channel and direct or inverse vortex-gas cascades in two-dimensional turbulence of astrophysical problems.

astro-ph↗

arXiv2007

Observations of Pulsar Wind Nebulae with the VERITAS Array of Imaging Atmospheric Cherenkov Telescopes

Many of the recently discovered galactic very high-energy (VHE) gamma-ray sources are associated with Pulsar Wind Nebulae, which is the most populous Galactic source category at TeV energies. The extended synchrotron nebulae of these objects observed in the X-ray band are a hallmark of the relativistic winds, generated by the young, energetic neutron stars, that interact with the matter ejected by the supernova explosion and the surrounding interstellar gas. Relativistic electrons, or protons, accelerated in the pulsar winds, or at their shock boundaries, interact with the magnetic field and low energy seed photons to produce the observed VHE gamma-ray emission. The VERITAS array of four imaging atmospheric Cherenkov telescopes was designed to study astrophysical sources of gamma rays in the energy domain from about 100 GeV up to several tens of TeV. The sensitivity of the VERITAS array allows detailed studies of the morphology and spectral features of gamma-ray emission from PWNe. Three northern sky PWNe, G75.2+0.1, G106.6+2.9, and 3C58, were observed with VERITAS during 2006. No evidence for TeV gamma-ray emission at the position of the pu lsar associated with these PWNe is demonstrated.

astro-ph↗

arXiv2007

Laser Atmospheric Studies with VERITAS

As a calibrated laser pulse propagates through the atmosphere, the amount of Rayleigh-scattered light arriving at the VERITAS telescopes can be calculated precisely. This technique was originally developed for the absolute calibration of ultra-high-energy cosmic-ray fluorescence telescopes but is also applicable to imaging atmospheric Cherenkov telescopes (IACTs). In this paper, we present two nights of laser data taken with the laser at various distances away from the VERITAS telescopes and compare it to Rayleigh scattering simulations.

astro-ph↗

arXiv2007

Gravitational lensing

This is a short and biased review of gravitational lensing with emphasis on the radio and especially VLBI aspects. We briefly explain the basic idea and give a short sketch of the discovery of the first lens, before we more systematically discuss the general fields that can be studied with lensing. We intentionally omit the details the average lensing expert would like to see and instead try to give a very general overview addressed to the radio astronomer working in some other field. The lens B0218+357 is presented as an example to show many aspects of lensing in a case that already has led to interesting results but still has additional potential for the future.

astro-ph↗

arXiv2007

The VERITAS standard data analysis

VERITAS is an array of Imaging Atmospheric Cherenkov Telescopes designed for very high energy gamma ray (E>100,GeV) observations of astrophysical sources. The experiment began its scientific observation program in the 2006/2007 observing season. We describe here the analysis chain for reducing the data, reconstructing the direction and energy of incident gamma rays and the rejection of background cosmic rays.

astro-ph↗

arXiv2007

Establishing a connection between high-power pulsars and very-high-energy gamma-ray sources

In the very-high-energy (VHE) gamma-ray wave band, pulsar wind nebulae (PWNe) represent to date the most populous class of Galactic sources. Nevertheless, the details of the energy conversion mechanisms in the vicinity of pulsars are not well understood, nor is it known which pulsars are able to drive PWNe and emit high-energy radiation. In this paper we present a systematic study of a connection between pulsars and VHE gamma-ray sources based on a deep survey of the inner Galactic plane conducted with the High Energy Stereoscopic System (H.E.S.S.). We find clear evidence that pulsars with large spin-down energy flux are associated with VHE gamma-ray sources. This implies that these pulsars emit on the order of 1% of their spin-down energy as TeV gamma-rays.

astro-ph↗

arXiv2007

Monte Carlo studies of the VERITAS array of Cherenkov telescopes

VERITAS is a system of four imaging Cherenkov telescopes located at the Fred Lawrence Whipple Observatory in southern Arizona. We present here results of detailed Monte Carlo simulations of the array response to extensive air showers. Cherenkov image and shower parameter distributions are calculated and show good agreement with distributions obtained from observations of background cosmic rays and high-energy gamma-rays. Cosmic-ray and gamma-ray rates are accurately predicted by the simulations. The energy threshold of the 3-telescope system is about 150 GeV after gamma-hadron separation cuts; the detection rate after gamma-selection cuts for the Crab Nebula is 7.5 gamma's/min. The three-telescope system is able to detect a source with a flux equivalent to 10% of the Crab Nebula flux in 1.2 h of observations (5 sigma detection).

astro-ph↗

arXiv2008

Deep study of the fast bipolar outflows in pre-PNe from CO mm-wave line emission

High spatial resolution images of PNe have shown their extremely complex morphology. However, the circumstellar envelopes of their progenitors, the AGB stars, are strikingly spherical. In order to understand the carving processes leading to axisymmetric nebulae, we are carrying out a study of a large sample of pre-PNe. Our emission model of the nebular molecular gas (12CO & 13CO) will allow us to determine important physical parameters (mass, linear momentum, kinetic energy) of the fast bipolar and slow spherical nebular components separately. We will study in an innovative way the properties for each source individually, and put our results in an evolutionary context with the help of the data obtained by us and collected from the literature.

astro-ph↗

arXiv2007

VEGAS, the VERITAS Gamma-ray Analysis Suite

VERITAS, the Very Energetic Radiation Imaging Telescope Array System, is an array of four 12 m diameter imaging atmospheric Cherenkov telescopes for gamma-ray astronomy above 100 GeV currently in operation in Arizona. The VERITAS Collaboration has developed VEGAS, the VERITAS Gamma-ray Analysis Suite, a data-analysis software package for the processing of single- and multiple-telescope data produced by the array. The package consists of a core of six stages as well as visualisation and diagnostic components. It has been developed in C++ using modern objected-oriented design patterns to be highly flexible, configurable and extendable. VEGAS utilises CERN's ROOT data-analysis framework and runs on Linux and Mac OS X systems. The architecture and structure of the VEGAS package will be described in detail while the data analysis algorithms are described in additional papers.

astro-ph↗

arXiv2007

Discovery of the candidate pulsar wind nebula HESS J1718-385 in very-high-energy gamma-rays

Motivated by recent detections of pulsar wind nebulae in very-high-energy (VHE) gamma rays, a systematic search for VHE gamma-ray sources associated with energetic pulsars was performed, using data obtained with the H.E.S.S. (High Energy Stereoscopic System) instrument. The search for VHE gamma-ray sources near the pulsar PSR J1718-3825 revealed the new VHE gamma-ray source HESS J1718-385. We report on the results from the HESS data analysis of this source and on possible associations with the pulsar and at other wavelengths. We investigate the energy spectrum of HESS J1718-385 that shows a clear peak. This is only the second time a VHE gamma-ray spectral maximum from a cosmic source was observed, the first being the Vela X pulsar wind nebula.

astro-ph↗

arXiv2007

The VERITAS Trigger System

The VERITAS gamma-ray observatory, situated in southern Arizona, is an array of four 12m diameter imaging Cherenkov telescopes, each with a 499-pixel photomultiplier-tube camera. The instrument is designed to detect astrophysical gamma rays at energies above 100 GeV. At the low end of the VERITAS energy range, fluctuations in the night sky background light and single muons from cosmic-ray showers constitute significant backgrounds. VERITAS employs a three-tier trigger system to reduce the rate of these background events: an initial trigger which acts at the single pixel level, a pattern trigger which acts on the relative timing and pixel level, a pattern trigger which acts on the relative timing and distribution of pixel-level triggers within a single telescope camera, and an array-level trigger which requires simultaneous observation of an air-shower event in multiple telescopes. This final coincidence requirement significantly reduces the rate of background events, particularly those due to single muons. In this paper, the implementation of all levels of the VERITAS trigger system is discussed and their joint performance is characterized.

astro-ph↗

arXiv2008

Solar System Constraints on f(G) Dark Energy

Corrections to solar system gravity are derived for f(G) gravity theories, in which a function of the Gauss-Bonnet curvature term is added to the gravitational action. Their effects on Newton's law, as felt by the planets, and on the frequency shift of signals from the Cassini spacecraft, are both determined. Despite the fact that the Gauss-Bonnet term is quadratic in curvature, the resulting constraints are substantial. It is shown that they practically rule out f(G) as a natural explanation for the late-time acceleration of the universe. Possible exceptions are when f(G) reduces to something very close to a cosmological constant, or if the form of the function f is exceptionally fine-tuned.

hep-th↗