Simon P. Swordy

VERITAS: The Very Energetic Radiation Imaging Telescope Array System

Abstract A next generation atmospheric Cherenkov observatory is described which uses the Whipple Observatory gamma-ray telescope as a prototype. An array of seven imaging telescopes will be deployed such that they will permit the maximum versatility and will give the highest sensitivity in the 50 GeV-50 TeV band (with maximum sensitivity from 100 GeV to 10 TeV). In this band critical measurements of natures most powerful accelerators will be made.The Very Energetic Radiation Imaging Telescope Array System (VERITAS) represents an important step forward in the study of extreme astrophysical processes in the universe. It combines the power of the atmospheric Cherenkov imaging technique using a large optical reflector with the power of stereoscopic observatories using arrays of separated telescopes looking at the same shower. The seven identical telescopes in VERITAS, each of aperture 10 m, will be deployed in a filled hexagonal pattern of side 80 m; each telescope will have a camera consisting of 499 pixels with a field of view of 3.5 deg VERITAS will substantially increase the catalog of very high energy (E>100GeV) gamma-ray sources and greatly improve measurements of established sources.

Cosmic-ray Proton and Helium Spectra from the First CREAM Flight

Cosmic-ray proton and helium spectra have been measured with the balloon-borne Cosmic Ray Energetics And Mass experiment flown for 42 days in Antarctica in the 2004–2005 austral summer season. High-energy cosmic-ray data were collected at an average altitude of �38.5 km with an average atmospheric overburden of �3.9 g cm −2 . Individual elements are clearly separated with a charge resolution of �0.15 e (in charge units) and �0.2 e for protons and helium nuclei, respectively. The measured spectra at the top of the atmosphere are represented by power laws with a spectral index of 2.66 ± 0.02 for protons from 2.5 TeV to 250 TeV and –2.58 ± 0.02 for helium nuclei from 630 GeV nucleon −1 to 63 TeV nucleon −1 . They are harder than previous measurements

Cosmic-Ray Electrons and Positrons from 1 to 100 GeV: Measurements with HEAT and Their Interpretation

Measurements of the energy spectra of negative electrons and positrons have been performed with the High-Energy Antimatter Telescope (HEAT) in two balloon flights—1994 May from Fort Sumner, NM, and 1995 August from Lynn Lake, Manitoba. We present the combined data set from these two flights, covering the energy range 1-100 GeV. We compare our data with results from other groups and discuss the data in the context of diffusive propagation models. There is some evidence that primary electrons above 10 GeV and cosmic-ray nuclei exhibit the same energy spectrum at the source, but that the source spectrum becomes harder at lower energy. Within the experimental uncertainties, the intensity of positrons is consistent with a purely secondary origin, due to nuclear interactions in interstellar space.

A MULTIWAVELENGTH VIEW OF THE TeV BLAZAR MARKARIAN 421: CORRELATED VARIABILITY, FLARING, AND SPECTRAL EVOLUTION

We report results from an intensive multiwavelength monitoring campaign on the TeV blazar Mrk 421 over the period of 2003-2004. The source was observed simultaneously at TeV energies with the Whipple 10 m telescope and at X-ray energies with the Rossi X-Ray Timing Explorer (RXTE) during each clear night within the Whipple observing windows. Supporting observations were also frequently carried out at optical and radio wavelengths to provide simultaneous or contemporaneous coverages. The large amount of simultaneous data has allowed us to examine the variability of Mrk 421 in detail, including cross-band correlation and broadband spectral variability, over a wide range of flux. The variabilities are generally correlated between the X-ray and gamma-ray bands, although the correlation appears to be fairly loose. The light curves show the presence of flares with varying amplitudes on a wide range of timescales at both X-ray and TeV energies. Of particular interest is the presence of TeV flares that have no coincident counterparts at longer wavelengths, because the phenomenon seems difficult to understand in the context of the proposed emission models for TeV blazars. We have also found that the TeV flux reached its peak days before the X-ray flux did during a giant flare (or outburst) in 2004 (with the peak flux reaching ~135 mcrab in X-rays, as seen by the RXTE ASM, and ~3 crab in gamma rays). Such a difference in the development of the flare presents a further challenge to both the leptonic and hadronic emission models. Mrk 421 varied much less at optical and radio wavelengths. Surprisingly, the normalized variability amplitude in the optical seems to be comparable to that in the radio, perhaps suggesting the presence of different populations of emitting electrons in the jet. The spectral energy distribution of Mrk 421 is seen to vary with flux, with the two characteristic peaks moving toward higher energies at higher fluxes. We have failed to fit the measured spectral energy distributions (SEDs) with a one-zone synchrotron self-Compton model; introducing additional zones greatly improves the fits. We have derived constraints on the physical properties of the X-ray/gamma-ray flaring regions from the observed variability (and SED) of the source. The implications of the results are discussed.

The first VERITAS telescope

Abstract The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic Radiation Imaging Telescope Array System) has been in operation since February 2005. We present here a technical description of the instrument and a summary of its performance. The calibration methods are described, along with the results of Monte Carlo simulations of the telescope and comparisons between real and simulated data. The analysis of TeV γ-ray observations of the Crab Nebula, including the reconstructed energy spectrum, is shown to give results consistent with earlier measurements. The telescope is operating as expected and has met or exceeded all design specifications.

VERITAS Observations of the γ-Ray Binary LS I +61 303

LS I +61 303 is one of only a few high-mass X-ray binaries currently detected at high significance in very high energy γ-rays. The system was observed over several orbital cycles (between 2006 September and 2007 February) with the VERITAS array of imaging air Cerenkov telescopes. A signal of γ-rays with energies above 300 GeV is found with a statistical significance of 8.4 standard deviations. The detected flux is measured to be strongly variable; the maximum flux is found during most orbital cycles at apastron. The energy spectrum for the period of maximum emission can be characterized by a power law with a photon index of -->Γ = 2.40 ± 0.16stat± 0.2sys and a flux above 300 GeV corresponding to 15%-20% of the flux from the Crab Nebula.

Energy spectra and composition of primary cosmic rays

New results are described on the energy spectra and relative abundances of primary cosmic ray nuclei from carbon to iron. The measurement was performed on the Spacelab-2 mission of the Space Shuttle Challenger in 1985, and extends to energies beyond 1 TeV per amu. The data indicate that the cosmic ray flux arriving near earth becomes enriched with heavier nuclei, most notably iron, as energy increases. Extrapolating to the source, with a simple leaky box model of galactic propagation with rigidity-dependent containment time, relative abundances of the elements are obtained that are quite similar to those reported at lower energy. In particular, the depletion of elements with high first ionization potential relative to the local galactic abundances, seems to persist in the cosmic ray source well up to TeV energies. A single power-law energy spectrum about E exp {minus}2.1 provides a good description of the observed spectra of most elemental species. 33 refs.

New measurement of the cosmic-ray positron fraction from 5 to 15 GeV

We present a new measurement of the cosmic-ray positron fraction at energies between 5 and 15 GeV with the balloon-borne HEAT-pbar instrument in the spring of 2000. The data presented here are compatible with our previous measurements, obtained with a different instrument. The combined data from the three HEAT flights indicate a small positron flux of nonstandard origin above 5 GeV. We compare the new measurement with earlier data obtained with the HEAT-e(+/-) instrument, during the opposite epoch of the solar cycle, and conclude that our measurements do not support predictions of charge sign dependent solar modulation of the positron abundance at 5 GeV.

Measurements of the cosmic ray positron fraction from 1-GeV to 50-GeV

Two measurements of the cosmic-ray positron fraction as a function of energy have been made using the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument. The first flight took place from Fort Sumner, New Mexico, in 1994 and yielded results above the geomagnetic cutoff energy of 4.5 GeV. The second flight, from Lynn Lake, Manitoba, in 1995, permitted measurements over a larger energy interval, from 1 to 50 GeV. We present results on the positron fraction based on data from the Lynn Lake flight and compare these with the previously published results from the Fort Sumner flight. The results confirm that the positron fraction does not increase with energy above ≈ 10 GeV, although a small excess above purely secondary production cannot be ruled out. At low energies the positron fraction is slightly larger than that reported from measurements made in the 1960s. This effect could possibly be a consequence of charge dependence in the level of solar modulation.

Relative abundances of secondary and primary cosmic rays at high energies

New results on the energy spectra of the cosmic-ray nuclei boron, carbon, nitrogen, and oxygen up to energies around 1 TeV per amu are described. The measurements were performed on the Spacelab 2 mission of the Space Shuttle in 1985. Carbon and oxygen are essentially primary cosmic rays, while boron is purely secondary, and nitrogen has secondary as well as primary contributions. Therefore, the relative abundances of these nuclei provide sensitive information on the propagation of cosmic rays through the Galaxy. It is found that the flux of the secondary cosmic rays continues to decrease relative to that of the primaries over the energy range covered with this observation, and that the mean escape length near 1 TeV per amu is about 1 g/sq cm. 36 refs.