arXiv subjects

S. Westerhoff

Publications and source records attributed to S. Westerhoff.

45 records · Page 3Linked to original sources

arXiv1999

Search for a TeV Component of GRBs using the Milagrito Detector

Observing gamma ray bursts (GRBs) in the TeV energy range can be extremely valuable in providing insight to GRB radiation mechanisms and in constraining source distances. The Milagrito detector was an air shower array which used the water Cherenkov technique to search for TeV sources. Data from this detector was analyzed to look for a TeV component of GRBs coincident with low energy -rays detected by the BATSE instrument on the Compton Gamma Ray Observatory. A sample of 54 BATSE GRBs which were in the field of view of the Milagrito detector during its lifetime (February 1997 to May 1998) was used.

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arXiv1999

Search for Short Duration Bursts of TeV Gamma Rays with the Milagrito Telescope

The Milagrito water Cherenkov telescope operated for over a year. The most probable gamma-ray energy was ~1 TeV and the trigger rate was as high as 400 Hz. We have developed an efficient technique for searching the entire sky for short duration bursts of TeV photons. Such bursts may result from "traditional" gamma-ray bursts that were not in the field-of-view of any other instruments, the evaporation of primordial black holes, or some as yet undiscovered phenomenon. We have begun to search the Milagrito data set for bursts of duration 10 seconds. Here we will present the technique and the expected results. Final results will be presented at the conference.

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arXiv1999

Detection of 6 November 1997 Ground Level Event by Milagrito

Solar Energetic Particles from the 6 November 1997 solar flare/CME(coronal mass ejection) with energies exceeding 10 GeV have been detected by Milagrito, a prototype of the Milagro Gamma Ray Observatory. While particle acceleration beyond 1 GeV at the Sun is well established, few data exist for protons or ions beyond 10 GeV. The Milagro observatory, a ground based water Cherenkov detector designed for observing very high energy gamma ray sources, can also be used to study the Sun. Milagrito, which operated for approximately one year in 1997/98, was sensitive to solar proton and neutron fluxes above ~5- 10 GeV. Milagrito operated in a scaler mode, which was primarily sensitive to muons, low energy photons, and electrons, and the detector operated in a mode sensitive to showers and high zenith angle muons. In its scaler mode, Milagrito registered a rate increase coincident with the 6 November 1997 ground level event observed by Climax and other neutron monitors. A preliminary analysis suggests the presence of >10 GeV particles.

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arXiv1999

Study of the Shadows of the Moon and the Sun with VHE Cosmic Rays

Milagrito, a prototype for the Milagro detector, operated for 15 months in 1997-8 and collected 8.9 billion events. It was the first extensive air shower (EAS) array sensitive to showers intiated by primaries with energy below 1 TeV. The shadows of the sun and moon observed with cosmic rays can be used to study systematic pointing shifts and measure the angular resolution of EAS arrays. Below a few TeV, the paths of cosmic rays coming toward the earth are bent by the helio- and geo-magnetic fields. This is expected to distort and displace the shadows of the sun and the moon. The moon shadow, offset from the nominal (undeflected) position, has been observed with high statistical significance in Milagrito. This can be used to establish energy calibrations, as well as to search for the anti-matter content of the VHE cosmic ray flux. The shadow of the sun has also been observed with high significance.

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arXiv1999

An All-Sky Search for Steady VHE Gamma-Ray Sources

The Milagrito water Cherenkov detector in the Jemez Mountains near Los Alamos, New Mexico took data from February 1997 to April 1998. Milagrito served as a prototype for the larger Milagro detector, which has just begun operations. Milagrito was the first large-aperture gamma-ray detector with sensitivity to gamma rays below 1 TeV. We report here on a search for steady emission from point sources over most of the northern sky using data from Milagrito.

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arXiv1998

Conceptual Design of a Next-Generation All-Sky Gamma-Ray Telescope Operating at TeV Energies

The next generation all-sky monitor operating at TeV (0.1 - 30 TeV) energies should be capable of performing a continuous high sensitivity sky survey, and detecting transient sources, such as AGN flares, with high statistical significance on timescales of hours to days. We describe an instrument concept for a large area, wide aperture, TeV ground-based Gamma-ray telescope. The conceptual design is comprised of 20000 sqm scintillator-based pixels arranged in a densely packed mosaic. In addition to fast timing, the dense array of pixels also provides a unique imaging capability; the shower image, the spatial distribution of secondary particles at ground level, can be exploited to identify and reject hadronic backgrounds. The good angular resolution and background rejection capabilities lead directly to high sensitivity, while retaining a large field of view, large effective area, and high duty cycle. An instrument such as the one described here complements the narrow field of view air-Cherenkov telescopes and could, in conjunction with future space-based instruments, extend the energy range for continuous monitoring of the Gamma-ray sky from MeV through TeV energies.

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arXiv1998

An Upper Limit on the Infrared Background Density from HEGRA data on Mkn501

The energy spectrum of Mkn501 in the TeV energy regime, as measured by the HEGRA (High Energy Gamma Ray Astronomy) Cerenkov telescopes during its low state in 1995/96 and during a fraction of the 1997 outburst in the TeV energy regime, is shown to place stringent upper limits on the still unknown infrared photon density in the energy region between 3 x 10**(-3) and 3 xt 10**(-1) eV. Assuming two different shapes for the unknown infrared photon spectrum in this energy range we calculate upper limits on the infrared photon density on the basis of the power-law fit obtained for the observed spectrum up to the maximum energy.

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arXiv1997

Results on AGN observations with HEGRA

Data taken with the HEGRA (High Energy Gamma Ray Astronomy) air shower array on La Palma is used to search for an excess of TeV $γ$-rays from nearby AGN. We report on the detection of two blazars (Mrk 421 and Mrk 501) with the two prototype Cherenkov telescopes, and an analysis based on data taken with the scintillator and air Cherenkov array. Using the source stacking method we find preliminary evidence for 50 TeV $γ$-rays from the superposition of nearby blazars.

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arXiv1996

Beacons at the Gamma Ray Horizon

Blazars with redshifts z<0.1 are likely candidates for detection at energies in the range 300 GeV - 50 TeV with Cerenkov telescopes and scintillator arrays. We present gamma-ray flux predictions for a sample of 15 nearby flat-spectrum radio sources fitting the proton blazar model of Mannheim (1993a) to their observed broad-band spectral energy distributions. At high energies, we use fluxes or flux limits measured by ROSAT, CGRO and the Whipple Observatory to constrain their spectra. We take into account absorption of the gamma-rays by pair production with low energy photons of the diffuse infrared-to-optical photon background produced by galaxies (cosmic absorption) and with low energy synchrotron photons of the blazar radiation field (internal absorption). Typically, the theoretical spectra decrease much faster above TeV (photon index s~3) than between GeV and TeV (s~2) owing to internal absorption. The predicted fluxes are confronted with flux limits in the 20-50 TeV energy range obtained by the High Energy Gamma Ray Astronomy (HEGRA) experiment. Without cosmic absorption, the fluxes are about equal to the current sensitivity of HEGRA. Improved gamma/hadron separation techniques could render a detection by HEGRA possible, if cosmic absorption by the far-infrared background at wavelengths ~100 mu is not exceedingly strong.

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