K. Lyons

Very High Energy Gamma Rays from PKS 2155–304

The close X-ray-selected BL Lac PKS 2155-304 has been observed using the University of Durham Mark 6 very high energy (VHE) gamma-ray telescope during 1996 September/October/November and 1997 October/November. VHE gamma rays with energy more than 300 GeV were detected from this object with a time-averaged integral flux of (4.2±0.7stat±2.0sys)×10−11 cm-2 s-1. There is evidence for VHE gamma-ray emission during our observations in 1996 September and 1997 October/November, with the strongest emission being detected in 1997 November, when the object was producing the largest flux ever recorded in high-energy X-rays and it was detected as a source of gamma rays of energy greater than 100 MeV. The VHE and X-ray fluxes show evidence of a correlation.

PKS 2155-304 - a source of VHE γ-rays

Abstract The close X-ray selected BL Lac PKS 2155-304 has been observed using the University of Durham Mark 6 very high energy (VHE) gamma ray telescope during 1996 September/October/November and 1997 October/November. VHE gamma rays with energy > 300 GeV were detected with a time-averaged integral flux of (4.2 ± 0.7stat ± 2.0sys) × 10−11 cm−2 s−1. There is evidence for VHE gamma ray emission during our observations in 1996 September and 1997 October/November. The strongest emission was detected in 1997 November, when the object was producing a large flux of high-energy X-rays and was detected in > 100 MeV gamma-rays.

VERY HIGH ENERGY GAMMA RAYS FROM YOUNG PULSARS AND SUPERNOVA REMNANTS IN THE SOUTHERN HEMISPHERE

Observations have been made with the University of Durham Mark 6 telescope of a number of southern hemisphere supernova remnants and young pulsars (Vela pulsar, PSR B1055-52, PSR J1105-6107, PSR J0537-6910, and PSR B0540-69). No very high energy gamma-ray emission, either steady or pulsed, has been detected from these objects. The implications of these results for theories of high-energy gamma-ray production in plerions and young pulsars are discussed.

A Search for Very High Energy Gamma Rays from Active Galactic Nuclei Visible from the Southern Hemisphere

Observations have been made, using the University of Durham Mark 6 gamma-ray telescope, of the very high energy gamma-ray emission from a number of active galactic nuclei visible from the Southern Hemisphere. Limits are presented to the very high energy gamma-ray emission from 1ES 0323+022, PKS 0829+046, 1ES 1101-232, Cen A, PKS 1514-24, RX J10578-275, and 1ES 2316-423, both for steady long-term emission and for outbursts of emission on timescales of 1 day.Observations have been made, using the University of Durham Mark 6 gamma ray telescope, of the very high energy gamma ray emission from a number of active galactic nuclei visible from the Southern hemisphere. Limits are presented to the VHE both for steady long-term emission and for outbursts of emission on timescales of 1 day. A) — galaxies: active — gamma rays: observations

Geomagnetic effects on atmospheric Cerenkov images

Atmospheric Cerenkov telescopes are used to detect electromagnetic showers from primary -rays of energy ~300 GeV up to 10 TeV and to discriminate these from cascades due to hadrons using the Cerenkov images. The geomagnetic field affects the development of showers and is shown to diffuse and distort the images. When the component of the field normal to the shower axis is sufficiently large (>0.4 G) the performance of -ray telescopes may be affected, although corrections should be possible.

Sensing atmospheric conditions using MIR radiometers

MIR (8–14 μm) radiometers have been used to sense the temperature of the sky as a measure of cloud and other obscurants in the field of an atmospheric Cerenkov telescope. Comparisons between the MIR signature and the performance of the Mark 6 gamma ray telescope are reported.

The correction of atmospheric Cerenkov images for the effect of the geomagnetic field

Imaging atmospheric Cerenkov telescopes use the differences in shape and orientation of the images of development of cascades to distinguish between very high energy gamma rays and the cosmic ray background. We have previously shown that these images may be distorted by the effects of the geomagnetic field, which decreases the sensitivity of a telescope. In this paper we propose an analysis technique which corrects for (in first order) the effects of the geomagnetic field on the orientation of gamma ray images. Applying this technique to observations of gamma ray sources, we demonstrate that these corrections reduce the effect of the geomagnetic field and improve the sensitivity of the University of Durham Mark 6 telescope.

OBSERVATIONS OF TEV GAMMA RAYS FROM MARKARIAN 501 AT LARGE ZENITH ANGLES

TeV gamma rays from the blazar Markarian 501 have been detected with the University of Durham Mark 6 atmospheric Cerenkov telescope using the imaging technique at large zenith angles. Observations were made at zenith angles in the range 70°-73° during July and August 1997 when Markarian 501 was undergoing a prolonged and strong flare.

PKS 2155-304—A source of very high energy gamma rays

The University of Durham Mark 6 very high energy (VHE) γ-ray telescope has been used to observe the X-ray selected BL lac PKS 2155-304 in 1996, 1997 and 1998. The data show evidence for the emission of VHE γ-rays with E>300 GeV in 1996 September and 1997 October/November. There is evidence to support a correlation between the X-ray emission and VHE γ-ray emission. The strongest VHE emission was observed in 1997 November, when the object was producing the strongest high-energy X-ray emission ever recorded, and emission was also detected at E>100 MeV.

Very high energy gamma rays from Cen X-3

Cen X-3 is a well-studied high-mass accreting X-ray binary and a variable source of high energy gamma rays from 100 MeV to 1 TeV. The object has been extensively monitored with the University of Durham Mark 6 telescope. Results of observations, including those taken in 1998 and 1999, are reported. There is no evidence for time variability in the VHE data. There is also no evidence for correlation of the VHE flux with the X-ray flux detected by BATSE and RXTE/ASM. A search for periodic emission, at or close to the X-ray spin period, in the VHE data yielded a 3σ upper limit to the pulsed flux of 2.0×10−12 cm−12 s−1.