K. E. Turver

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.

Transient emission of ultra-high energy pulsed γ rays from Crab pulsar PSR0531

A new experiment has been established to measure the energy spectrum of ultra-high energy (UHE) γ rays (E> 2,000 GeV) from a number of celestial objects. During the commissioning of the equipment several potential sources were studied, including the Crab pulsar PSR0531. UHE γ rays from this object have been observed in previous experiments to be practically all pulsed1–4, but no single observation has been of high statistical significance, and the flux values have been difficult to reproduce5. The current observations are based on 34 h of exposure between 25 September and 2 November 1981, and show overall an integral pulsed flux in agreement with our estimate of the average of previous data; most important, they also show strong evidence for a burst of pulsed emission of ∼15 min duration. There had been previous suggestions that the Crab pulsar is variable at the energy we used and at slightly lower energies1,3, but at time scales of months or days. The present work is the first to demonstrate short duration (15 min) emission and to offer an explanation of previous apparently discordant results.

350 GeV gamma rays from AE Aqr

Abstract We report here the results of VHE γ-ray observations made during 1990 and 1991 using the Durham University Mark III and IV VHE γ-ray telescopes at Narrabri, N.S.W., Australia. There are 4 separate datasets, each obtained from observations over typically 10 days during a lunar dark period. The dataset from October 1990 exhibits periodicity at a chance probability of 1 × 10 -4 , and a similar effect is noted in the dataset from August 1991 at 1.6 × 10 −2 . In both cases, most of the power is in the second harmonic; the period is constant over at least 10 days and similar to that seen at optical wavelengths. No significant signal was detected in the September 1991 and October 1991 datasets. In addition, a strong and highly significant outburst lasting about 1 minute on 13 October 1990 and detected by two independent telescopes is reported. This outburst shows the 33 s periodic pulse structure and the pulses are in phase with those observed during the strongest sustained periodic signal in our data, from an observation lasting 3 h and taken 48 h previously. A second, weaker burst was also observed during August 1991.

TeV gamma rays from PSR 1706-44

Observations made with the University of Durham Mark 6 atmospheric Cerenkov telescope conrm that PSR B1706-44 is a very high energy -ray emitter. There is no indication from our dataset that the very high energy -rays are pulsed, in contrast to the ndings at 300 GeV is estimated to be (3:90:7stat)10 11 cm 2 s 1 .

Centaurus X-3: A Source of High-Energy Gamma Rays

Centaurus X-3 is a well studied high-mass accreting X-ray binary and a variable source of high-energy gamma rays with energies from 100 MeV to 1 TeV. Previous results have suggested that the origin of the gamma rays is not the immediate vicinity of the neutron star but is sited in the accretion disk, perhaps in an accretion wake. The Durham Mark 6 gamma-ray telescope has been used to measure the gamma-ray flux from Centaurus X-3 with much higher sensitivity than previous ground-based measurements. The flux above ~400 GeV was measured to be (2 ± 0.3) × 10-11 cm-2 s-1 and appears constant over a period of 2-3 months. In 10 hours of observations there is no evidence for periodicity in the detected gamma rays at the X-ray spin period either from a site in the region of the neutron star or from any other potential site in the orbit.

Very high energy gamma rays from x-ray binary pulsars

A number of categories of celestial very high energy (> 0.2 TeV) gamma ray sources have now been established. The most surprising discovery has been that one of the categories was x-ray binary sources containing pulsars. The authors describe the ground based atmospheric Cerenkov light techniques used to detect such very high energy (VHE) gamma rays, and review the evidence for emission from x-ray binary pulsars. An attempt is made to establish patterns in the observed emissions as an aid to the understanding of the gamma ray production. Finally, the various models which have been put forward to explain the origin of these gamma rays are reviewed.

A burst of pulsed VHE gamma rays from AE Aquarii

Abstract We report the results of observations of AE Aquarii made on 25 occasions between July 1992 and October 1993 with the University of Durham very high energy (VHE) gamma-ray telescopes at Narrabri, N.S.W., Australia. The observations lasted for a total of 78 h and no persistent low-level pulsed emission was detected above an energy threshold of 350 GeV. However, an excess of pulsed VHE γ-rays was observed on 1993 October 11. The burst lasted for approximately 4200 s and occurred at phase 0.62–0.74 in the 9.88 h orbital cycle. The count rate excess was significant at the 0.99997 level and the periodicity was at the second harmonic of the 33 s period The hadron-induced background in the burst data has been suppressed by applying simple imaging techniques to those events recorded by an individual telescope and using trigonometric height of maximum analysis for the events recorded by two telescopes. These two independent results together provide evidence at a probability of chance occurrence of 10−7, after allowing for degrees of freedom, for the emission of a burst of pulsed γ-rays from AE Aquarii with threshold energy > 350 GeV, which show the characteristic features of γ-ray induced cascades. There is also some evidence to suggest that the brightness spectrum of Cerenkov events recorded during the burst is harder than that recorded in a sample of the background. The flux during the burst was approximately 4% of the total cosmic ray background at E> 350 GeV, corresponding to a peak pulsed luminosity of 1032 erg s−1

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.

TeV gamma rays from Geminga

The bright gamma -ray source. Geminga has been identified as an X-ray and gamma -ray pulsar with a pulse period of 237 ms. Geminga was observed with the University of Durham very high-energy (VHE) gamma -ray telescopes at Dugway, Utah, in 1983. The analysis of these data has revealed pulsations at the X-ray/ gamma -ray pulse period, with the VHE gamma -ray pulses being in phase with the lower-energy gamma -ray pulses. The VHE and HE gamma -ray light curves are connected with a significance for positive correlation of 6*10-4. The VHE gamma -ray signal corresponds to a flux of about 3*10-11 cm-2 s-1 at energies greater than 1000 GeV, and a luminosity of 6*1030 erg s-1.

New measurements of the 12. 6 millisecond pulsar in Cygnus X-3

Evidence for a 12.59 ms pulsar in Cygnus X-3 is presented on the basis of TeV gamma-ray observations. Evidence for pulsed emission at a phase in the 4.8 hr cycle and with a pulsar period and secular period derivative are compatible with earlier measurements (Chadwick et al., 1985). The conservative overall Rayleigh probability of uniformity of phase for this new result is 1.7 x 10 to the -6th. Data from observations of Cygnus X-3 from 1981 to 1985 are analyzed using a new X-ray ephemeris of the 4.8 hr X-ray cycle. This suggests that Cygnus X-3 is producing sporadic very high energy gamma rays at a fixed time in the 4.8 hr X-ray cycle. 28 refs.