E. W. Lincoln

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.

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.

The effect of the geomagnetic field on TeV gamma -ray detection

Very high energy (VHE) gamma -ray astronomy using the ground based atmospheric Cerenkov technique is an important new field of high energy astrophysics and a range of VHE gamma -ray sources has been detected at various levels of significance by a number of experimental groups. Outstanding evidence for gamma -rays from the Crab Nebula was obtained by the Whipple collaboration. This involved the use of a sophisticated technique based on computer simulations to select gamma -ray-like events from the proton induced background When this same technique was applied to data from other sources detected using an earlier version of the Whipple telescope, the evidence for a gamma -ray signal disappeared. The inability to reconcile the Whipple result from the Crab Nebula with both the failure of the Whipple camera to detect other objects and the detections made with other non-imaging telescopes has caused some concern at a critical time in the evolution of this branch of high energy astrophysics. In an attempt to reconcile these results, the authors have made a detailed study of the effects of the geomagnetic field on the electron-photon cascades. They have simulated the Cerenkov light produced by individual protons and gamma -rays at different orientations in the atmosphere, and made measurements of the sensitivity of the observations to the geomagnetic field. Indications are that these geomagnetic effects may be a cause of the differing results.

The University of Durham Southern Hemisphere VHE gamma ray telescope

The design of very high energy (VHE) gamma ray telescopes is discussed and it is shown that a triple-mirror telescope operating in threefold fast coincidence provides an effective instrument. The design, construction and operation of a large VHE gamma ray telescope with an energy threshold of 300 GeV (at the zenith) and incorporating several novel features is described. The use of a coincidence ‘guard ring’ technique is shown to result in the rejection of about 50% of the off-source (proton) events which form a high background and limit the sensitivity of such telescopes.

VHE Gamma Ray Emission from Centaurus X-3

The high mass X-ray binary Centaurus X-3 has been observed using the Narrabri 250 GeV gamma ray telescope and the observations show evidence for pulsed VHE gamma ray emission, significant at the 4.2 SD level. The detected gamma ray signal in the total dataset comprises a broad pulse at precisely the contemporary X-ray period. The VHE emission is strongest for a few hours around the ascending node in the 2.1 d binary orbit. The time averaged energy flux in VHE gamma rays (> 250 GeV) is estimated to be ~ 1036 erg s-1 with a peak luminosity of 3×1036 erg s-1.

SN1987a - updated limit to 400 GeV gamma ray emission using the narrabri VHE gamma ray telescope - May 1989

Abstract A search for 400 GeV gamma rays from SN 1987a has been conducted using the Narrabri gamma ray telescope, starting with a fortuitous observation within 5 hours of the core collapse in 1987 February. A more extensive program of observations up to 1988 April has been reported by us 1 . A luminosity limit of 4 × 10 38 erg s −1 (significant at the 3 SD level) was obtained for the emission of gamma rays of energy > 400 GeV immediately following the collapse. Subsequent observations throughout 1987 indicate continued validity of this limit. In 1988 January–February, as the possibility of the penetration of VHE gamma rays increased, a series of observations provided a limit to the luminosity of 7×10 37 erg s −1 . We here report the results of continuing observations up to 1989 May.

The University of Durham New VHE Gamma Ray Telescopes

The specification and performance of two new VHE gamma ray telescopes are described. The larger of the telescopes (MK III) has a threshold of 240 GeV and is at present en route for a series of observations in the Southern hemisphere. The MK IV telescope now nearing completion has a slightly higher threshold (300 GeV) and will be used in 1987 for Northern hemisphere observations.

A ground based installation for the detection of gamma rays with energy 50–2000 GeV

This paper describes the University of Durham ground‐based gamma ray programme for the 1990s, and improvements being made to the Cerenkov Observatory at Narrabri, N. S. W.

TeV gamma rays from millisecond pulsars

We summarize here the results of the VHE gamma ray study of millisecond pulsars made over seven years by the Durham group. In addition to reporting observations made recently with the Mark III and IV telescopes at Narrabri, Australia, we consider the application of new analysis techniques to earlier data taken using telescopes at Dugway, USA. Pulsed VHE γ‐rays have been detected from three of the eight millisecond pulsars observed; we compare our results with the predictions of a theory of VHE emission from millisecond pulsars.

400 Gev gamma rays from the X-ray pulsar 1E2259+586

Abstract We report observations of the X-ray pulsar 1E2259+586 made in 1988 October using the very high energy gamma ray telescope operating in La Palma. We find evidence significant at the 5.3 × 10 −5 level for the emission of 400 GeV gamma rays pulsating at 6.9790s. The flux of gamma rays is (2.0 ± 0.8) × 10 −10 cm −2 s −1 corresponding to VHE gamma ray luminosity of (5.3 ± 2.1) × 10 35 erg s −1 .