arXiv subjects

P. G. Edwards

Publications and source records attributed to P. G. Edwards.

134 records · Page 8Linked to original sources

arXiv1997

A 5 GHz Southern Hemisphere VLBI Survey of Compact Radio SOurces - I

We report the results of a 5 GHz southern hemisphere VLBI survey of compact extragalactic radio sources. These observations were undertaken with the SHEVE array plus Shanghai station in November 1992. A sample of 22 sources was observed and images of 20 of them were obtained. Of the 20 sources imaged, 15 showed core-jet structure, one had a two-sided jet and 4 had only single compact cores. Eleven of the 16 core-jet (including one two-sided jet) sources show some evidence of bent jets. No compact doubles were found. A comparison with previous images and the temporal variability of the radio flux density showed evidence for superluminal motion in 4 of the sources. Five sources were high energy (>100 MeV) gamma-ray sources. Statistical analysis showed the dominance of highly polarized quasars among the detected gamma-ray sources, which emphasizes the importance of beaming effect in the gamma-ray emission.

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arXiv1997

Very High Energy Gamma Rays from the Vela Pulsar Direction

We have observed the Vela pulsar region at TeV energies using the 3.8 m imaging Cherenkov telescope near Woomera, South Australia between January 1993 and March 1995. Evidence of an unpulsed gamma-ray signal has been detected at the 5.8 sigma level. The detected gamma-ray flux is (2.9 +/- 0.5 +/- 0.4) x 10^-12 photons cm^-2 sec^-1 above 2.5 +/- 1.0 TeV and the signal is consistent with steady emission over the two years. The gamma-ray emission region is offset from the Vela pulsar position to the southeast by about 0.13 deg. No pulsed emission modulated with the pulsar period has been detected and the 95 % confidence flux upper limit to the pulsed emission from the pulsar is (3.7 +/- 0.7) x 10^-13 photons cm^-2 sec^-1 above 2.5 +/- 1.0 TeV.

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arXiv1997

Observations of Pulsars, PSR 1509-58 and PSR 1259-63, by CANGAROO 3.8 m Telescope

The data for VHE (~TeV) gamma rays from young gamma-ray pulsar PSR1509-58 observed in 1996 with the CANGAROO 3.8m Cerenkov imaging telescope are presented, as well as the additional data from March to June of 1997. The high spin-down luminosity of the pulsar and the plerionic feature around the pulsar observed with radio and X-rays suggest that VHE gamma-ray emission is quite likely above the sensitivity of the CANGAROO telescope. The CANGAROO results on other pulsars, such as PSR1259-63, are also presented. PSR1259-63 is a highly eccentric X-ray binary system, which includes a high mass Be companion star, and a preliminary analysis on the data taken 4 months after the periastron in 1994 suggests emission of VHE gamma rays.

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arXiv1997

Very High Energy Gamma Rays from the Vela Pulsar/Nebula

We have observed the Vela pulsar region at TeV energies using the 3.8 m imaging Cherenkov telescope near Woomera, South Australia every year since 1992. This is the first concerted search for pulsed and unpulsed emission from the Vela region, and the imaging technique also allows the location of the emission within the field of view to be examined. A significant excess of gamma-ray-like events is found offset from the Vela pulsar to the southeast by about 0.13deg. The excess shows the behavior expected of gamma-ray images when the asymmetry cut is applied to the data. There is no evidence for the emission being modulated with the pulsar period -- in contrast to earlier claims of signals from the Vela pulsar direction.

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arXiv1997

TeV Gamma-ray Observations of Southern AGN with the CANGAROO 3.8m Telescople

Since 1992 the CANGAROO 3.8m imaging telescope has been used to search for sources of TeV gamma-rays. Results are presented here for observations of four Southern Hemisphere BL-Lacs - PKS0521-365, PKS2316-423, PKS2005-489 and EXO0423-084. In addition to testing for steady DC emission, a night by night burst excess search has been performed for each source.

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arXiv1997

Observation of Spectrum of TeV Gamma Rays up to 60 TeV from the Crab at the Large Zenith Angles

The CANGAROO experiment has observed gamma-ray above 7TeV from the Crab pulsar/nebula at large zenith angle in Woomera, South Australia. We report the CANGAROO data taken in 1992, 1993 and 1995, from which it appears that the energy spectrum extends at least up to 50 TeV. The observed integral spectrum is (8.4+-1.0) x 10^{-13}(E/7 TeV)^(-1.53+-0.15)cm^{-2}s^{-1} between 7 TeV and 50 TeV. In November 1996, the 3.8m mirror was recoated in Australia, and its reflectivity was improved to be about 90% as twice as before. Due to this recoating, the threshold energy of ~4 TeV for gamma rays has been attained in the observation of the Crab at large zenith angle. Here we also report the preliminary result taken in 1996.

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arXiv1997

TeV Gamma Ray Emission from Southern Sky Objects and CANGAROO Project

We report recent results of the CANGAROO Collaboration on very high energy gamma ray emission from pulsars, their nebulae, SNR and AGN in the southern sky. Observations are made in South Australia using the imaging technique of detecting atmospheric Cerenkov light from gamma rays higher than about 1 TeV. The detected gamma rays are most likely produced by the inverse Compton process by electrons which also radiate synchrotron X-rays. Together with information from longer wavelengths, our results can be used to infer the strength of magnetic field in the emission region of gamma rays as well as the energy of the progenitor electrons. A description of the CANGAROO project is also given, as well as details of the new telescope of 7 m diameter which is scheduled to be in operation within two years.

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arXiv1996

Discovery of a Sub-Parsec Radio Counterjet in the Nucleus of Centaurus A

A sub-parsec scale radio counterjet has been detected in the nucleus of the closest radio galaxy, Centaurus A (NGC 5128), with VLBI imaging at 2.3 and 8.4 GHz. This is one of the first detections of a VLBI counterjet and provides new constraints on the kinematics of the radio jets emerging from the nucleus of Cen A. A bright, compact core is seen at 8.4 GHz, along with a jet extending along P.A. 51 degrees. The core is completely absorbed at 2.3 GHz. Our images show a much wider gap between the base of the main jet and the counterjet at 2.3 GHz than at 8.4 GHz and also that the core has an extraordinarily inverted spectrum. These observations provide evidence that the innermost 0.4-0.8 pc of the source is seen through a disk or torus of ionized gas which is opaque at low frequencies due to free-free absorption.

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