title: A search for radio pulses of astrophysical origin
Creators: name: Meikle, William Peter Stuart
Publisher: name: University of Glasgow
Year: 1973

description: The traditional use of long integration times by both optical and radio astronomers has, until recently, precluded the exploration of transient astrophysical events. With the discovery of the pulsars, it became apparent that there might exist an important field of investigation involving phenomena of short duration. This thesis is a description of a search for isolated bursts ( &lt;103 s duration) of radio emission from cosmic sources. Conventional generating mechanisms such as black-body or synchrotron processes are unlikely to give rise to detectable pulses of duration &lt; 10 s if the emission is isotropic. However, if the energy is "beamed" towards the earth or if the source moves at relativistic velocities then the observation of- much greater energies is possible, Specific sources which might give rise to detectable bursts of radio emission are supernovae, novae, black holes and pulsars. In addition, assuming that the coincidences detected by Weber are due to gravitational waves of astrophysical origin, if only?10-21 of the energy present in these events were radiated into a 1 MHz band of the VHF spectrum then this emission would be easily observable. It is interesting to note that in 1949 F.G. Smith discovered 10-20 s bursts ox MHz radiation, apparently of extraterrestrial origin. No further investigation of these events was wade until the commencement of the present work. Initial observations were carried out simultaneously at Cambridge, Dublin, Glasgow, Harwell and Jodrell Fank using simple dipole inter-ferometers. By searching for events which occurred in coincidence at several stations, this "spaced-receiver" technique made possible the elimination of local interference pulses. The principal operating frequency was 151.5 MHz, although some stations set up additional lower frequency systems in order to investigate possible dispersion effects. Dispersion measures to potential sources were likely to be in the range 10-103 cm-3 pc and it is shown that the corresponding optimum bandwidths and integration times lay in the ranges 0.1-1 MHz and 0.01-1 s respectively. The threshold sensitivity at 151.5 MHz was 2 x 10-22 W m-2 Hz-1. At 151.5 MHz, 8.5% of the celestial sphere was observed for 3,620 hours with a sensitivity which was never poorer than the above threshold. No radio pulses of astrophysical origin were detected. Corresponding upper limits for emission from novae and supernovae are 1023 J Hz-1 and 1027 J Hz-1respectively. This assumes isotropic emission within a timescale &lt;1 s. The limit for supernova emission is about 10 less than the energy predicted by the shock ejection model of Colgate and Noerdlinger. No significant coincidences were obtained at any of the lower frequencies. In view of the possibility that Weber's events originated in either the Galactic Centre (GC) or the Crab Nebula, observations of these objects were made at 151-5 KHz using antennae of greater gain than the simple dipoles. The GC was observed for 350 hours at a thres hold of and the Crab for 605 hours at a threshold of No pulses were detected from either object. It can therefore be concluded that if Weber's events are caused by gravitational -21 radiation from these objects, then less than 10 of this energy appears in a 1 MHz band at 151.5 MHz and this places a strong constraint on possible generating mechanisms. During the GC observations one highly significant coincidence occurred which may have been caused by the eruption of Nova Scuti 1970 or Supernova 1970g. The upper limit on pulsed emission from the Crab Pulsar NP 0532 was consistent with a pulse height spectrum subsequently determined by other workers. The negative VHF results might have been caused by screening or high dispersion in plasmas surrounding possible sources. In order to reduce this potential problem, observations using a microwave radiometer tuned to 10 GHz were instigated. This device could automatically "track" any astronomical object in the declination range -31°? +90° for several months without resetting. Searches were made for pulses from the GC and supernovae in the Coma Cluster of galaxies as well as from the suggested black holes in Beta Lyrae and Cygnus X-1 No events exceeding a flux of 10-21 W m-2 Hz-1 were found. The overall negative results may have several explanations such as insufficient sensitivity or severe electromagnetic screening. Since this work began, several other investigators have conducted searches for rare radio pulses. It is possibly significant that the only positive effect reported so far, v/as in an investigation having x102 -103 better sensitivity than those given above. It seems likely that if any success is to be achieved in this field, it will be necessary to employ considerably greater collecting areas than those used in the present work.