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Durham E-Theses Atmospheric Cerenkov astronomy of cataclysmic variables & other potential gamma ray sources Bradbury, Stella Marie How to cite: Bradbury, Stella Marie (1993) Atmospheric Cerenkov astronomy of cataclysmic variables & other potential gamma ray sources, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/5632/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk 2 Atmospheric Cerenkov Astronomy of Cataclysmic Variables & Other Potential Gamma Ray Sources by Stella Marie Bradbury, B.Sc. The copyright of this thesis rests with the author. No quotation from it should be pubhshed without his prior written consent and information derived from it should be acknowledged. A thesis submitted to the University of Durham in accordance with the regulations for admittance to the degree of Doctor of Philosophy Department of Physics November 1993 University of Durham 2 2 FEB m ABSTRACT Recent developments in the application of the atmospheric Cerenkov technique to 7-ray astronomy are reviewed here. These include new methods of signal to noise enhancement and the increasing diversity of stellar systems positively identified as Very High Energy 7-ray sources. Four Cataclysmic Variable systems were observed using the University of Durham Atmospheric Cerenkov telescopes during the course of 1990 and 1991. The statistical analysis performed in the search for a 7-ray signal, above a threshold energy of approximately 0-4 TeV, from three of these objects, H0253+193, EF Eridani and VW Hydri, is described here. The results of this brief survey are discussed in the context of current ideas as to the mechanisms by which Very High Energy 7-rays may be emitted from accreting binary star systems of this type. The analysis techniques applied to Cataclysmic Variable data were extended to an x-ray binary system, Sgr X-7. For comparison, the analysis of data recorded on two radio pulsars, PSR 1855+09 and PSR 1509-58, having more accurately known pulse signatures than the accreting systems is also described here, together with that of the globular cluster 47 Tucanae, which may emit a steady 7-ray flux; an upper limit is placed upon the level of Very High Energy 7-ray emission from this object. Extension of the Very High Energy 7-ray source catalogue will require a further improvement beyond the current signal to noise ratios of atmospheric Cerenkov telescopes. Some features characteristic of the atmospheric Cerenkov emission triggered by Very High Energy 7-rays as opposed to other cosmic ray particles, which could be exploited in an attempt to reduce interference from the latter, are reviewed. The first attempt to obtain directional information from the relative time of arrival of a Cerenkov flash at the telescopes at the University of Durham Southern Hemisphere site, and thus isolate an anisotropic 7-ray flux is reported here. CONTENTS 1) An lutroductioii to Gamma Ray Astronomy 1 1.1 preamble 1 1.2 7-ray production mechanisms 2 1.2.1 particle - electro-magnetic field interactions 3 (i) bremsstrahlung radiation 3 (ii) magneto-bremsstrahlung radiation 4 (iii) curvature radiation 4 (iv) inverse Compton scattering 5 1.2.2 meson decay 5 1.3 celestial origins 6 1.3.1 7-ray absorption 6 1.3.2 favoured sources 8 1.4 observational methods 8 1.4.1 satellite and balloon borne instruments 9 (i) Low Energy 7-ray detectors 9 (ii) High Energy 7-ray detectors 10 (iii) the Compton Gamma Ray Observatory 11 1.4.2 atmospheric particle cascades 11 (i) Very High Energy 7-ray detectors 12 (ii) Ultra High Energy 7-ray detectors 12 2) The Atmospheric Cerenkov Technique 14 2.1 Cerenkov radiation mechanism 14 2.2 Cerenkov radiation as a trace of cosmic rays 17 2.2.1 integral Cerenkov detectors 17 2.2.2 Cerenkov light from extensive air showers 19 2.3 astronomy by the atmospheric Cerenkov technique 21 2.3.1 telescope design 22 (i) the Dugway experiment 23 (ii) aperture rejection 24 (iii) the imaging technique 25 2.3.2 contemporary atmospheric Cerenkov observations 27 3) The Durham Southern Hemisphere Installation 30 3.1 the site 30 3.2 the Mark III telescope 30 3.2.1 mechanical design 31 3.2.2 the detector packages 31 3.2.3 modes of operation 32 (i) tracking mode 33 (ii) chopping mode 33 (iii) drift scans 34 3.2.4 signal processing 34 3.3 the Mark IV telescope 36 3.3.1 history 36 3.3.2 design 37 3.4 the Mark V telescope 37 3.4.1 mechanical design 37 3.4.2 the detector packages 38 3.4.3 signal processing 39 3.5 operation of a three-telescope observatory 40 3.5.1 remote surveillance 40 3.5.2 absolute timekeeping 40 3.5.3 performance and environment monitoring 41 (i) steering 42 (ii) detector performance 42 (iii) environmental conditions 43 3.5.4 burst confirmation 43 4) Data Processing and Analysis Techniques 45 4.1 routine data processing 45 4.1.1 conversion to a format suitable for analysis 46 4.1.2 routine corrections to event times 46 (i) clock corrections 47 (ii) reduction to the solar system barycentre 47 4.1.3 fire pattern selection 48 4.1.4 data archives 49 4.2 quality control 49 4.3 analysis techniques 51 4.3.1 a test for unpulsed emission 52 4.3.2 searching for unknown periodicities 55 4.3.3 tests for uniformity of phase 57 (i) the Rayleigh test 59 (ii) the Z2ni test 60 (ill) amendment for a dataset of finite length 61 (iv) correction for number of trials 62 4.3.4 a test for goodness of fit 63 4.3.5 periodic emission from binary star systems 65 (i) transformation of event times 65 (ii) orbital sampling 69 4.3.6 combining a set of discrete observations 70 (i) retention of relative phase 70 (ii) aggregate probability from individual datasets 71 4.4 calculation of flux and flux limits 72 4.4.1 a method of flux estimation 73 (i) unpulsed emission 73 (ii) the Rayleigh test 73 (ill) the test for goodness of fit 74 4.4.2 flux limits from the Durham database 75 5) Signal to Noise Enhancement Techniques 77 5.1 the distribution of Cerenkov photons 77 5.1.1 air shower propagation 77 5.1.2 air shower morphology 79 5.1.3 the Cerenkov flash 80 (i) the light pool 80 (ii) the time profile 82 5.2 techniques of 7-ray signal enhancement 84 5.2.1 a test of aperture rejection 84 5.2.2 pulse integral threshold selection 88 5.3 the inter-telescope timing experiment 90 5.3.1 projected time delays 91 5.3.2 method of inter-telescope time delay measurement 93 5.3.3 data analysis and results 94 5.3.4 conclusions and recommendations 99 6) Cataclysmic Variables as Sources of VHE 7-rays 101 6.1 an introduction to Cataclysmic Variables 101 6.1.1 the formation of degenerate stars 101 6.1.2 the cataclysmic variable population 103 6.2 mass transfer in binary systems 105 6.2.1 stellar wind accretion 106 6.2.2 mass transfer by Roche lobe overflow 107 6.2.3 production of accretion columns and discs 109 (i) disc formation 109 (ii) accretion in a strong magnetic field 111 6.2.4 observational implications 113 6.3 VHE 7-ray emission mechanisms 114 6.3.1 curvature radiation 114 6.3.2 accretion driven 7-ray production 116 (i) first order Fermi acceleration 116 (ii) plasma turbulence 120 (iii) dynamo models 123 6.3.3 encounters with matter 127 (i) 7-ray production 127 (ii) particle - particle collision sites 128 6.3.4 loss mechanisms 130 (i) synchrotron emission 130 (ii) absorption by pair production 131 6.3.5 the 7-ray light curve 132 6.4 the Durham cataclysmic variable survey 134 6.4.1 AE Aquarii, an intermediate polar system 134 (i) the Potchefstroom report 134 (ii) the Durham observations 135 6.4.2 H0253+193, an intermediate polar system 136 (i) background information 136 (ii) the dataset 138 (ill) analysis and results 138 (iv) summary 141 6.4.3 EF Eridani, a polar system 142 (i) background information 142 (ii) the dataset 144 (iii) analysis and results 144 (iv) summary 147 6.4.4 VW Hydri, a dwarf nova 148 (i) background information 148 (ii) the dataset 150 (iii) analysis and results 150 (iv) summary 151 6.5 conclusions 152 7) VHE 7-rays in the Vicinity of Neutron Stars 155 7.1 x-ray binaries 155 7.2 an introduction to pulsars 157 7.2.1 x-ray pulsars 158 7.2.2 radio pulsars 159 (i) braking mechanisms 160 (ii) VHE 7-ray production 162 7.3 Sgr X-7, a low mass x-ray binary 165 7.3.1 background information 165 7.3.2 the dataset 166 7.3.3 analysis and results 167 (i) the August 1987 data 167 (ii) the June 1991 to September 1992 data 171 7.3.4 discussion 172 7.4 PSR 1855+09, a binary radio pulsar 174 7.4.1 background information 174 7.4.2 previous analysis in Durham 176 7.4.3 the 1987 to 1992 dataset 177 7.4.4 recent analysis and results 178 (i) a test for periodicity at the second harmonic 178 (ii) orbital phase dependency 179 (iii) a test for a narrow pulse profile 181 7.4.5 discussion 183 7.5 PSR 1509-58, an isolated pulsar 185 7.5.1 background information 185 7.5.2 previous VHE 7-ray observations 187 7.5.3 results from the Compton Gamma Ray Observatory 188 7.5.4 the dataset 189 7.5.5 recent
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