Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1982 Part I: Ultra high energy gamma rays from Cygnus X-3; Part II: Properties of the galactic gamma ray source population Christopher Paul Godfrey Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Astrophysics and Astronomy Commons Recommended Citation Godfrey, Christopher Paul, "Part I: Ultra high energy gamma rays from Cygnus X-3; Part II: Properties of the galactic gamma ray source population " (1982). Retrospective Theses and Dissertations. 7500. https://lib.dr.iastate.edu/rtd/7500 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. 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These prints are available upon request from the Dissertations Customer Services Department. 5. Some pages in any document may have indistinct print. In all cases the best available copy has been filmed. Un ms International 300N.Zeeb Road Ann Arbor, Ml 48106 8224218 Godfrey, Christopher Paul PART I - ULTRA HIGH ENERGY GAMMA RAYS FROM CYGNUS X-3. PART II - PROPERTIES OF THE GALACTIC GAMMA RAY SOURCE POPULATION Iowa State University PH.D. 1982 University Microfilms I nt©rnâti0 nBI 300 N. Zeeb Road, Ann Arbor. MI 48106 Part I - Ultra high energy gamma rays from Cygnus X-5 Part II - Properties of the galactic gamma ray source population Christopher Paul Godfrey A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Major: Physics Approved: Signature was redacted for privacy. In Charge of Major Work Signature was redacted for privacy. Signature was redacted for privacy. Iowa State University Ames, Iowa 1982 ii TABLE OF CONTENTS PREFACE la PART I. ULTRA HIGH ENERGY GAMMA RAYS lb FROM CYGNUS X-3 INTRODUCTION 2 THE ATMOSPHERIC CERENKOV TECHNIQUE 5 PREVIOUS RESULTS ON CYGNUS X-3 10 DESCRIPTION OF THE OBSERVING SITE 14 THE DATA ACQUISITION SYSTEM 18 OBSERVING PROCEDURE 25 INITIAL TESTING 30 CALCULATION OF THE PHASE 39 DATA ANALYSIS PROCEDURE 42 THE ENERGY THRESHOLD AND GAMMA RAY FLUX 57 DISCUSSION AND CONCLUSIONS 60 PART II. PROPERTIES OF THE GALACTIC GAMMA 67 RAY SOURCE POPULATION INTRODUCTION 68 THE TOTAL EMISSIVITY DISTRIBUTION 72 THE DISCRETE SOURCE CONTRIBUTION 77 DISCUSSION 83 CONCLUSIONS 86 BIBLIOGRAPHY 37 ACKNOWLEDGEMENTS 93 la PREFACE This thesis is divided into two parts, each part corresponding to a separate research project. The first deals with the detection of gamma rays of energy greater than 500 GeV from Cygnus X-3, a periodic X-ray source, via the atmospheric Cerenkov technique. The observa­ tions were carried out at the Jet Propulsion Laboratory Edwards Test Station, Edwards AFB, California, from August 28 to September 6, 1981. The twin 11 meter dia­ meter mirrors at the solar research facility of the Jet Propulsion Laboratory were used in the observations. The results reveal periodic gamma ray emission from Cygnus X-3 modulated at the same period and in phase with the X-ray emission. The second part describes the results of a study of the 100 MeV sources in the galaxy as seen by the COS-B satellite. The observed gamma ray flux distri­ bution may be deconvolved to find the distribution of emissivity in the galaxy. This leads to estimates of the fraction of the total emission originating in dis­ crete sources, the total number of sources in the galaxy, and their combined luminosity. lb PART I. ULTRA HIGH ENERGY GAMMA RAYS FROM CYGNTJS X-3 2 I. INTRODUCTION The gamma ray region of the electromagnetic spec­ trum has been the last to be exploited as a channel for astronomical investigation. Gamma ray astronomy was originally expected to be a very productive enterprise (Morrison 1958), but the techniques proved extremely dif­ ficult. Above approximately 5 MeV the dominant inter­ action between photons and matter is via pair production and the gamma rays are converted high in the atmosphere, at mass column densities of several gm/cm , So to de­ tect cosmic gamma rays one must not only use detectors employing massive target materials, but these detectors must be carried above the atmosphere by balloons, rockets, and satellites. The weight of the material nec­ essary to convert the photon to an e*-e" pair in a rel­ atively small volume constrains the size of the detectors. As a result, gamza ray astronomy has been slow to devel­ op as an experimental science. Several attempts to detect cosmic gamma rays with relatively simple balloon-borne detectors in the late 1950s, and the 1960s, all failed. However, during the 19bOs, the experimental techniques and hardware were sig­ nificantly improved. This led to the detection of gamma rays with energies above 50 MeV by Kraushaar et al. (1972) 3 from the galactic plane with the peak flux coming from the region of the galactic center. Little progress could be made from this point because of the short observing time available from this type of balloon or rocket based experiment. In late 1972, the Second Small Astronomy Satellite (SAS-2) was launched. The gamma ray instru­ ment aboard had a much better angular resolution (2-5°) than previous ones (10-30°) and an order of magnitude increase in sensitivity (Derdeyn et a2. 1972), The results obtained by SAS-2 provided a vastly improved and refined picture of the galaxy in gamma rays above 55 MeV (Pichtel st 1975)* A similar instrument, COS-B, was launched in 1975 by the European Space Agency which has further refined our understanding of gamma ray astrophysics. (Mayer-Hasselwander et 1982 ), Even with this level of activity, the low flux at these energies has limited the total number of gamma rays above 100 KeV ever detect- ed to approximately 10 , An additional motivation for the study of cosmic gamma rays comes from the related field of cosmic ray physics. The production of high energy photons requires the presence of particles of even higher energies. Since most cosmic rays are electrically charged, and interstellar and intergalactic space are permeated with 4 magnetic fields, cosmic rays are deflected from their orig­ inal trajectories and arrive at the earth more or less isotropically. However, the gaimna rays co-produced with or produced by the particles are undeflected. So their arrival direction is the same as the direction to the gamma ray source and to a possible cosmic ray source as well. At the present time, gamma ray observations repre­ sent the only tool available for investigation of the origin of the cosmic rays. The gamma ray emission is composed of two differ­ ent components, one due to diffuse processes and one due to localized sources. Approximately 25 discrete sources have been found by the SAS-2 (Kniffen et 1978) and COS-B (Swanenberg et 1981) satellites, most of which have not been identified with objects at other wave­ lengths. In general the mechanisms for emission by these sources are totally uncertain. The spatial distribution and luminosity function of the sources are also unknown. In this work, one of these sources, Cygnus X-3, a variable X-ray source, is studied at ultra high ener­ gies using the atmospheric Cerenkov technique. 5 II. THE ATMOSPHERIC CERENKOV TECHNIQUE A high energy gamma ray or particle entering the earth's atmosphere initiates an electromagnetic cascade shower very high in the atmosphere. The electrons and positrons in the shower that have energies above the Cerenkov threshold (approximately 50 MeV in air) produce Cerenkov radiation into a narrow cone whose axis is the trajectory of the emitting particle. If the energy of the primary charged particle or photon is less than about 10^^ eV the secondary particles produced in the shower are absorbed in the atmosphere, but the optical Cerenkov light penetrates the atmosphere reaching ground level.