HIGH ENERGY GAMMA-RAY ASTRONOMY OBSERVATIONS OF GEMINGA WITH THE VERITAS ARRAY by Gary Marvin Finnegan A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics Department of Physics and Astronomy The University of Utah December 2012 Copyright c Gary Marvin Finnegan 2012 All Rights Reserved The University of Utah Graduate School STATEMENT OF DISSERTATION APPROVAL This dissertation of Gary Marvin Finnegan has been approved by the following supervisory committee members: David B. Kieda , Chair August 22, 2012 Date Approved Peter W. Alfeld , Member August 22, 2012 Date Approved Carleton Detar , Member August 22, 2012 Date Approved Stephane L. Le Bohec , Member August 22, 2012 Date Approved R. Wayne Springer , Member August 22, 2012 Date Approved and by David B. Kieda , Chair of the Department of Physics and Astronomy and by Charles A. Wright of the Graduate School. ABSTRACT The closest known super-nova remnant and pulsar is Geminga. The Geminga pulsar is the first pulsar to have ever been detected initially by gamma rays and the first pulsar in a class of radio-quiet pulsars. In 2007, the Milagro collaboration detected a large angularly extended ( 2.6◦) emission of high energy gamma rays ( 20 T eV ), that was positionally ∼ ∼ coincident with Geminga. The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is a ground- based observatory with four imaging Cherenkov telescopes with an energy range between 100 GeV to more than 30 TeV. The imaging Cherenkov telescopes detect the Cherenkov light from charged particles in electromagnetic air showers initiated by high energy particles such as gamma rays and cosmic rays. Most gamma-ray sources detected by VERITAS are point like sources, which have an angular extension smaller than the angular resolution of the telescopes ( 0.1◦). For a point source, the background noise can be measured in the ∼ same field of view (FOV) as the source. For an angularly extended object, such as Geminga, an external FOV from the source region must be used to estimate the background noise, to avoid contamination from the extended source region. In this dissertation, I describe a new analysis procedure that is designed to increase the observation sensitivity of angularly extended objects like Geminga. I apply this procedure to a known extended gamma-ray source, Boomerang, as well as Geminga. The results indicate the detection of very high energy emission from the Geminga region at the level of a four percent of the Crab nebula with a weighted average spectral index of 2.8 0.2. − ± A possible extension less than one degree wide is shown. This detection however awaits a confirmation by the VERITAS collaboration. LIST OF TABLES 1.1 Gamma-ray initiated extensive air shower parameters . ................. 20 2.1 Hillas image parameters definitions . ................. 27 3.1 Comparison of Vela, Geminga, and the Crab pulsars . .............. 49 4.1 Data selection criteria cuts for the 2007 Geminga data. Ton and Toff are the duration of the total observations for the ON and OFF off runs respectively. 53 4.2 Table summary of the distribution study of significances in the FOV . 57 4.3 Data selection criteria cuts for the 2010 Geminga data. Ton and Toff are the duration of the total observations for the ON and OFF off runs respectively. 57 4.4 Data selection criteria cuts for the 2011 Geminga data. Ton and Toff are the duration of the total observations for the ON and OFF off runs respectively. 59 4.5 Summary table the 2007, 2010, and 2011 data. .............. 60 5.1 Comparison of luminosities (E > 300GeV ) and age of Vela, Crab, and Geminga 70 LIST OF FIGURES 1.1 The electromagnetic spectrum. The top bar shows which photon windows are open for ground level observations. Other diagrams illustrate the size, frequency and temperature of the corresponding photon. Credit: NASA (via https://mynasadata.larc.nasa.gov) . ................ 2 1.2 Composite X-ray image of the Tycho SNR taken by the Chandra X-ray obser- vatory. The blue filaments show the extrememly high energy electrons. The red and green remnant material is multimillion degree debris from the SNR. Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al. ........... 5 1.3 Adiagramofapulsar ............................... ............. 7 1.4 A diagram of different pulsar models.................. ............... 10 1.5 Spectral energy distribution of the Crab pulsar . ................. 12 1.6 IC 443 very high energy spectrum..................... .............. 13 1.7 IC443excessmap .................................. ............. 14 1.8 A diagram of a theoretical model of a radio-loud AGN. The broad and narrow lines refer to radio emission. The active galaxy would surround this object and is not shown in the diagram. Credit: NASA/GSFC/C.M. Urry & P. Padovani 17 1.9 Gamma ray initiated electromagnetic shower . ................ 20 1.10 A proton initiated electromagnetic air shower . ................... 22 1.11 A comparison of a 320 GeV gamma ray EAS and a 1 TeV proton EAS...... 23 1.12 The wavefront of the Cherenkov radiation from an EAS. (a) shows the diagram of the Cherenkov light emitted by the charged particle. (b) shows the light cone that is swept out by the wave front of the Cherenkov light............ 24 2.1 The VERITAS telescopes. Credit: Steve Criswell, Smithsonian Astrophysical Observatory ........................................ ........... 25 2.2 Hillas Parameters................................ ................ 27 2.3 A single event as seen by the VERITAS telescopes with the fitted ellipse. The ellipse has a major axis equal to the RMS length, and the minor axis equal to the RMS width. The solid line is the major axis. The dashed line is the distance parameter. The X is the centroid of the camera and the Cherenkov image. The star is the reconstructed arrival directions of the primary particle. 28 2.4 The reflected regions and ring background models . ............... 31 2.5 An example of the acceptance function of the camera. The number of events drops off towards the edge of the field of view. Cherenkov images that originate close to the edge of the field of view have shower images that are normally truncated at the edge, and become more difficult to reconstruct their geometric properties. The drop is caused by the inability to reconstruct events that fall neartheedgeofthefieldofview. ........................ .......... 32 2.6 Orbit mode radial exposure profile. .................. ................ 33 2.7 Two-dimensional exposure map of a single orbit mode run. The longer the exposure is, the darker the color in the map. .............. ............ 34 2.8 Woble mode radial exposure profile (East to West or North to South axis). 35 2.9 Two-dimensional exposure map of four wobble mode runs. The longer expo- sure is the darker the color in the map.................... ............ 36 2.10 Excess map of the Crab Nebula from an orbit mode observation. .......... 37 2.11 Significance map of the Crab Nebula. Black circle: 68% containment of the source. ............................................ ............ 40 2.12 TheVERITAS sensitivity .......................... ............... 41 2.13 The angular resolution of VERITAS observations . ................. 42 3.1 The ROSAT periodograms of the Geminga data . ............ 45 3.2 XMM-Newton image of Geminga (top) showing the discovery of twin tails. The motion of Geminga across the sky is indicated, showing the trailing tails of the neutron star. Image courtesy of P.A. Caraveo (INAF/IASF), Milan and ESA. ............................................... .......... 46 vi 3.3 Fermi and Milagro spectral energy distribution of Geminga............... 47 4.1 Pre-trials Significance distribution of the 2007 wobble data .............. 52 4.2 The significance of each bin for the 2007 data in the Geminga region. 54 4.3 The excess events for each bin for the 2007 data in the Geminga region. 55 4.4 FADC digital counts histogram per event from the 2007 data ............. 56 4.5 Histograms of the significance for each map bin of a simulated gamma-ray sky 58 4.6 Simulated significance histograms of an extended and point-like source. Figure (a) shows the extended source and figure (b) shows the point-like source. The red line is the gaussian fit to the distribution and the blue is the simulated data. 59 4.7 A histogram of the the excess events (OFF events subtracted from the ON events) from the Crab nebula spectrum analysis. ............. 60 4.8 The excess map of the 2010 ON-OFF data. ........... 61 4.9 The significance map of the 2010 ON-OFF data. ............ 62 4.10 FADC digital counts histogram per event from the 2010 data ............. 63 4.11 The excess map of the 2011 ON-OFF data. ............ 64 4.12 The significance map of the 2011 ON-OFF data . ............. 65 4.13 FADC digital counts histogram per event from the 2011 data ............. 66 5.1 Spectral energy distribution of the Geminga pulsar (< 0.1T eV ) and the Geminga diffusesource. ...................................... ............ 69 vii Dedication Page ACKNOWLEDGMENTS Thanks to ... CONTENTS ABSTRACT ................................................... ..... iii LIST OF TABLES ................................................... iv LIST OF FIGURES ................................................. v ACKNOWLEDGMENTS ............................................ ix CHAPTERS 1. INTRODUCTION TO HIGH ENERGY GAMMA-RAY ASTRONOMY 1 1.1 Introduction .................................... .............