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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

Very high-energy gamma rays from the Crab nebula and pulsar.

Kwok, Ping Wai. January 1989 (has links)
This project is to search for Very High Energy (VHE) (10¹¹ eV to 10¹⁴ eV) gamma rays from the Crab nebula and pulsar using the atmospheric Cherenkov imaging technique. The technique uses an array of 37 photomultiplier tubes to record the images of the Cherenkov light pulses generated by energetic particles in the air showers initiated by VHE gamma rays or charged cosmic rays. Gamma-ray-like events are selected from numerous cosmic-ray events based on the predicted properties of the image, such as the size, shape, and orientation with respect to the axis of the detector. A steady weak flux of VHE gamma rays from the Crab is detected at high statistical significance (9 sigma), which is not usually achieved in VHE gamma-ray astronomy. No strong evidence of pulsed emission is found when the same data is folded at the Crab pulsar's radio ephemeris. The angular resolution of the technique cannot separate the emission coming from the nebula from that from the pulsar. Although it is generally believed that the unpulsed emission is coming from the nebula, there may be an unpulsed component coming at only a couple of light cylinder radii away from the pulsar too. Using the outer gap model of pulsar, the spectrum is derived and is found to be compatible with the observations.
32

A REVERSE SHOCK IN GRB 160509A

Laskar, Tanmoy, Alexander, Kate D., Berger, Edo, Fong, Wen-fai, Margutti, Raffaella, Shivvers, Isaac, Williams, Peter K. G., Kopač, Drejc, Kobayashi, Shiho, Mundell, Carole, Gomboc, Andreja, Zheng, WeiKang, Menten, Karl M., Graham, Melissa L., Filippenko, Alexei V. 08 December 2016 (has links)
We present the second multi-frequency radio detection of a reverse shock in a gamma-ray burst. By combining our extensive radio observations of the Fermi-Large Area Telescope gamma-ray burst 160509A at z - 1.17 up to 20 days after the burst with Swift X-ray observations and ground-based optical and near-infrared data, we show that the afterglow emission comprises distinct reverse shock and forward shock contributions: the reverse shock emission dominates in the radio band at. less than or similar to 10 days, while the forward shock emission dominates in the X-ray, optical, and near-infrared bands. Through multi-wavelength modeling, we determine a circumburst density of n(0) approximate to 10(-3) cm(-3), supporting our previous suggestion that a low- density circumburst environment is conducive to the production of long-lasting reverse shock radiation in the radio band. We infer the presence of a large excess X-ray absorption column, N-H approximate to 1.5. x 10(22) cm(-2), and a high rest-frame optical extinction, A(V) approximate to 3.4 mag. We identify a jet break in the X-ray light curve at t(jet) approximate to 6 days, and thus derive a jet opening angle of theta(jet) approximate to 4 degrees, yielding a beaming-corrected kinetic energy and radiated gamma-ray energy of E-K approximate to 4 x 10(50) erg and E-gamma approximate to 1.3 x 10(51) erg ( 1-10(4) keV, rest frame), respectively. Consistency arguments connecting the forward shocks and reverse shocks suggest a deceleration time of t(dec) approximate to 460 s approximate to T-90, a Lorentz factor of Gamma( t(dec)) approximate to 330, and a reverse-shock-to-forward-shock fractional magnetic energy density ratio of R-B equivalent to is an element of(B, RS)/is an element of(B, FS) approximate to 8. Our study highlights the power of rapid-response radio observations in the study of the properties and dynamics of gamma-ray burst ejecta.
33

Gamma-Ray Burst Science in the Era of IACT Arrays

Weiner, Ori Michael January 2017 (has links)
In this thesis, we explore and improve on the science of gamma-ray bursts with particular attention to the very-high-energy regime. We begin by discussing Imaging Atmospheric Cherenkov Telescope Arrays (IACTs), which are the primary instruments of observation at very-high-energy gamma rays. We focus on a particular, state-of-the-art IACT array in southern Arizona: Very Energetic Radiation Imaging Telescope Array System (VERITAS). We then discuss the science of gamma-ray bursts and what can be learned from IACT observations. We follow with a couple of technical improvements, allowing one to better search for and characterize gamma-ray bursts with IACTs. The techniques focus on statistical methods for detection of transient sources, as well as angular reconstruction at arbitrary zenith angles of observation. We then use one of our new statistical methods to search for a signal in VERITAS observations of gamma-ray burst locations, with tests designed to search for particular bursts as well as for hints of emission in the entire sample of observations. We conclude that there is no evidence for a signal and follow with a discussion of the particularly interesting non-detection of GRB 150323A. We discuss the implications of this non-detection on the energetics and ambient environment of this burst. We conclude that the VERITAS observations might indicate gamma-ray bursts taking place in the dense wind of Wolf-Rayet stars.
34

Cyclotron line formation in a radiation-driven outflow /

Isenberg, Michael. January 1997 (has links)
Thesis (Ph. D.)--University of Chicago, Dept. of Physics, June 1997. / Includes bibliographical references. Also available on the Internet.
35

A search for very high energy gamma rays from PSR1706-44 using the Atmospheric Cerenkov Imaging Technique / by Gavin Peter Rowell.

Rowell, Gavin Peter January 1995 (has links)
Includes bibliographical references. / ix, 159 p. : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1995
36

A search for celestial sources of very high energy gamma-ray emission using the atmospheric Cerenkov technique / Stephen Dennis Elton

Elton, Stephen Dennis January 1989 (has links)
Bibliography: leaves 187-200 / x, 200 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1990
37

A search for celestial sources of very high energy gamma-ray emission using the atmospheric Cerenkov technique /

Elton, Stephen Dennis. January 1989 (has links) (PDF)
Thesis (Ph. D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1990. / Includes bibliographical references (leaves 187-200).
38

The multichromatic wavelet transformation as a source identification tool for GLAST /

Robinson, Sean M. January 2004 (has links)
Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 92-96).
39

A study of the optical afterglows of gamma-ray bursts

Tam, Pak-hin. January 2005 (has links)
published_or_final_version / abstract / toc / Physics / Master / Master of Philosophy
40

Short gamma-ray bursts resulting from phase-induced collapse of neutron stars

Tian, Xiaolei., 田小磊. January 2008 (has links)
published_or_final_version / Physics / Master / Master of Philosophy

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