Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phdt........12c&link_type=abstract
Proquest Dissertations And Theses 2008. Section 0030, Part 0606 180 pages; [Ph.D. dissertation].United States -- California: Un
Astronomy and Astrophysics
Astrophysics
Gamma-Rays, Tev, Egret, Milagro Observatory
Scientific paper
Very high energy gamma-rays can he used to understand some of the most powerful astrophysical objects in the universe, such as active galactic nuclei, supernova remnants and pulsar wind nebula. EGRET is one of the four instruments of the Compton Gamma Ray Observatory (CGRO) project which detected gamma-ray emission from 30 keV to 30 GeV. EGRET covered an energy range between 20 MeV and 30 GeV. The third EGRET catalog contained 271 new gamma-ray sources with energies above 100 MeV. The 271 sources in this catalog include the single 1991 solar flare, the Large Magellanic Cloud, five pulsars, one radio galaxy, and 66 high-confidence identification of blazars. In addition, 27 lower confidence potential blazar identifications are noted. The rest of sources, 170 out of 271, have not yet been identified with known objects.
Located in northern New Mexico, the Milagro gamma-ray observatory employs a water-Cherenkov technique to continuously monitor the northern sky for TeV gamma-ray emission from astrophysical sources. The instrument has a large field of view, which covers the entire overhead sky (~ 2 sr) and has a high duty cycle (> 90%). More than seven years of Milagro data are used to search for TeV gamma-ray emission from the EGRET sources in the northern sky. Milagro's new gamma-hadron separation variable, A 4 , coupled with the weighting analysis technique significantly improves the sensitivity of the Milagro detector. In my thesis I present the flux and flux upper limits of 129 EGRET sources which are in the Milagro field of view using two different gamma-hadron separation parameters, X 2 and A 4 . The median energies for X 2 and A 4 are 7 TeV and 25 TeV respectively. Constrains on the flux of these sources at 7 TeV and 25 TeV are computed assuming a differential photon spectrum of a power law with spectral index a = -2.3. For 18 EGRET unidentified sources I compare the Milagro flux (if s > 2.0) and flux upper limits (if s < 2.0) with the flux measured by EGRET at 100 MeV and the flux upper limits measured by the Whipple observatory at 350 GeV. The comparison of Milagro measurements, EGRET and Whipple extrapolation to Milagro energies using the spectral indices taken from 3EG catalog are presented. Because the typical point spread function of EGRET is large (~ 5°), Monte Carlo simulations are used to find out an optimum analysis technique to search EGRET sources in Milagro data.
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