Measuring energy spectra of TeV gamma-ray emission from the Cygnus region of our galaxy with Milagro.

摘要

High energy gamma rays can provide fundamental clues to the origins of cosmic rays. In this thesis, TeV gamma-ray emission from the Cygnus region is studied. Previously the Milagro experiment detected five TeV gamma-ray sources in this region and a significant excess of TeV gamma rays whose origin is still unclear. To better understand the diffuse excess the separation of sources and diffuse emission is studied using the latest and most sensitive data set of the Milagro experiment. In addition, a newly developed technique is applied that allows the energy spectrum of the TeV gamma rays to be reconstructed using Milagro data. No conclusive statement can be made about the spectrum of the diffuse emission from the Cygnus region because of its low significance of 2.2 sigma above the background in the studied data sample. The entire Cygnus region emission is best fit with a power law with a spectral index of alpha=2.40 (68% confidence interval: 1.35--2.92) and a exponential cutoff energy of 31.6 TeV (10.0--251.2 TeV). In the case of a simple power law assumption without a cutoff energy the best fit yields a spectral index of alpha=2.97 (68% confidence interval: 2.83--3.10). Neither of these best fits are in good agreement with the data. The best spectral fit to the TeV emission from MGRO J2019+37, the brightest source in the Cygnus region, yields a spectral index of alpha=2.30 (68% confidence interval: 1.40--2.70) with a cutoff energy of 50.1 TeV (68% confidence interval: 17.8--251.2 TeV) and a spectral index of alpha=2.75 (68% confidence interval: 2.65--2.85) when no exponential cutoff energy is assumed. According to the present analysis, MGRO J2019+37 contributes 25% to the differential flux from the entire Cygnus at 15 TeV.