University of Pennsylvania
Department of Physics and Astronomy
High Energy Physics Seminar

Abstract

The majority of galactic gamma rays are produced by interaction of cosmic rays with matter. This results in a diffuse radiation concentrated in the galactic plane where the flux of cosmic rays and the density of material (mostly atomic, molecular and ionized hydrogen) is high. Gamma emission from the plane has indeed been detected in the energy range up to 30 GeV by space-based detectors. Above 1 GeV, the observed intensity is notably higher than expected in simple models, possibly implying an enhancement at the TeV region as well. Observations at TeV energies, for which the flux is too low for satellite detection, can be done with ground based telescopes. Milagro is a large aperture water Cherenkov detector for extensive air showers, collecting data from a solid angle of more than two steradians in the overhead sky at energies near 1 TeV. A 2000-2001 data set from Milagro has been used to search for the emission of diffuse gamma rays from the galactic disk. An excess has been observed from the region of the Milagro inner Galaxy defined by $l \in (20^{\circ}, 100^{\circ})$ and $|b| < 5^{\circ}$ with the significance $2.3 \cdot 10^{-4}$. Under the assumption that EGRET measurements in 10-30 GeV range can be extended to TeV region with a simple power law energy spectrum, the integral gamma ray flux with energies above 1 TeV for the region of inner Galaxy is measured to be $F(>1 TeV) = (9.5 \pm 2.0) \cdot 10^{-10} \; cm^{-1}\; sr^{-1} \; s^{-1}$ with spectral index $\alpha_{\gamma} = 2.59 \pm 0.07$. Extrapolation of the EGRET measurements between 1 and 30 GeV for the region of inner Galaxy using constant power law spectral index is incompatible with the Milagro data. This indicates softening of the spectrum at energy between 10 GeV and 1 TeV. General overview of the Milagro detector will also be presented.