Search for gamma-ray emission from galactic plane with Milagro

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. The interactions producing gamma rays include, among others, the decay of π0's produced in spallation reactions. 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 ∈ (20°, 100°) and |b| < 5° with the significance 2.3 · 10 −4. The emission from the region of the Milagro outer Galaxy defined by l ∈ (140°, 220°) and |b| < 5° is not inconsistent with being that of background only. 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(>1TeV) = (9.5 ± 2.0) · 10−10 cm−1 sr−1 s−1 with spectral index α_γ = 2.59 ± 0.07. The 99.9% upper limit for the diffuse emission in the region of outer Galaxy is set at F(>1TeV) < 4.5 · 10 −10 cm−1 sr −1 s−1 using a differential spectral index of α_γ = 2.49. The upper limit for the outer Galaxy is consistent with the extrapolation of EGRET measurements between 1 and 30 GeV. 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. These observations may be used to constrain some models of Galactic gamma ray emission.