arXiv subjects
Publications and source records attributed to L. Tibaldo.
During its first three years of operation, the Fermi Gamma-ray Space Telescope has provided an unprecedented view of the high energy gamma-ray sky, and also performed direct measurements of the cosmic-ray leptons and searches for signals from dark matter. In this paper we present a short overview of some highlight results, shedding new light on the high-energy side of the Universe.
Diffuse gamma-ray emission arising from interactions of cosmic rays with the interstellar gas traces the densities of both of them throughout the Milky Way. We discuss the results obtained from the analysis of Fermi LAT observations in the region of Cassiopeia and Cepheus, towards the Perseus spiral arm. We find that the gamma-ray emissivity of local gas is consistent with expectations based on the cosmic-ray spectra measured at the Earth. The emissivity decreases from the Gould Belt to the Perseus arm, but the measured gradient is flatter than predictions by a propagation model based on a cosmic-ray source distribution peaking in the inner Galaxy as suggested by pulsars. The Xco=N(H2)/W(CO) conversion factor moderately increases by a factor ~2 from the Gould Belt to the Perseus arm. The presence of additional gas not properly traced by HI and CO surveys in the Gould Belt is suggested by the correlation between gamma rays and thermal emission from cold interstellar dust.
We have used measurements by the Fermi Large Area Telescope (LAT) to study the interstellar gamma-ray emission in a region of the second Galactic quadrant, at 100 deg < l < 145 deg and -15 deg < b < 30 deg. This region encompasses the prominent Gould-Belt clouds of Cassiopeia, Cepheus, and the Polaris flare, as well as large atomic and molecular complexes at larger distances in several spiral arms. The good kinematic separation in velocity between the local, Perseus, and outer arms, and the presence of massive complexes in each, make this region very well suited to probe the gamma-ray emission from the interstellar medium beyond the solar circle. The unprecedented sensitivity and angular resolution of the LAT provide improved constraints on the gradient of the cosmic-ray densities and on the increase of the CO-to-H2 conversion factor, Xco, in the outer Galaxy.