The cosmic-ray and gas content of the Cygnus region as measured inγ-rays by theFermiLarge Area Telescope

Abstract

Context. The Cygnus region hosts a giant molecular-cloud complex that actively forms massive stars. Interactions of cosmic rays with interstellar gas and radiation fields make it shine at γ-ray energies. Several γ-ray pulsars and other energetic sources are seen in this direction. Aims. In this paper we analyze the γ-ray emission measured by the Fermi Large Area Telescope (LAT) in the energy range from 100 MeV to 100 GeV in order to probe the gas and cosmic-ray content on the scale of the whole Cygnus complex. The γ-ray emission on the scale of the central massive stellar clusters and from individual sources is addressed elsewhere. Methods. The signal from bright pulsars is greatly reduced by selecting photons in their off-pulse phase intervals. We compare the diffuse γ-ray emission with interstellar gas maps derived from radio/mm-wave lines and visual extinction data. A general model of the region, including other pulsars and γ-ray sources, is sought. Results. The integral H i emissivity above 100 MeV averaged over the whole Cygnus complex amounts to [2.06 ± 0.11 (stat.), where the systematic error is dominated by the uncertainty on the H i opacity to calculate its column densities. The integral emissivity and its spectral energy distribution are both consistent within the systematics with LAT measurements in the interstellar space near the solar system. The average X CO = N(H 2 )/W CO ratio is found to be [1.68 ± 0.05 (stat.) (K km s −1 ) −1 , consistent with other LAT measurements in the Local Arm. We detect significant γ-ray emission from dark neutral gas for a mass corresponding to ∼40% of what is traced by CO. The total interstellar mass in the Cygnus complex inferred from its γ-ray emission amounts to 8 +5 −1 × 10 6 M at a distance of 1.4 kpc. Conclusions. Despite the conspicuous star formation activity and high masses of the interstellar clouds, the cosmic-ray population in the Cygnus complex averaged over a few hundred parsecs is similar to that of the local interstellar space.

Selected Topics in Cosmic Ray Physics

“…These findings were qualitatively confirmed by an independent analysis of the Fermi-LAT data [146]. The weak dependence of the CR density on Galactocentric distance R for R > 5 kpc is the well known CR gradient problem [147,148,149]: the CR density drops much slower than proportional to the density of sources. These findings are difficult to reconcile with the standard approach to CR propagation, which is based upon solving the transport equation under the assumption that the diffusive properties are the same in the whole propagation volume [150].…”

Massive stars as major factories of Galactic cosmic rays

“…The Cygnus region is located inside the Local Arm, although the Perseus and the outer arms also contribute to the total gas content in the line of sight. To select the gas content related to Cygnus Cocoon itself, we separate, following 49 , two regions contributing to the total gas. In the H I and CO maps, we assign the gas with V LSR < −20 km/s to be Local arm and those with V LSR > −20 km/s to be outer arms.…”

Correlated GeV–TeV Gamma-Ray Emission from Extended Sources in the Cygnus Region

“…Fermi-LAT is a satellite-based gamma ray observatory, sensitive to gamma-ray emission in the 20 MeV-300 GeV range 1 . Fermi-LAT observation of the Cygnus region revealed an extended excess after subtraction known point sources, extended emission from Gamma Cygni and background using a diffuse emission model specifically developed for the Cygnus region in the energy range of 1 GeV -100 GeV [6,7]. This excess emission is detected with 10.1σ significance above 1 GeV and is best described by a Gaussian width of σ = 2.0 • ± 0.2 • .…”