The cosmic-ray puzzle and the census of the interstellar medium: the Fermi LAT view of Cassiopeia, Cepheus and the Perseus arm

TL;DR

This study uses Fermi LAT gamma-ray data to analyze cosmic-ray interactions with interstellar gas in specific Galactic regions, revealing discrepancies with models and suggesting additional gas components.

Contribution

It provides new insights into cosmic-ray distribution and interstellar medium properties in the Perseus arm, challenging existing propagation models.

Findings

Gamma-ray emissivity aligns with local cosmic-ray spectra

Emissivity gradient is flatter than model predictions

Xco factor increases moderately from Gould Belt to Perseus arm

Abstract

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.