The contribution of unresolved sources on the gamma-ray spectrum of molecular clouds

Abstract

Fermi-LAT observations of the large-scale diffuse gamma-ray emission unveiled that in some locations, towards the inner Galaxy, the spectrum of this component is harder than the local one, measured at the Earth position. If this signal is associated with the “truly” diffuse emission produced by the interaction of cosmic rays (CRs) with the interstellar medium (ISM) then its observed spectral features can be interpreted as indirect evidence of a CR spectral hardening toward the Galactic center. However, in order to correctly interpret the data, the contribution from unresolved sources has to be taken into account. Newly developed theoretical models showed that the cumulative flux produced by unresolved pulsar wind nebulae (PWNe) added to the “truly” diffuse emission significantly shapes the spectrum of the large-scale diffuse emission, challenging the CR spectral hardening hypothesis. In light of the recent results, we discuss the effect of unresolved PWNe on the observed spectra of the diffuse emission with a particular focus on molecular clouds (MCs). We analyze the influence of unresolved sources on clouds of different sizes and locations. Finally, we provide a prescription on how to choose the regions to target in order to have an unbiased determination of the “truly” diffuse emission.