Low Cosmic Ray Density In Polaris Flare

Cosmic rays(CRs)lose their spatial information in the propagation in the interstellar magnetic field.In this regard,γ-rays,which is the secondary product in the interaction between CRs and the ambient gas,can provide us with important information on the CR distributions.In this work,we reported the γ-ray observation towards the giant molecular cloud Polaris Flare.Together with the dust column density map,we derived the CR density and spectrum in this cloud.Compared with the CR measured locally,the CR density in the Polaris Flare is significantly lower and the spectrum is softer.Such a different CR spectrum reveals either a rather large gradient of CR distribution in the direction perpendicular to the Galactic plane or a suppression of CR inside molecular clouds.