Positron excess from cosmic ray interactions in galactic molecular clouds

Abstract

The recent data on cosmic ray positron flux measured near the Earth by the Alpha Magnetic pectrometer(AMS-02) experiment extends to TeV energy. The recently observed data by AMS-02 clearly onfirms that the positron flux rises with energy and shows a peak near a few hundred GeV. This rising sitron flux cannot be explained by interactions of cosmic rays with interstellar hydrogen gas. Pulsars, dark matter and many other innovative physical scenarios have been studied to explain this rising of the positron flux, lso known as positron excess. In this paper, our goal is to study whether secondary production due to cosmic ray interactions in nearby Galactic Molecular Clouds (GMCs) can contribute significantly to the observed ositron spectrum on Earth. Due to the progress in multi-wavelength astronomy, many new GMCs have been discovered in our Galaxy recently. Using large scale CO survey, 1064 GMCs were detected in the Galaxy, which reside in the Galactic plane. Alongside that, very recent survey implemented the optical/IR dust extinction measurements, to trace 567 GMCs within 4 kpc of Earth, also residing in the Galactic plane. We use the pdated list of GMCs reported in recent papers, which are distributed in the Galactic plane, to find the econdary positrons produced in them in interactions of cosmic rays with molecular hydrogen. Moreover, by analyzingthe Fermi-LAT data, new GMCs have been discovered near the Galactic plane. We also include some of these GMCs closest to the Earth where cosmic ray interactions are producing secondaries. It has been speculated earlier that cosmic rays may be reaccelerated in some GMCs. We select 7 GMCs out of 567 GMCs recently reported, within 4 kpc of Earth, where reacceleration due to magnetized turbulence is assumed. We include a small hardened component of secondary positrons, produced from interaction of reaccelerated CRs in those 7 GMCs. We use publicly available codeDRAGONfor our simulation setup to study CR ropagation in the Galaxy and show that the observed positron spectrum can be well explained in the energy range of 1 to 1000 GeV by our self consistent model.