Diffusion of Cosmic Rays and theGamma‐Ray Large Area Telescope: Phenomenology at the 1–100 GeV Regime

Abstract

This paper analyzes astrophysical scenarios that may be detected at the upper end of the energy range of the GammaRay Large Area Space Telescope (GLAST ), as a result of cosmic-ray (CR) diffusion in the interstellar medium (ISM). Hadronic processes are considered the source of -ray photons from localized molecular enhancements nearby accelerators. Two particular cases are presented: (1) the possibility of detecting spectral energy distributions (SEDs) with maxima above 1 GeV, which may be constrained by detection or nondetection at very high energies (VHEs) with observations by ground-based Cerenkov telescopes, and (2) the possibility of detecting V-shaped, inverted spectra, due to confusion of a nearby (to the line of sight) arrangement of accelerator/ target scenarios with different characteristic properties. We show that finding these signatures (in particular, a peak at the 1Y100 GeV energy region) indicates the underlying mechanism producing the -rays that is realized by nature, which accelerator (age and relative position to the target cloud) and under which diffusion properties CRs propagate.

Broad-band non-thermal emission from molecular clouds illuminated by cosmic rays from nearby supernova remnants

“…Similar concave or ‘V‐shaped’ spectra have been recently obtained by Rodriguez Marrero et al (2008) in a different context in which two molecular clouds are assumed to be located in the proximity of a CR accelerator. If the two clouds happen to be located at different distances from the CR accelerator but within an angular separation smaller than the FERMI angular resolution, then they would appear as a single GeV source, and the superposition of their emission might result in concave spectra.…”

“…The prediction of V‐shaped spectra that we make in this paper is more general than the one by Rodriguez Marrero et al (2008), since it represents an intrinsic feature of a single molecular cloud which is located close to a CR source. The V‐shaped gamma‐ray spectrum reflects the shape of the underlying CR spectrum which is the superposition of the steep spectrum of the background CRs and the hard spectrum of CRs coming from the nearby SNR.…”

“…If the two clouds happen to be located at different distances from the CR accelerator but within an angular separation smaller than the FERMI angular resolution, then they would appear as a single GeV source, and the superposition of their emission might result in concave spectra. However, Rodriguez Marrero et al (2008) did not include in their calculations the contribution to the emission coming from the ubiquitous galactic CR background, which in most cases dominates over the contribution of CRs from the nearby SNR, at least for what concerns the GeV emission from the cloud (see e.g. Fig.…”

FERMILARGE AREA TELESCOPE OBSERVATIONS OF SUPERNOVA REMNANTS INTERACTING WITH MOLECULAR CLOUDS

“…Lee et al (2008) model the interaction of particles accelerated by the diffusive shock of an SNR with arbitrary matter distributions outside the remnant blast wave, including an appropriate treatment of the escape and diffusion of these particles. These authors, as well as other works (Gabici & Aharonian 2007;Rodriguez Marrero et al 2008;Gabici et al 2008), show that clouds of dense material in close proximity to SNRs can have an important, and even dominant contribution to the γ -ray flux from these regions. It is important to note, however, that the escaping particles are predominantly from the high-energy portion of the spectrum; the observed spectra, which extend to low energies, may be problematic for such a scenario.…”

Gamma-ray signatures of cosmic ray acceleration, propagation, and confinement in the era of CTA

“…Since the CR background is known not to vary a lot throughout the Galaxy 2 , the detection of a MC in gamma rays with a flux significantly larger than the one expected for a passive cloud of the same mass would indicate that a source of CRs is present in the vicinity of or inside the cloud. Thus, gamma-ray-bright MCs can be used, in principle, to locate the sources of CRs [64,65,66,16,67,68]. For these reasons, an investigation of the capability of CTA to detect MCs is of paramount importance.…”