Anisotropic cosmic ray diffusion and gamma-ray production close to supernova remnants, with an application to W28

Abstract

Cosmic rays that escape their acceleration site interact with the ambient medium and produce gamma rays as the result of inelastic proton-proton collisions. The detection of such diffuse emission may reveal the presence of an accelerator of cosmic rays, and also constrain the cosmic ray diffusion coefficient in its vicinity. Preliminary results in this direction have been obtained in the last years from studies of the gamma-ray emission from molecular clouds located in the vicinity of supernova remnants, which are the prime candidate for cosmic ray production. Hints have been found for a significant suppression of the diffusion coefficient with respect to the average one in the Galaxy. However, most of these studies rely on the assumption of isotropic diffusion, which may not be very well justified. Here, we extend this study to the case in which cosmic rays that escape an accelerator diffuse preferentially along the magnetic field lines. As a first approximation, we further assume that particles are strongly magnetized and that their transport perpendicular to the magnetic field is mainly due to the wandering of the field lines. The resulting spatial distribution of runaway cosmic rays around the accelerator is, in this case, strongly anisotropic. An application of the model to the case of the supernova remnant W28 demonstrates how the estimates of the diffusion coefficient from gamma-ray observations strongly depend on the assumptions made on the isotropy (or anisotropy) of diffusion. For higher levels of anisotropy of the diffusion, larger values of the diffusion coefficient are found to provide a good fit to data. Thus, detailed models for the propagation of cosmic rays are needed in order to interpret in a correct way the gamma-ray observations.

A young supernova remnant illuminating nearby molecular clouds with cosmic rays

“…MC-J1729 can dominate the TeV emission outside the SNR assuming isotropic diffusion by adopting a reasonable three-dimensional molecular cloud structure. There is no need to invoke anisotropic CR diffusion (Jokipii 1966;Nava & Gabici 2013;Malkov et al 2013;Giacinti et al 2013) to boost the CR density in a particular direction.…”

Anisotropic cosmic ray diffusion and its implications for gamma-ray astronomy

“…[16] modeled the diffusion of CRs close to their source by an anisotropic diffusion tensor, resulting in faster diffusion along a given axis. Given the sensitivity and angular resolution of present gammaray observatories, and the amount of poorly constrained astrophysical parameters, such a treatment is justified.…”

“…Ref. [16] addressed this question by assuming strongly anisotropic diffusion around sources in one direction. Recently, [17] confirmed our findings [10] and discussed the implications for local e ± sources.…”

THE TeV GAMMA-RAY MILKY WAY

“…Such diffusive effects have been used to explain the GeV to TeV gamma-ray spectral shapes from several mature SNR/MC sources (e.g. Nava & Gabici (2013)), and the diffuse TeV emission towards the inner few 100 pc of our galaxy (Aharonian et al, 2006). Recent assessments of the combined atomic and molecular ISM towards young TeV Shell SNRs (Fukui et al, 2012;Fukuda et al, 2014) offer observational evidence for a hadronic component.…”

“…CTA's sensitivity is expected to reveal the more diffuse TeV emission surrounding many sources, for example from cosmic-rays escaping SNRs (Malkov et al, 2013;Nava & Gabici, 2013;Acero et al , 2013), and cosmic-rays entering dense cores of molecular clouds (Nicholas et al, 2012). Hence, improved arc-minute resolution Galactic plane surveys in CO (Burton et al, 556 G. ROWELL 2013;Braiding et al, 2014), and mapping of TeV sources in dense gas tracers such as CS and NH 3 (Nicholas et al, 2012;Maxted et al, 2012;Voisin et al, 2014) such as those provided by the Mopra telescope will also be necessary to perform the next stage of TeV/ISM comparisons that CTA will provide in the coming years.…”