Cosmic ray transport in the proximity of pulsars and the formation of gamma-ray halos

Abstract

The detection of an extended multi-TeV gamma ray emission around the Geminga and Monogem pulsars by the HAWC collaboration provides a unique tool to investigate the transport properties of cosmic rays in the turbulent magnetized interstellar medium. Previous analyses of such emission in the framework of pure isotropic diffusion lead to infer a suppression of the diffusion coefficient in the pulsars region by at least two orders of magnitude compared to typical values found in models of Galactic cosmic ray propagation. In this work, we investigate the transition from the ballistic to the diffusive regime, and show that, when such transition is taken into account, a good fit to the HAWC −ray data around Geminga and Monogem is obtained with typical values of the interstellar diffusion coefficient.

Slow diffusion is necessary to explain the gamma-ray pulsar halos

“…The fast-diffusion scenario with D 0 = 2 × 10 29 cm 2 s −1 also gives an acceptable fit, which is in agreement with the result of Ref. [3]. The latter diffusion coefficient is consistent with the Galactic typical value derived by B/C data [13].…”

“…We find that the fast-diffusion scenario can give an acceptable fit to the γ-ray profile of the Geminga halo, which is in agreement with the result of Ref. [3]. In relativistic diffusion, the…”

“…We find that if the effect of ballistic propagation is taken into account the profile of the γ-ray halo around Geminga can be roughly fitted and the result of Ref. [3] is repeated. However, the same effect can not account for the γ-ray halo profile around J0622+3749 and slow diffusion is still necessary in this case.…”

“…Recently, a paper by Recchia et al [3] points out another plausible scenario, in which the pulsar halo may be explained without a slow-diffusion environment. It is well known that the nonrelativistic diffusion equation has the superluminal problem.…”

“…The method used in Ref. [3] can be considered as an approximate form of the Jüttner propagator to describe the ballistic and quasi-ballistic propagation.…”