Cosmic-Ray Propagation in Light of the Recent Observation of Geminga

Abstract

The High Altitude Water Cherenkov (HAWC) telescope recently observed extended emission around the Geminga and PSR B0656+14 pulsar wind nebulae (PWNe). These observations have been used to estimate cosmic-ray (CR) diffusion coefficients near the PWNe that appear to be more than two orders of magnitude smaller than that typically derived for the interstellar medium from the measured abundances of secondary species in CRs. Two-zone diffusion models have been proposed as a solution to this discrepancy, where the slower diffusion zone (SDZ) is confined to a small region around the PWN. Such models are shown to successfully reproduce the HAWC observations of the Geminga PWN while retaining consistency with other CR data. It is found that the size of the SDZ influences the predicted positron flux and the spectral shape of the extended γ-ray emission at lower energies that can be observed with the Fermi Large Area Telescope (Fermi-LAT). If the two observed PWNe are not unique, then it is likely that there are similar pockets of slow diffusion around many CR sources elsewhere in the Milky Way. The consequences of such picture for Galactic CR propagation is explored.

Deciphering Residual Emissions: Time-dependent Models for the Nonthermal Interstellar Radiation from the Milky Way

“…In this paper we take the next steps to address this issue using the latest release of the GALPROP CR propagation code 1 (Porter et al 2017;Jóhannesson et al 2018), which has been enhanced with new capabilities to facilitate efficient modelling for time/space discretised CR sources and propagation in the ISM. Two smooth CR source density models are used for this investigation: a 3D distribution (SA50) that has an axisymmetric disc and spiral arm components with 50% of the CR luminosity injected by each, and an axisymmetric disc only distribution (SA0) for all of the CR injected power.…”

“…The transport equations are solved on the ζ grid accounting for the change in first and second derivatives. Without loss of generality, we use the SA50 model (Porter et al 2017;Jóhannesson et al 2018) for the underlying source density distribution from which the discretised injection regions are sampled. The SA50 density model assumes 50% of the injected CR luminosity is provided by an axisymmetric disc component following Yusifov & Küçük (2004) and the other 50% is from the 4-arm spiral of Robitaille et al (2012).…”

“…The propagation model parameters are as determined by Jóhannesson et al (2019) using the 3D neutral gas (atomic and molecular) distribution model described by Jóhannesson et al (2018) with 90% hydrogen and 10% helium by number, and the ionised gas distribution described by the hybrid HII model included in the GALPROP code that is based on the NE2001 model of Cordes & Lazio (2002) and the work of Gaensler et al (2008). The tuning procedure follows that of Porter et al (2017) and Jóhannesson et al (2018) where the size of the CR halo is set to 6 kpc and the parameters are adjusted to reproduce recent CR data. Solar modulation is accounted for by using the force-field approximation, one modulation potential value for each observation period.…”

“…The procedure is then iterated until convergence. The only difference to Porter et al (2017) and Jóhannesson et al (2018) are the data: elemental spectra on Be, C, and O from AMS-02 has been added to the fit as well as elemental spectra from ACE-CRIS, while data from HEAO-C3 and PAMELA has been removed (for the full list of CR data used see Table 1 of Jóhannesson et al 2019).…”

The cosmic-ray content of the Orion-Eridanus superbubble

“…The stellar density in the superbubble and the results of superbubble MHD simulations (Ntormousi et al 2017) suggest a typical injection scale L * ≈ 10 pc and interior magnetic-field strengths of order 1-3 µG. In these conditions, the diffusion coefficient spans from These values are respectively 200 and 20 times smaller at 10 GeV than the average uniform coefficient inferred in the Milky Way using the GALPROP propagation code and elemental spectra from AMS-02 and ACE/CRIS (Jóhannesson et al 2019). The size of the TeV haloes found around the nearby Geminga and PSR B0656+14 pulsars requires slow diffusion between 1.5 and 7.0 × 10 26 (E/10 GeV) cm 2 /s (Abeysekara et al 2017).…”