Obtaining the diffusion coefficient for cosmic ray propagation in the Galactic Centre Ridge through time-dependent simulations of their gamma-ray emission

Recent observations by the H.E.S.S. collaboration of the Galactic Centre region have revealed what appears to be gamma-ray emission from the decay of pions produced by interactions of recently accelerated cosmic rays with local molecular hydrogen clouds. Synthesizing a 3-D hydrogen cloud map from the available data and assuming a diffusion coefficient of the form kappa(E) = kappa_0(E/E0)^delta, we performed Monte Carlo simulations of cosmic ray diffusion for various propagation times and values of kappa_0 and delta. By fitting the model gamma-ray spectra to the observed one we were able to infer the value of the diffusion coefficient in that environment (kappa = 3.0 +/- 0.2 kpc^2 Myr^-1 for E = 10^12.5 eV and for total propagation time 10^4 yr) as well as the source spectrum (2.1 < gamma < 2.3). Also, we found that proton losses can be substantial, which justifies our approach to the problem.Comment: 7 pages, 7 figures, accepted for publication in Astroparticle Physic

One-zone SSC model for the core emission of Centaurus A revisited

Aims: We investigate the role of the second synchrotron self-Compton (SSC) photon generation to the multiwavelength emission from the compact regions of sources that are characterized as misaligned blazars. For this, we focus on the nearest high-energy emitting radio galaxy Centaurus A and we revisit the one-zone SSC model for its core emission. Methods: We have calculated analytically the peak luminosities of the first and second SSC components by, first, deriving the steady-state electron distribution in the presence of synchrotron and SSC cooling and, then, by using appropriate expressions for the positions of the spectral peaks. We have also tested our analytical results against those derived from a numerical code where the full emissivities and cross-sections were used. Results: We show that the one-zone SSC model cannot account for the core emission of Centaurus A above a few GeV, where the peak of the second SSC component appears. We, thus, propose an alternative explanation for the origin of the high energy ($\gtrsim 0.4$ GeV) and TeV emission, where these are attributed to the radiation emitted by a relativistic proton component through photohadronic interactions with the photons produced by the primary leptonic component. We show that the required proton luminosities are not extremely high, e.g. $\sim 10^{43}$ erg/s, provided that the injection spectra are modelled by a power-law with a high value of the lower energy cutoff. Finally, we find that the contribution of the core emitting region of Cen A to the observed neutrino and ultra-high energy cosmic-ray fluxes is negligible.Comment: 12 pages, 6 figures, 3 tables, accepted for publication in A&

On the Close Correspondence between Storm-time ULF Wave Power and the POES VLF Chorus Wave Amplitude Proxy

Ground-based Pc5 ULF wave power in multiple ground-based meridians is compared to the VLF wave amplitude proxy, derived from POES precipitation, for the 33 storms studied by Li et al. [2015]. The results reveal common L-shell and time profiles for the ULF waves and VLF proxy for every single storm, especially at $L\leq 6$, and identical discrimination between efficient and inefficient radiation belt electron acceleration. The observations imply either ULF waves play a role in driving precipitation which is falsely interpreted as VLF wave power in the proxy, ULF waves drive VLF waves (the reverse being energetically unfeasible), or both have a common driver with nearly identical L-shell and time-dependence. Global ground-based ULF wave power coherence implies a small number of meridians can be used to estimate storm-time radial diffusion coefficients. However, the strong correspondence between ULF wave power and VLF wave proxy complicates causative assessments of electron acceleration.Comment: 16 pages, 4 figures, 1 table, published in Geophysical Research Letters. Supporting Information is included as a PDF in the source file