Prof Subir Sarkar

Abstract:

Gamma-ray data from Fermi-LAT show a bi-lobular structure extending up to 50 degrees above and below the Galactic centre, coincident with a possibly related structure in the ROSAT X-ray map which presumably originated in some energy release close to the centre a few million years ago. It has been argued that the gamma-rays arise due to inverse Compton scattering of relativistic electrons accelerated at plasma shocks present in the bubbles. We explore the alternative possibility that the relativistic electrons undergo stochastic 2nd-order Fermi acceleration in the entire volume of the bubbles by plasma wave turbulence. This turbulence is generated behind the outer shock and propagates into the bubble volume, leading to a non-trivial spatial variation of the electron spectral index. Rather than a constant volume emissivity as predicted in other models we find an almost constant surface brightness in gamma-rays and also reproduce the observed sharp edges of the bubbles. We comment on possible cross-checks in other channels.

Abstract:

We compare predictions for high energy neutrino and anti-neutrino deep inelastic scattering cross-sections within the conventional DGLAP formalism of next-to-leading order QCD, using the latest parton distribution functions such as CT10, HERAPDF1.5 and MSTW08 and taking account of PDF uncertainties. From this we derive a benchmark cross-section and uncertainty which is consistent with the results obtained earlier using the ZEUS-S PDFs. We advocate the use of this for analysing data from neutrino telescopes, in order to facilitate comparison between their results.

Abstract:

We discuss the recently observed `excesses' in cosmic ray electron and positron fluxes which have been widely interpreted as signals of dark matter. By considering the production and acceleration of secondary electrons and positrons in nearby supernova remnants, we predict an additional, harder component that becomes dominant at high energies. The unknown spatial distribution of the supernova remnants introduces a stochastic uncertainty which we estimate analytically. Fitting the prediction for different source distributions to the total electron + positron flux measured by Fermi--LAT fixes all free parameters and allows us to `postdict' the rise in the positron fraction seen by PAMELA. A similar rise in the B/C ratio is predicted at high energies.