Prof Subir Sarkar

Abstract:

We discuss recent observations of high energy cosmic ray positrons and electrons in the context of hadronic interactions in supernova remnants, the suspected accelerators of galactic cosmic rays. Diffusive shock acceleration can harden the energy spectrum of secondary positrons relative to that of the primary protons (and electrons) and thus explain the rise in the positron fraction observed by PAMELA above 10 GeV. We normalize the hadronic interaction rate by holding pion decay to be responsible for the gamma-rays detected by HESS from some SNRs. By simulating the spatial and temporal distribution of SNRs in the Galaxy according to their known statistics, we are able to then fit the electron (plus positron) energy spectrum measured by Fermi. It appears that IceCube has good prospects for detecting the hadronic neutrino fluxes expected from nearby SNRs.

Abstract:

The excess in the positron fraction measured by PAMELA has been interpreted as due to annihilation or decay of dark matter in the Galaxy. More prosaically it has been ascribed to direct production of positrons by nearby pulsars or due to pion production during diffusive shock acceleration of hadronic cosmic rays in nearby sources. We point out that measurements of secondary cosmic ray nuclei can discriminate between these possibilities. New data on the titanium-to-iron ratio support the hadronic source model above and enable a prediction for the boron-to-carbon ratio at energies above 100 GeV.

Abstract:

Precision measurements of anisotropies in the cosmic microwave background and of the clustering of large-scale structure have supposedly established that the universe is presently dominated by "dark energy" which has negative pressure and behaves similarly to a cosmological constant. This is based on the assumption that the primordial density perturbation has a nearly scale-invariant power-law spectrum and that the dark matter consists of "cold" particles. However there are theoretical and observational indications that the spectrum is not scale-free and it is known that sub-eV mass neutrinos contribute a small component of hot dark matter. This would be sufficient to fit the same observational data without requiring any dark energy. © EAS, EDP Sciences 2009.