Silvia Mollerach

The toes of the ultra high energy cosmic ray spectrum

We study the effects of the galactic magnetic field on the ultra high energy cosmic ray propagation. We show that the deflections of the cosmic ray trajectories can have many important implications such as (de)magnification of the cosmic ray fluxes by lensing effects (which can modify the spectrum of individual sources), the formation of multiple images of a source or the existence of regions of the sky to which the Earth is almost blind. The appearance of image pairs is related to the existence of critical curves in the magnification maps, which divide regions in the sky where the images have opposite parities. The results are pictorially illustrated as the stretching and folding of a `sheet describing the sky seen on Earth. Making use of the most energetic AGASA events we emphasize the need to know the cosmic ray composition and the structure of the magnetic field when attempting to do detailed cosmic ray astronomy.

Signatures of galactic magnetic lensing upon ultra high energy cosmic rays

We analyse several implications of lensing by the regular component of the galactic magnetic field upon the observed properties of ultra high energy cosmic rays. Magnetic fields deflect cosmic ray trajectories, causing flux (de)magnification, formation of multiple images of a single source, and time delays. We derive the energy dependence of these effects near the caustics at which the flux amplification of a point source diverges. We show that the large magnification of images around caustics leads to an amplification bias, which can make them dominate the flux in some energy ranges. We argue that clustering in the arrival directions of UHECRs of comparable energy may be due to magnetic lensing around caustics. We show that magnetic lensing can also significantly alter the observed composition of cosmic rays at the highest energies. We also show that the time delay between events from a single image may monotonically decrease with decreasing energy in the neighborhood of a caustic, opposite to its behaviour in normal regions. Lensing effects in the magnetic field model considered are significant for cosmic rays with ratio between energy and electric charge E/Z between 1018 and 5 × 1019 eV approximately. Similar effects may also occur for higher E/Z values if the galactic magnetic field is stronger or more extended than what is assumed here, or in the magnetic field of source galaxies, or even in intergalactic fields.

B polarization of the CMB from Faraday rotation

We study the effect of Faraday rotation due to a uniform magnetic field on the polarization of the cosmic microwave background. Scalar fluctuations give rise only to parity-even E-type polarization of the cosmic microwave background. However in the presence of a magnetic field, a nonvanishing parity-odd B-type polarization component is produced through Faraday rotation. We derive the exact solution for the E and B modes generated by scalar perturbations including the Faraday rotation effect of a uniform magnetic field, and evaluate their cross correlations with temperature anisotropies. We compute the angular autocorrelation function of the B-modes in the limit that the Faraday rotation is small. We find that uniform primordial magnetic fields of present strength around B{sub 0}=10{sup -9} G rotate E-modes into B-modes with amplitude comparable to those due to the weak gravitational lensing effect at frequencies around {nu}=30 GHz. The strength of B-modes produced by Faraday rotation scales as B{sub 0}/{nu}{sup 2}. We evaluate also the depolarizing effect of Faraday rotation upon the cross correlation between temperature anisotropy and E-type polarization.

Fermion production during preheating after hybrid inflation

At the end of inflation, the coherent oscillations of the inflaton field may resonantly amplify the long wavelength modes of both bosons and fermions coupled to it. We study the resonant production of both kinds of particles during preheating in a model of hybrid inflation. The coherent time evolution of the inflaton and the Higgs fields after inflation induce a very different production of fermions depending on whether they are coupled to the Higgs or to the inflaton. For reasonable values of the model parameters, the fermion production through parametric resonance can be very efficient. We study the relative growth of the fermion and boson energy densities during preheating in hybrid models. During the initial stage of preheating, fermion production dominates the relative energy density, while the exponential growth of bosonic modes soon takes over.

Magnetic lensing of extremely high energy cosmic rays in a galactic wind

We show that in the model of Galactic magnetic wind recently proposed to explain the extremely high energy (EHE) cosmic rays so far observed as originating from a single source (M87 in the Virgo cluster), the magnetic field strongly magnifies the fluxes and produces multiple images of the source. The apparent position on Earth of the principal image moves, for decreasing energies, towards the galactic south. It is typically amplified by an order of magnitude at E/Z ~ 2 × 1020 eV, but becomes strongly demagnified below 1020 eV. At energies below E/Z ~ 1.3 × 1020 eV, all events in the northern galactic hemisphere are due to secondary images, which have huge amplifications ( > 102). This model would imply strong asymmetries between the north and south galactic hemispheres, such as a (latitude dependent) upper cut-off value below 2 × 1020 eV for CR protons arriving to the south and lower fluxes in the south than in the north above 1020 eV. The large resulting magnifications reduce the power requirements on the source, but the model needs a significant tunning between the direction to the source and the symmetry axis of the wind. If more modest magnetic field strengths were assumed, a scenario in which the observed EHE events are heavier nuclei whose flux is strongly lensed becomes also plausible and would predict that a transition from a light composition to a heavier one could take place at the highest energies.

On the disintegration of cosmic ray nuclei by solar photons

We discuss in detail the possibility of observing pairs of simultaneous parallel air showers produced by the fragments of cosmic ray nuclei which disintegrated in collisions with solar photons. We consider scenarios with different cosmic ray compositions, exploring the predicted rates for existing and planned detectors and looking for methods to extract information on the initial composition from the characteristics of the signal. In particular, we find that fluorescence detectors, such as HiRes or the Telescope Array, due to their low threshold ( ~ 1017 eV) and large area ( ~ 104 km2) may observe several events per year if cosmic rays at those energies are indeed heavy nuclei. The possibility of exploiting the angular orientation of the plane containing the two showers to further constrain the cosmic ray composition is also discussed.

Magnetic lensing of ultra high energy cosmic rays

We discuss several effects due to lensing of ultra high energy cosmic rays by the regular component of the galactic magnetic field. Large flux magnification around caustics can be a significant source of clustering in the arrival directions of UHECRs of comparable energy. We also discuss lensing effects in a hypothetical galactic magnetic wind model recently proposed to explain the extremely high energy cosmic rays so far observed as originating from a single source (M87). This model implies large flux magnifications, which reduce the power requirements on the source, and a significant asymmetry in the expected flux between the north and south galactic hemispheres.

Gravitational lensing as folds in the sky

We revisit the gravitational lensing phenomenon using a new visualization technique. It consists in projecting the observers sky into the source plane, what gives rise to a folded and stretched surface. This provides a clear graphical tool to visualize some interesting well-known effects, such as the development of multiple images of a source, the structure of the caustic curves, the parity of the images and their magnification as a function of the source position.