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Diego Harari

Publications and source records attributed to Diego Harari.

29 records · Page 2Linked to original sources
arXiv2002

Lensing of ultra-high energy cosmic rays in turbulent magnetic fields

We consider the propagation of ultra high energy cosmic rays through turbulent magnetic fields and study the transition between the regimes of single and multiple images of point-like sources. The transition occurs at energies around $E_c\simeq Z~41 {\rm EeV}(B_{rms}/5 μ{\rm G}) (L/ 2 {\rm kpc})^{3/2}\sqrt{50 {\rm pc}/L_c}$, where $L$ is the distance traversed by the CR's with electric charge $Ze$ in the turbulent magnetic field of root mean square strength $B_{rms}$ and coherence length $L_c$. We find that above $2 E_c$ only sources located in a fraction of a few % of the sky can reach large amplifications of its principal image or start developing multiple images. New images appear in pairs with huge magnifications, and they remain amplified over a significant range of energies. At decreasing energies the fraction of the sky in which sources can develop multiple images increases, reaching about 50% for $E>E_c/2$. The magnification peaks become however increasingly narrower and for $E<E_c/3$ their integrated effect becomes less noticeable. If a uniform magnetic field component is also present it would further narrow down the peaks, shrinking the energy range in which they can be relevant. Below $E\simeq E_c/10$ some kind of scintillation regime is reached, where many demagnified images of a source are present but with overall total magnification of order unity. We also search for lensing signatures in the AGASA data studying two-dimensional correlations in angle and energy and find some interesting hints.

astro-ph↗
arXiv2001

Dimming of Supernovae by Photon-Pseudoscalar Conversion and the Intergalactic Plasma

Recently Csaki, Kaloper and Terning (hep-ph/0111311) suggested that the observed dimming of distant type Ia supernovae may be a consequence of mixing of the photons with very light axions. We point out that the effect of the plasma, in which the photons are propagating, must be taken into account. This effect changes the oscillation probability and renders the dimming frequency-dependent, contrary to observations. One may hope to accommodate the data by averaging the oscillations over many different coherence domains. We estimate the effect of coherence loss, either due to the inhomogeneities of the magnetic field or of the intergalactic plasma. These estimates indicate that the achromaticity problem can be resolved only with very specific, and probably unrealistic, properties of the intergalactic medium.

hep-ph↗
arXiv2001

Constraining non standard recombination: A worked example

We fit the BOOMERANG, MAXIMA and COBE/DMR measurements of the cosmic microwave background anisotropy in spatially flat cosmological models where departures from standard recombination of the primeval plasma are parametrized through a change in the fine structure constant $α$ compared to its present value. In addition to $α$ we vary the baryon and dark matter densities, the spectral index of scalar fluctuations, and the Hubble constant. Within the class of models considered, the lack of a prominent second acoustic peak in the measured spectrum can be accomodated either by a relatively large baryon density, by a tilt towards the red in the spectrum of density fluctuations, or by a delay in the time at which neutral hydrogen formed. The ratio between the second and first peak decreases by around 25% either if the baryon density $Ω_bh^2$ is increased or the spectral index $n$ decreased by a comparable amount, or if neutral hydrogen formed at a redshift $z_*$ about 15% smaller than its standard value. We find that the present data is best fitted by a delay in recombination, with a lower baryon density than the best fit if recombination is standard. Our best fit model has $z_*= 900$, $Ω_bh^2=0.024$, $Ω_mh^2=0.14$, $H_0=49$ and $n=1.02$. Compatible with present data at 95% confidence level $780< z_*<1150$, $0.018<Ω_bh^2<0.036$, $0.07< Ω_m h^2 < 0.3$ and $0.9<n<1.2$.

astro-ph↗
arXiv2000

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.

astro-ph↗
arXiv2000

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 $E/Z\sim 2\times 10^{20}$ eV, but becomes strongly demagnified below $10^{20}$ eV. At energies below $E/Z\sim 1.3\times 10^{20}$ eV, all events in the northern galactic hemisphere are due to secondary images, which have huge amplifications ($>10^2$). This model would imply strong asymmetries between the north and south galactic hemispheres, such as a (latitude dependent) upper cut-off value below $2\times 10^{20}$ eV for CR protons arriving to the south and lower fluxes in the south than in the north above $10^{20}$ 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.

astro-ph↗
arXiv2000

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.

astro-ph↗
arXiv1999

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.

astro-ph↗
arXiv1996

Depolarization of the cosmic microwave background by a primordial magnetic field and its effect upon temperature anisotropy

We estimate the depolarizing effect of a primordial magnetic field upon the cosmic microwave background radiation due to differential Faraday rotation across the last scattering surface. The degree of linear polarization of the CMB is significantly reduced at frequencies around and below 30 GHz $(B_* /10^{-2}{\rm Gauss})^{1/2}$, where $B_*$ is the value of the primordial field at recombination. The depolarizing mechanism reduces the damping of anisotropies due to photon diffusion on small angular scales. The $l\approx 1000$ multipoles of the CMB temperature anisotropy correlation function in a standard cold dark matter cosmology increase by up to 7.5\% at frequencies where depolarization is significant.

astro-ph↗
arXiv1993

Quantum Electrodynamics in the Gravitational Field of a Cosmic String

We evaluate the cross section for electron-positron pair production by a single high energy photon in the space-time of a static, straight cosmic string. Energy and momentum conservation precludes this process in empty Minkowski space. It happens around a cosmic string, in spite of the local flatness of the metric, as a consequence of the conical structure of space. Previous results based on a simplified model with scalar fields are here extended to the realistic QED case. Analytic expressions are found in three different regimes: near the threshold, at energies much larger than the electron rest mass M, and at ultrahigh energies, much larger than $M/δ$, with $δ$ the string mass per unit length in Planck units.

astro-ph↗
arXiv1993

Polarization of the Microwave Background in Inflationary Cosmology

We evaluate the large scale polarization of the cosmic microwave background induced, via Thomson scattering prior to decoupling, by nearly scale-invariant spectra of scalar and tensor metric perturbations, such as those predicted by most inflationary models of the early Universe. We solve the radiative transfer equation for the polarized photon distribution function analytically, for wavelengths of order or larger than the horizon size at decoupling, assuming no reionization. The induced polarization is proportional to the redshift induced by the perturbations at decoupling and to the duration of the decoupling transition. Normalizing the spectra to the quadrupole anisotropy measured by the COBE satellite, the expected degree of linear polarization is $P\lsim 10^{-7}$, more than two orders of magnitude smaller than current bounds. The dependence of $P$ with the tilt in the spectrum away from scale invariance is too weak to be of observational relevance or to provide a sensitive discrimination between scalar and tensor fluctuations. The variation of $P$ with the angular scale is significant.

astro-ph↗