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Silvia Mollerach

Publications and source records attributed to Silvia Mollerach.

At least 37 records · Page 2Linked to original sources
arXiv2009

Source position reconstruction and constraints on the galactic magnetic field from ultra-high energy cosmic rays

We study the possibility to reconstruct the position of ultra-high energy cosmic ray sources and some properties of the magnetic field along the line of sight towards them in the case that several events from the same source are detected. By considering an illustrative model for the galactic magnetic field, including both a regular and a turbulent component, we estimate the accuracy that can be achieved in the reconstruction. We analyse the effect of the experimental energy and angular resolutions on these results. We show that if about ten events with energies above 30 EeV are detected coming from the same source, it should be possible to reconstruct the source position with an accuracy of 0.5$^{\circ}$ and the integral of the orthogonal component of the magnetic field along the line of sight with an accuracy of 0.6 $μ$G kpc Z$^{-1}$ (with Z the electric charge of the particles).

astro-ph.HE↗
arXiv2009

Search for clustering of ultra high energy cosmic rays from the Pierre Auger Observatory

We present the results of a search for clustering among the highest energy events detected by the surface detector of the Pierre Auger Observatory between 1 January 2004 and 31 August 2007. We analyse the autocorrelation function, in which the number of pairs with angular separation within a given angle is compared with the expectation from an isotropic distribution. Performing a scan in energy above 30 EeV and in angles smaller than 30 degrees, the most significant excess of pairs appears for E > 57 EeV and for a wide range of separation angles, between 9 and 22 degrees. An excess like this has a chance probability of about 2% to arise from an isotropic distribution and appears at the same energy threshold at which the Pierre Auger Observatory has reported a correlation of the arrival directions of cosmic rays with nearby astrophysical objects.

astro-ph.HE↗
arXiv2008

Kolmogorov-Smirnov test as a tool to study the distribution of ultra-high energy cosmic ray sources

We analyze in detail the two-dimensional Kolmogorov-Smirnov test as a tool to learn about the distribution of the sources of the ultra-high energy cosmic rays. We confront in particular models based on AGN observed in X rays, on galaxies observed in HI and isotropic distributions, discussing how this method can be used not only to reject isotropy but also to support or reject specific source models, extending results obtained recently in the literature.

astro-ph↗
arXiv2007

Studies of clustering in the arrival directions of cosmic rays detected at the Pierre Auger Observatory above 10 EeV

If clustering of the arrival directions of ultra high energy cosmic rays is discovered, this would provide important information about their origin, composition, and the galactic and extragalactic magnetic fields. We present here the analysis of the autocorrelation function of the data from the Pierre Auger Observatory as a function of the angular scale and the energy threshold. We compare our results with the signals found by previous experiments.

astro-ph↗
arXiv2007

B-mode polarization of the CMB from the second-order photon quadrupole

We study a new contribution to the polarization of the Cosmic Microwave Background induced at the epoch of recombination by the second-order quadrupole moment of the photon distribution. At second order in perturbation theory the quadrupole moment is not suppressed by the inverse of the optical depth in the tight coupling limit, as it happens at first order in perturbation theory. We concentrate on the B-mode CMB polarization and find that such a novel contribution constitutes a contamination in the detection of the primordial tensor modes if the tensor to scalar ratio r is smaller than a few x 10^{-5}. The magnitude of the effect is larger than the B-mode due to secondary vector/tensor perturbations and the analogous effect generated during the reionization epoch, while it is smaller than the contamination produced by the conversion of polarization of type E into type B, by weak gravitational lensing. However the lensing signal can be cleaned, making the secondary modes discussed here the actual contamination limiting the detection of small amplitude primordial gravitational waves if r is below \simeq 10^{-5}.

astro-ph↗
arXiv2006

Detecting filaments in the ultra-high energy cosmic ray distribution

We propose and test new statistical tools to study the distribution of cosmic rays based on the use of the Minimal Spanning Tree. The method described is particularly sensitive to filamentary structures, as those expected to arise from strong sources of charged cosmic rays which get deflected by intervening magnetic fields. We also test the method with data available from the AGASA and SUGAR surface detector arrays.

astro-ph↗
arXiv2005

A new method to search for a cosmic ray dipole anisotropy

We propose a new method to determine the dipole (and quadrupole) component of a distribution of cosmic ray arrival directions, which can be applied when there is partial sky coverage and/or inhomogeneous exposure. In its simplest version it requires that the exposure only depends on the declination, but it can be easily extended to the case of a small amplitude modulation in right ascension. The method essentially combines a $χ^2$ minimization of the distribution in declination to obtain the multipolar components along the North-South axis and a harmonic Rayleigh analysis for the components involving the right ascension direction.

astro-ph↗
arXiv2004

B polarization of the CMB from Faraday rotation

We study the effect of Faraday rotation due to a homogeneous magnetic field on the polarization of the cosmic microwave background (CMB). Scalar fluctuations give rise only to parity-even E-type polarization of the CMB. However in the presence of a magnetic field, a non-vanishing 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 primordial magnetic fields of present strength around $B_0=10^{-9}$G rotate E-modes into B-modes with amplitude comparable to those due to the weak gravitational lensing effect at frequencies around $ν=30$ GHz. The strength of B-modes produced by Faraday rotation scales as $B_0/ν^2$. We evaluate also the depolarizing effect of Faraday rotation upon the cross correlation between temperature anisotropy and E-type polarization.

astro-ph↗
arXiv2004

Statistics of cluster multiplicity and the nature of UHE cosmic ray sources

We study in detail the properties of clusters of ultra high energy cosmic ray events, looking in particular to their angular correlation function, to the relative frequency of clusters with different multiplicities (e.g. doublets vs. triplets or quadruplets), as well as the way in which these quantities should evolve for different ultra high energy cosmic rays source scenarios as a function of the experimental exposure achieved. We identify some useful tools which can help to characterise the nature of the cosmic ray sources in a more precise way after a modest increase in statistics will be achieved in the very near future, even before strong signals from individual sources become eventually observable.

astro-ph↗
arXiv2004

CMB polarization from secondary vector and tensor modes

We consider a novel contribution to the polarization of the Cosmic Microwave Background induced by vector and tensor modes generated by the non-linear evolution of primordial scalar perturbations. Our calculation is based on relativistic second-order perturbation theory and allows to estimate the effects of these secondary modes on the polarization angular power-spectra. We show that a non-vanishing B-mode polarization unavoidably arises from pure scalar initial perturbations, thus limiting our ability to detect the signature of primordial gravitational waves generated during inflation. This secondary effect dominates over that of primordial tensors for an inflationary tensor-to-scalar ratio $r<10^{-6}$. The magnitude of the effect is smaller than the contamination produced by the conversion of polarization of type E into type B, by weak gravitational lensing. However the lensing signal can be cleaned, making the secondary modes discussed here the actual background limiting the detection of small amplitude primordial gravitational waves.

astro-ph↗
arXiv2003

Decaying neutron propagation in the Galaxy and the Cosmic Ray anisotropy at 1 EeV

We study the cosmic ray arrival distribution expected from a source of neutrons in the galactic center at energies around 1 EeV and compare it with the anisotropy detected by AGASA and SUGAR. Besides the point-like signal in the source direction produced by the direct neutrons, an extended signal due to the protons produced in neutron decays is expected. This associated proton signal also leads to an excess in the direction of the spiral arm. For realistic models of the regular and random galactic magnetic fields, the resulting anisotropy as a function of the energy is obtained. We find that for the anisotropy to become sufficiently suppressed below E\sim 10^{17.9}eV, a significant random magnetic field component is required, while on the other hand, this also tends to increase the angular spread of the associated proton signal and to reduce the excess in the spiral arm direction. The source luminosity required in order that the right ascension anisotropy be 4% for the AGASA angular exposure corresponds to a prediction for the point-like flux from direct neutrons compatible with the flux detected by SUGAR. We also analyse the distinguishing features predicted for a large statistics southern observatory.

astro-ph↗
arXiv2003

Cosmic ray spectrum and anisotropies from the knee to the second knee

We consider the scenario in which the knee in the cosmic ray spectrum is due to a change in the escape mechanism of cosmic rays from the Galaxy from one dominated by transverse diffusion to one dominated by drifts. We show that this scenario explains not only the changes in spectral slope at the knee and at the second knee, but can also account for the main characteristics of the observed energy dependent anisotropy amplitude and phase of first harmonic in the energy range between $10^{15}$ and $10^{18}$ eV. This provides a useful handle to distinguish this diffusion/drift model from other scenarios proposed to explain the knee in the spectrum.

astro-ph↗
arXiv2002

Cosmic ray drift, the second knee and galactic anisotropies

We show that the second knee in the cosmic ray spectrum (i.e. the steepening occurring at $E\simeq 4\times 10^{17}$ eV) could be related to drift effects affecting the heaviest nuclear component, the iron group nuclei, in a scenario in which the knee at $3\times 10^{15}$ eV indicates the onset of drift effects in the lighter proton component. We also study the anisotropies resulting from diffusion and drift currents in the Galaxy, showing their potential relevance to account for the AGASA observations at $E\sim 10^{18}$ eV, before the extragalactic component becomes dominant.

astro-ph↗
arXiv2002

Astrophysical magnetic field reconstruction and spectroscopy with ultra high energy cosmic rays

The next generation of ultra high energy cosmic ray experiments will probably detect several dozens of events clustered around the direction towards each of the most powerful extragalactic sources. We develop a method which could make possible to reconstruct, from the arrival directions and energies of the clustered events, the strength and coherence properties of the magnetic field along the line of sight towards the sources. The method exploits peculiar signatures arising from magnetic lensing effects, such as the strong flux magnification of multiple images around caustics. We also discuss how to obtain information about the cosmic ray composition, and apply this method to samples of simulated data.

astro-ph↗
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↗
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↗