Searcharxiv⌕ Search

arXiv subjects

Angela V. Olinto

Publications and source records attributed to Angela V. Olinto.

At least 55 records · Page 3Linked to original sources
arXiv2012

Cosmic Rays at the highest energies

After a century of observations, we still do not know the origin of cosmic rays. I will review the current state of cosmic ray observations at the highest energies, and their implications for proposed acceleration models and secondary astroparticle fluxes. Possible sources have narrowed down with the confirmation of a GZK-like spectral feature. The anisotropy observed by the Pierre Auger Observatory may signal the dawn of particle astronomy raising hopes for high energy neutrino observations. However, composition related measurements point to a different interpretation. A clear resolution of this mystery calls for much larger statistics than the reach of current observatories.

astro-ph.HE↗
arXiv2012

Newly-born pulsars as sources of ultrahigh energy cosmic rays

Newly-born pulsars offer favorable sites for the injection of heavy nuclei, and for their further acceleration to ultrahigh energies. Once accelerated in the pulsar wind, nuclei have to escape from the surrounding supernova envelope. We examine this escape analytically and numerically, and discuss the pulsar source scenario in light of the latest ultrahigh energy cosmic ray (UHECR) data. Our calculations show that, at early times, when protons can be accelerated to energies E>10^20 eV, the young supernova shell tends to prevent their escape. In contrast, because of their higher charge, iron-peaked nuclei are still accelerated to the highest observed energies at later times, when the envelope has become thin enough to allow their escape. Ultrahigh energy iron nuclei escape newly-born pulsars with millisecond periods and dipole magnetic fields of ~10^(12-13) G, embedded in core-collapse supernovae. Due to the production of secondary nucleons, the envelope crossing leads to a transition of composition from light to heavy elements at a few EeV, as observed by the Auger Observatory. The escape also results in a softer spectral slope than that initially injected via unipolar induction, which allows for a good fit to the observed UHECR spectrum. We conclude that the acceleration of iron-peaked elements in a reasonably small fraction (< 0.01%) of extragalactic rotation-powered young pulsars would reproduce satisfactorily the current UHECR data. Possible signatures of this scenario are also discussed.

astro-ph.HE↗
arXiv2011

Ultrahigh Energy Cosmic Rays and Neutrinos

The observation of neutrinos from cosmic accelerators will be revolutionary. High energy neutrinos are closely connected to ultrahigh energy cosmic rays and their sources. Cosmic ray sources are likely to produce neutrinos and the propagation of ultrahigh cosmic rays from distant sources can generate PeV to ZeV neutrinos. We briefly review recent progress on the observations of ultrahigh energy cosmic rays and their implications for the future detections of high energy neutrinos.

astro-ph.HE↗
arXiv2011

The Astrophysics of Ultrahigh Energy Cosmic Rays

The origin of the highest energy cosmic rays is still unknown. The discovery of their sources will reveal the workings of the most energetic astrophysical accelerators in the universe. Current observations show a spectrum consistent with an origin in extragalactic astrophysical sources. Candidate sources range from the birth of compact objects to explosions related to gamma-ray bursts or to events in active galaxies. We discuss the main effects of propagation from cosmologically distant sources including interactions with cosmic background radiation and magnetic fields. We examine possible acceleration mechanisms leading to a survey of candidate sources and their signatures. New questions arise from an observed hint of sky anisotropies and an unexpected evolution of composition indicators. Future observations may reach the necessary sensitivity to achieve charged particle astronomy and to observe ultrahigh energy photons and neutrinos, which will further illuminate the workings of the universe at these extreme energies. In addition to fostering a new understanding of high-energy astrophysical phenomena, the study of ultrahigh energy cosmic rays can constrain the structure of the Galactic and extragalactic magnetic fields as well as probe particle interactions at energies orders of magnitude higher than achieved in terrestrial accelerators.

astro-ph.HE↗
arXiv2010

Cosmogenic Neutrinos: parameter space and detectabilty from PeV to ZeV

While propagating from their source to the observer, ultrahigh energy cosmic rays interact with cosmological photon backgrounds and generate to the so-called "cosmogenic neutrinos". Here we study the parameter space of the cosmogenic neutrino flux given recent cosmic ray data and updates on plausible source evolution models. The shape and normalization of the cosmogenic neutrino flux are very sensitive to some of the current unknowns of ultrahigh energy cosmic ray sources and composition. We investigate various chemical compositions and maximum proton acceleration energies E_p,max which are allowed by current observations. We consider different models of source evolution in redshift and three possible scenarios for the Galactic to extragalactic transition. We summarize the parameter space for cosmogenic neutrinos into three regions: an optimistic scenario that is currently being constrained by observations, a plausible range of models in which we base many of our rate estimates, and a pessimistic scenario that will postpone detection for decades to come. We present the implications of these three scenarios for the detection of cosmogenic neutrinos from PeV to ZeV (10^14-21 eV) with the existing and upcoming instruments. In the plausible range of parameters, the narrow flux variability in the EeV energy region assures low but detectable rates for IceCube (0.06-0.2 neutrino per year) and the Pierre Auger Observatory (0.03-0.06 neutrino per year), and detection should happen in the next decade. If EeV neutrinos are detected, PeV information can help select between competing models of cosmic ray composition at the highest energy and the Galactic to extragalactic transition at ankle energies. With improved sensitivity, ZeV neutrino observatories, such as ANITA and JEM-EUSO could explore and place limits on the maximum acceleration energy.

astro-ph.HE↗
arXiv2006

Small Scale Anisotropy Predictions for the Auger Observatory

We study the small scale anisotropy signal expected at the Pierre Auger Observatory in the next 1, 5, 10, and 15 years of operation, from sources of ultra-high energy (UHE) protons. We numerically propagate UHE protons over cosmological distances using an injection spectrum and normalization that fits current data up to $\sim 10^{20}\eV$. We characterize possible sources of ultra-high energy cosmic rays (UHECRs) by their mean density in the local Universe, $\barρ = 10^{-r}$ Mpc$^{-3}$, with $r$ between 3 and 6. These densities span a wide range of extragalactic sites for UHECR sources, from common to rare galaxies or even clusters of galaxies. We simulate 100 realizations for each model and calculate the two point correlation function for events with energies above $4 \times 10^{19}\eV$ and above $10^{20}\eV$, as specialized to the case of the Auger telescope. We find that for $r\ga 4$, Auger should be able to detect small scale anisotropies in the near future. Distinguishing between different source densities based on cosmic ray data alone will be more challenging than detecting a departure from isotropy and is likely to require larger statistics of events. Combining the angular distribution studies with the spectral shape around the GZK feature will also help distinguish between different source scenarios.

astro-ph↗
arXiv2005

A closer look at the spectrum and small scale anisotropies of UHECRs

We present results of numerical simulations of the propagation of ultra high energy cosmic rays (UHECRs) over cosmological distances, aimed at quantifying the statistical significance of the highest energy data on the spectrum and small scale anisotropies as detected by the AGASA experiment. We assess the significance of the lack of a GZK feature and its compatibility with the reported small scale anisotropies. Assuming that UHECRs are protons from extragalactic sources, we find that the small scale anisotropies are incompatible with the reported spectrum at a probability level of $2 \times 10^{-5}$. Our analysis of the AGASA results shows the power of combining spectrum and small scale anisotropy data in future high statistics experiments, such as Auger.

astro-ph↗
arXiv2004

Neutralino Annihilation at the Galactic Center Revisited

The annihilation of neutralino dark matter in the Galactic Center (GC) may result in radio signals that can be used to detect or constrain the dark matter halo density profile or dark matter particle properties. At the Galactic Center, the accretion flow onto the central Black Hole (BH) sustains strong magnetic fields that can induce synchrotron emission by electrons and positrons generated in neutralino annihilations during advection onto the BH. Here we reanalyze the radiative processes relevant for the neutralino annihilation signal at the GC, with realistic assumptions about the accretion flow and its magnetic properties. We find that neglecting these effects, as done in previous papers, leads to the incorrent electron and photon spectra. We find that the magnetic fields associated with the flow are significantly stronger than previously estimated. We derive the appropriate equilibrium distribution of electrons and positron and the resulting radiation, considering adiabatic compression in the accretion flow, inverse Compton scattering off synchrotron photons (synchrotron self-Compton scattering), and synchrotron self-absorption of the emitted radiation. We derive the signal for a Navarro-Frenk-White (NFW) dark matter halo profile and a NFW profile with a dark matter spike due to the central BH. We find that the observed radio emission from the GC is inconsistent with the scenario in which a spiky distribution of neutralinos is present. We discuss several important differences between our calculations and those previously presented in the literature.

astro-ph↗
arXiv2004

Gamma-ray and synchrotron emission from neutralino annihilation in the Large Magellanic Cloud

We calculate the expected flux of gamma-ray and radio emission from the LMC due to neutralino annihilation. Using rotation curve data to probe the density profile and assuming a minimum disk, we describe the dark matter halo of the LMC using models predicted by N-body simulations. We consider a range of density profiles including the NFW profile, a modified NFW profile proposed by Hayashi et al.(2003) to account for the effects of tidal stripping, and an isothermal sphere with a core. We find that the gamma-ray flux expected from these models may be detectable by GLAST for a significant part of the neutralino parameter space. The prospects for existing and upcoming Atmospheric Cherenkov Telescopes are less optimistic, as unrealistically long exposures are required for detection. However, the effects of adiabatic compression due to the baryonic component may improve the chances for detection by ACTs. The maximum flux we predict is well below EGRET's measurements and thus EGRET does not constrain the parameter space. The expected synchrotron emission generally lies below the observed radio emission from the LMC in the frequency range of 19.7 to 8550 MHz. As long as <2x 10^-26 cm^3 s^-1 for a neutralino mass of 50 GeV, the observed radio emission is not primarily due to neutralinos and is consistent with the assumption that the main source is cosmic rays. We find that the predicted fluxes, obtained by integrating over the entire LMC, are not very strongly dependent on the inner slope of the halo profile, varying by less than an order of magnitude for the range of profiles we considered.

astro-ph↗
arXiv2004

Rapporteur talk for Ultra High Energy Cosmic Rays (HE 1.3, 1.4, 1.5): Messengers of the Extreme Universe

In this report I summarize contributions on the highest energy cosmic rays to the 28th International Cosmic Ray Conference in Tsukuba, Japan, Sections HE 1.3, 1.4 and 1.5 involved over 80 oral presentations and about 120 posters. This large body of work attests to the vitality of a field committed to resolving the mystery of the origin of ultra-high energy cosmic rays. These cosmic rays with energies well above $10^{18}$ eV are messengers of an unknown extremely high-energy universe.

astro-ph↗
arXiv2003

TeV black hole fragmentation and detectability in extensive air showers

In models with large extra dimensions, particle collisions with center-of-mass energy larger than the fundamental gravitational scale can generate nonperturbative gravitational objects. Since cosmic rays have been observed with energies above $10^{8}$ TeV, gravitational effects in the TeV energy range can, in principle, be observed by ultrahigh energy cosmic ray detectors. We consider the interaction of ultrahigh energy neutrinos in the atmosphere and compare extensive air showers from TeV black hole formation and fragmentation with standard model processes. Departures from the standard model predictions arise in the interaction cross sections and in the multiplicity of secondary particles. Large theoretical uncertainties in the black hole cross section weaken attempts to constrain TeV gravity based solely on differences between predicted and observed event rates. The large multiplicity of secondaries in black hole fragmentation enhances the detectability of TeV gravity effects. We simulate TeV black hole air showers using PYTHIA and AIRES, and find that black hole-induced air showers are quite distinct from standard model air showers. However, the limited amount of information registered by realistic detectors together with large air shower fluctuations limit in practice the ability to distinguish TeV gravity events from standard model events in a shower by shower case. We discuss possible strategies to optimize the detectability of black hole events and propose a few unique signatures that may allow future high statistics detectors to separate black hole from standard model events.

hep-ph↗
arXiv2003

Probing TeV gravity with extensive air-showers

Particle collisions with center-of-mass energy larger than the fundamental gravitational scale can generate non perturbative gravitational objects such as black holes and branes. In models with large extra dimensions, the fundamental gravitational scale may be around a TeV, making it possible for next generation particle colliders and ultra-high energy cosmic rays to produce such non perturbative gravitational objects. The decay of TeV gravitational objects is significantly different from standard model processes such that probes of these new ideas are within reach. We study the differences between standard model and TeV gravity interactions in extensive air showers (EAS) generated by ultra-high energy cosmic neutrinos. We show that discriminating TeV gravity from standard model interactions is generally difficult, but not impossible given a few unique signatures.

astro-ph↗
arXiv2003

The GZK Feature in the Spectrum of UHECRs: What is it Telling Us?

We developed a numerical simulation of the propagation of UHECR in the Cosmic Microwave Background (CMB) and we used it to determine the significance of the GZK feature in the spectrum of UHECR measured by AGASA and HiRes. We find that these two experiments are best fit by two different injection spectra in the region below 10^20 eV and that the error bars around the GZK feature are dominated by fluctuations which leave a determination of the GZK feature not attainable at present. In addition the comparison of the spectra of AGASA and HiRes suggests the presence of about a 30% systematic errors in the relative energy determination of the two experiments. Correcting for these systematics, the two experiments are brought in agreement at energies below 10^20 eV and, in this region, are best fit by an injection spectrum with spectral index 2.5-2.6. In the high energy region (above 10^20 eV) the two experiments maintain their disagreement, but only at the 2sigma level. Our results clearly show the need for much larger experiments such as Auger, EUSO, and OWL, that can increase the number of detected events by one or two orders of magnitude.

astro-ph↗
arXiv2003

On the statistical significance of the GZK feature in the spectrum of ultra high energy cosmic rays

The nature of the unknown sources of ultra-high energy cosmic rays can be revealed through the detection of the GZK feature in the cosmic ray spectrum, resulting from the production of pions by ultra-high energy protons scattering off the cosmic microwave background. Here we show that the GZK feature cannot be accurately determined with the small sample of events with energies $\sim 10^{20}$ eV detected thus far by the largest two experiments, AGASA and HiRes. With the help of numerical simulations for the propagation of cosmic rays, we find the error bars around the GZK feature are dominated by fluctuations which leave a determination of the GZK feature unattainable at present. In addition, differing results from AGASA and HiRes suggest the presence of $\sim 30%$ systematic errors that may be due to discrepancies in the relative energy determination of the two experiments. Correcting for these systematics, the two experiments are brought into agreement at energies below $\sim 10^{20}$ eV. After simulating the GZK feature for many realizations and different injection spectra, we determine the best fit injection spectrum required to explain the observed spectra at energies above $10^{18.5}$ eV. We show that the discrepancy between the two experiments at the highest energies has low statistical significance (at the 2 $σ$ level) and that the corrected spectra are best fit by an injection spectrum with spectral index $\sim 2.6$. Our results clearly show the need for much larger experiments such as Auger, EUSO, and OWL, that can increase the number of detected events by 2 orders of magnitude. Only large statistics experiments can finally prove or disprove the existence of the GZK feature in the cosmic ray spectrum.

astro-ph↗
arXiv2003

Low Statistics of EHECRs

The nature of the unknown sources of ultra-high energy cosmic rays can be revealed through the detection of the GZK feature in the cosmic ray spectrum. The only two experiments that have probed this energy range, AGASA and HiRes, have apparently conflicting results. HiRes measured a flux consistent with the GZK feature while AGASA reported a larger than expected flux of so-called Super-GZK particles. Here we emphasize that neither experiment has gathered the statistics necessary for making a definitive measurement of the GZK cutoff. The photo-pion production responsible for the GZK feature is stochastic for energies around the cutoff leading to large fluctuations of the spectrum for low statistics measurements. We show that the results from AGASA and HiRes results are within about $ 2 σ$ of one another by simulating 400 spectra for a range of input spectral indices normalized to the number of events above $10^{19}$ eV for each experiment. If a 15% systematic correction in energy is applied to both experiments, the agreement between the experiments improves considerably and the best fit input spectral index becomes $\sim 2.6$ for both data sets. Our results clearly show the need for much larger experiments such as Auger and EUSO, that can increase the number of detected events by 2 orders of magnitude. Only large statistics experiments can finally prove or disprove the existence of the GZK feature in the cosmic ray spectrum.

astro-ph↗
arXiv2002

Brane factories

We propose that higher-dimensional extended objects (p-branes) are created by super-Planckian scattering processes in theories with TeV scale gravity. As an example, we compute the cross section for p-brane creation in a (n+4)-dimensional spacetime with asymmetric compactification. We find that the cross section for the formation of a brane which is wounded on a compact submanifold of size of the fundamental gravitational scale is larger than the cross section for the creation of a spherically symmetric black hole. Therefore, we predict that branes are more likely to be created than black holes in super-Planckian scattering processes in these manifolds. The higher rate of p-brane production has important phenomenological consequences, as it significantly enhances possible detection of non-perturbative gravitational events in future hadron colliders and cosmic rays detectors.

hep-th↗
arXiv2002

The Future of Ultra High Energy Cosmic Rays

Contrary to earlier expectations, several cosmic ray events with energies above $10^{20}$ eV have been reported by a number of ultra-high energy cosmic ray observatories. According to the AGASA experiment, the flux of such events is well above the predicted Greisen-Zatsepin-Kuzmin cutoff due to the pion production of extragalactic cosmic ray protons off the cosmic microwave background. In addition to the relatively high flux of events, the isotropic distribution of arrival directions and an indication of small scale clustering strongly challenge all models proposed to resolve this puzzle. We discuss how the GZK cutoff is modified by the local distribution of galaxies and how astrophysical proton sources with soft injection spectra are ruled out by AGASA data. Sources with hard injection spectrum are barely allowed by the observed spectrum. If the most recent claims by AGASA that the highest energy events are due to clustered nuclei are confirmed, the most plausible explanation are astrophysical sources with very hard spectra such as extragalactic unipolar inductors. In addition, extragalactic magnetic fields need to be well below the current nano-Gauss upper limits. Alternatively, if the primaries are not nuclei, the need for new physics explanations is paramount. We present an overview of the theoretical proposals along with their most general signatures to be tested by upcoming experiments.

astro-ph↗