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E. Bernardini

Publications and source records attributed to E. Bernardini.

At least 397 records · Page 22Linked to original sources
arXiv2013

Improvement in Fast Particle Track Reconstruction with Robust Statistics

The IceCube project has transformed one cubic kilometer of deep natural Antarctic ice into a Cherenkov detector. Muon neutrinos are detected and their direction inferred by mapping the light produced by the secondary muon track inside the volume instrumented with photomultipliers. Reconstructing the muon track from the observed light is challenging due to noise, light scattering in the ice medium, and the possibility of simultaneously having multiple muons inside the detector, resulting from the large flux of cosmic ray muons. This manuscript describes work on two problems: (1) the track reconstruction problem, in which, given a set of observations, the goal is to recover the track of a muon; and (2) the coincident event problem, which is to determine how many muons are active in the detector during a time window. Rather than solving these problems by developing more complex physical models that are applied at later stages of the analysis, our approach is to augment the detectors early reconstruction with data filters and robust statistical techniques. These can be implemented at the level of on-line reconstruction and, therefore, improve all subsequent reconstructions. Using the metric of median angular resolution, a standard metric for track reconstruction, we improve the accuracy in the initial reconstruction direction by 13%. We also present improvements in measuring the number of muons in coincident events: we can accurately determine the number of muons 98% of the time.

astro-ph.IM↗
arXiv2013

Search for time-independent neutrino emission from astrophysical sources with 3 years of IceCube data

We present the results of a search for neutrino point sources using the IceCube data collected between April 2008 and May 2011 with three partially completed configurations of the detector: the 40-, 59- and 79-string configurations. The live-time of this data set are 1,040 days. An unbinned maximum likelihood ratio test was used to search for an excess of neutrinos above the atmospheric background at any given direction in the sky. By adding two more years of data with improved event selection and reconstruction techniques, the sensitivity was improved by a factor 3.5 or more with respect to the previously published results obtained with the 40-string configuration of IceCube. We performed an all-sky survey and a dedicated search using a catalog of \textit{a priori} selected objects observed by other telescopes. In both searches, the data are compatible with the background-only hypothesis. In the absence of evidence for a signal, we set upper limits on the flux of muon neutrinos. For an E$^{-2}$ neutrino spectrum, the observed limits are between 0.9 and $23.2\times 10^{-12}$ TeV$^{-1}$ cm$^{-2}$s$^{-1}$. We also report upper limits for neutrino emission from groups of sources which were selected according to theoretical models or observational parameters and analysed with a stacking approach.

astro-ph.HE↗
arXiv2013

First observation of PeV-energy neutrinos with IceCube

We report on the observation of two neutrino-induced events which have an estimated deposited energy in the IceCube detector of 1.04 $\pm$ 0.16 and 1.14 $\pm$ 0.17 PeV, respectively, the highest neutrino energies observed so far. These events are consistent with fully contained particle showers induced by neutral-current $ν_{e,μ,τ}$ ($\barν_{e,μ,τ}$) or charged-current $ν_{e}$ ($\barν_{e}$) interactions within the IceCube detector. The events were discovered in a search for ultra-high energy neutrinos using data corresponding to 615.9 days effective livetime. The expected number of atmospheric background is $0.082 \pm 0.004 \text{(stat)}^{+0.041}_{-0.057} \text{(syst)}$. The probability to observe two or more candidate events under the atmospheric background-only hypothesis is $2.9\times10^{-3}$ ($2.8σ$) taking into account the uncertainty on the expected number of background events. These two events could be a first indication of an astrophysical neutrino flux, the moderate significance, however, does not permit a definitive conclusion at this time.

astro-ph.HE↗
arXiv2013

The Simultaneous Low State Spectral Energy Distribution of 1ES 2344+514 from Radio to Very High Energies

[Abridged] Context. To construct and interpret the spectral energy distribution (SED) of BL Lacertae objects, simultaneous broad-band observations are mandatory. Aims. We present the results of a dedicated multi-wavelength study of the high-frequency peaked BL Lacertae (HBL) object and known TeV emitter 1ES 2344+514 by means of a pre-organised campaign. Methods. The observations were conducted during simultaneous visibility windows of MAGIC and AGILE in late 2008. The measurements were complemented by Metsähovi, RATAN-600, KVA+Tuorla, Swift and VLBA pointings. Additional coverage was provided by the ongoing long-term F-GAMMA and MOJAVE programs, the OVRO 40-m and CrAO telescopes as well as the Fermi satellite. The obtained SEDs are modelled using a one-zone as well as a self-consistent two-zone synchrotron self-Compton model. Results. 1ES 2344+514 was found at very low flux states in both X-rays and very high energy gamma rays. Variability was detected in the low frequency radio and X-ray bands only, where for the latter a small flare was observed. The X-ray flare was possibly caused by shock acceleration characterised by similar cooling and acceleration time scales. MOJAVE VLBA monitoring reveals a static jet whose components are stable over time scales of eleven years, contrary to previous findings. There appears to be no significant correlation between the 15 GHz and R-band monitoring light curves. The observations presented here constitute the first multi-wavelength campaign on 1ES 2344+514 from radio to VHE energies and one of the few simultaneous SEDs during low activity states. The quasi-simultaneous Fermi-LAT data poses some challenges for SED modelling, but in general the SEDs are described well by both applied models. The resulting parameters are typical for TeV emitting HBLs. Consequently it remains unclear whether a so-called quiescent state was found in this campaign.

astro-ph.HE↗
arXiv2013

Observation of the cosmic-ray shadow of the Moon with IceCube

We report on the observation of a significant deficit of cosmic rays from the direction of the Moon with the IceCube detector. The study of this "Moon shadow" is used to characterize the angular resolution and absolute pointing capabilities of the detector. The detection is based on data taken in two periods before the completion of the detector: between April 2008 and May 2009, when IceCube operated in a partial configuration with 40 detector strings deployed in the South Pole ice, and between May 2009 and May 2010 when the detector operated with 59 strings. Using two independent analysis methods, the Moon shadow has been observed to high significance (> 6 sigma) in both detector configurations. The observed location of the shadow center is within 0.2 degrees of its expected position when geomagnetic deflection effects are taken into account. This measurement validates the directional reconstruction capabilities of IceCube.

astro-ph.HE↗
arXiv2013

An Absence of Neutrinos Associated with Cosmic Ray Acceleration in Gamma-Ray Bursts

Gamma-Ray Bursts (GRBs) have been proposed as a leading candidate for acceleration of ultra high-energy cosmic rays, which would be accompanied by emission of TeV neutrinos produced in proton-photon interactions during acceleration in the GRB fireball. Two analyses using data from two years of the IceCube detector produced no evidence for this neutrino emission, placing strong constraints on models of neutrino and cosmic-ray production in these sources.

astro-ph.HE↗
arXiv2013

Search for Neutrinos from Annihilating Dark Matter in the Direction of the Galactic Center with the 40-String IceCube Neutrino Observatory

A search for muon neutrinos from dark matter annihilations in the Galactic Center region has been performed with the 40-string configuration of the IceCube Neutrino Observatory using data collected in 367 days of live-time starting in April 2008. The observed fluxes were consistent with the atmospheric background expectations. Upper limits on the self-annihilation cross-section are obtained for dark matter particle masses ranging from 100 GeV to 10 TeV. In the case of decaying dark matter, lower limits on the lifetime have been determined for masses between 200 GeV and 20 TeV.

hep-ex↗
arXiv2013

Measurement of the Atmospheric $ν_e$ flux in IceCube

We report the first measurement of the atmospheric electron neutrino flux in the energy range between approximately 80 GeV and 6 TeV, using data recorded during the first year of operation of IceCube's DeepCore low energy extension. Techniques to identify neutrinos interacting within the DeepCore volume and veto muons originating outside the detector are demonstrated. A sample of 1029 events is observed in 281 days of data, of which 496 $\pm$ 66(stat.) $\pm$ 88(syst.) are estimated to be cascade events, including both electron neutrino and neutral current events. The rest of the sample includes residual backgrounds due to atmospheric muons and charged current interactions of atmospheric muon neutrinos. The flux of the atmospheric electron neutrinos is consistent with models of atmospheric neutrinos in this energy range. This constitutes the first observation of electron neutrinos and neutral current interactions in a very large volume neutrino telescope optimized for the TeV energy range.

hep-ex↗
arXiv2013

Discovery of very high energy gamma-ray emission from the blazar 1ES 1727+502 with the MAGIC Telescopes

Motivated by the Costamante & Ghisellini (2002) predictions we investigated if the blazar 1ES 1727+502 (z=0.055) is emitting very high energy (VHE, E>100 GeV) gamma rays. We observed the BL Lac object 1ES 1727+502 in stereoscopic mode with the two MAGIC telescopes during 14 nights between May 6th and June 10th 2011, for a total effective observing time of 12.6 hours. For the study of the multiwavelength spectral energy distribution (SED) we use simultaneous optical R-band data from the KVA telescope, archival UV/optical and X-ray observations by instruments UVOT and XRT on board of the Swift satellite and high energy (HE, 0.1 GeV - 100 GeV) gamma-ray data from the Fermi-LAT instrument. We detect, for the first time, VHE gamma-ray emission from 1ES 1727+502 at a statistical significance of 5.5 sigma. The integral flux above 150 GeV is estimated to be (2.1\pm0.4)% of the Crab Nebula flux and the de-absorbed VHE spectrum has a photon index of (2.7\pm0.5). No significant short-term variability was found in any of the wavebands presented here. We model the SED using a one-zone synchrotron self-Compton model obtaining parameters typical for this class of sources.

astro-ph.HE↗
arXiv2013

Measurement of South Pole ice transparency with the IceCube LED calibration system

The IceCube Neutrino Observatory, approximately 1 km^3 in size, is now complete with 86 strings deployed in the Antarctic ice. IceCube detects the Cherenkov radiation emitted by charged particles passing through or created in the ice. To realize the full potential of the detector, the properties of light propagation in the ice in and around the detector must be well understood. This report presents a new method of fitting the model of light propagation in the ice to a data set of in-situ light source events collected with IceCube. The resulting set of derived parameters, namely the measured values of scattering and absorption coefficients vs. depth, is presented and a comparison of IceCube data with simulations based on the new model is shown.

astro-ph.IM↗
arXiv2013

Searches for high-energy neutrino emission in the Galaxy with the combined IceCube-AMANDA detector

We report on searches for neutrino sources at energies above 200 GeV in the Northern sky of the galactic plane, using the data collected by the South Pole neutrino telescopes IceCube and AMANDA. The galactic region considered here includes the Local Arm towards the Cygnus region and our closest approach to the Perseus Arm. The data have been collected between 2007 and 2009 when AMANDA was an integrated part of IceCube, which was still under construction and operated with 22-strings (2007-8) and 40-strings (2008-9) of optical modules deployed in the ice. By combining the larger IceCube detector with the lower energy threshold of the more compact AMANDA detector, we obtain an improved sensitivity at energies below $\sim$10 TeV with respect to previous searches. The analyses presented here are: a scan for point sources within the galactic plane; a search optimized for multiple and extended sources in the Cygnus region, which might be below the sensitivity of the point source scan; and studies of seven pre-selected neutrino source candidates. For one of them, Cygnus X-3, a time-dependent search for neutrinos in coincidence with observed radio and X-ray flares has been performed. No evidence of a signal is found, and upper limits are reported for each of the searches. We investigate neutrino spectra proportional to E$^{-2}$ and E$^{-3}$ to cover the entire range of possible spectra. The soft E$^{-3}$ spectrum results in an energy distribution similar to a source with cut-off below $\sim$50 TeV. For the considered region of the galactic plane, the 90% confidence level muon neutrino flux upper limits are in the range E$^3$dN/dE$\sim 5.4 - 19.5 \times 10^{-11} \rm{TeV^{2} cm^{-2} s^{-1}}$ for point-like neutrino sources in the energy region [180.0 GeV - 20.5 TeV]. These represent the most stringent upper limits for soft-spectra neutrino sources within the Galaxy reported to date.

astro-ph.HE↗
arXiv2012

Observation of Cosmic Ray Anisotropy with the IceTop Air Shower Array

We report on the observation of anisotropy in the arrival direction distribution of cosmic rays at PeV energies. The analysis is based on data taken between 2009 and 2012 with the IceTop air shower array at the South Pole. IceTop, an integral part of the IceCube detector, is sensitive to cosmic rays between 100 TeV and 1 EeV. With the current size of the IceTop data set, searches for anisotropy at the 10^-3 level can, for the first time, be extended to PeV energies. We divide the data set into two parts with median energies of 400 TeV and 2 PeV, respectively. In the low energy band, we observe a strong deficit with an angular size of about 30 degrees and an amplitude of (-1.58 +/- 0.46 (stat) +/- 0.52 (sys)) x 10^(-3) at a location consistent with previous observations of cosmic rays with the IceCube neutrino detector. The study of the high energy band shows that the anisotropy persists to PeV energies and increases in amplitude to (-3.11 +/- 0.38 (stat) +/- 0.96 (sys)) x 10^(-3).

astro-ph.HE↗
arXiv2012

Lateral Distribution of Muons in IceCube Cosmic Ray Events

In cosmic ray air showers, the muon lateral separation from the center of the shower is a measure of the transverse momentum that the muon parent acquired in the cosmic ray interaction. IceCube has observed cosmic ray interactions that produce muons laterally separated by up to 400 m from the shower core, a factor of 6 larger distance than previous measurements. These muons originate in high pT (> 2 GeV/c) interactions from the incident cosmic ray, or high-energy secondary interactions. The separation distribution shows a transition to a power law at large values, indicating the presence of a hard pT component that can be described by perturbative quantum chromodynamics. However, the rates and the zenith angle distributions of these events are not well reproduced with the cosmic ray models tested here, even those that include charm interactions. This discrepancy may be explained by a larger fraction of kaons and charmed particles than is currently incorporated in the simulations.

astro-ph.HE↗