Las Palmas de Gran Canaria, Spain
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Ahnen M.L.,ETH Zurich | Ansoldi S.,University of Udine | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | And 152 more authors.
Astronomy and Astrophysics | Year: 2016

During February-March 2014, the MAGIC telescopes observed the high-frequency peaked BL Lac 1ES 1011+496 (z = 0:212) in flaring state at very-high energy (VHE, E > 100 GeV). The flux reached a level of more than ten times higher than any previously recorded flaring state of the source. Aims. To describe the characteristics of the flare presenting the light curve and the spectral parameters of the night-wise spectra and the average spectrum of the whole period. From these data we aim to detect the imprint of the extragalactic background light (EBL) in the VHE spectrum of the source, to constrain its intensity in the optical band. Methods. We analyzed the gamma-ray data from the MAGIC telescopes using the standard MAGIC software for the production of the light curve and the spectra. To constrain the EBL, we implement the method developed by the H.E.S.S. collaboration, in which the intrinsic energy spectrum of the source is modeled with a simple function (≤ 4 parameters), and the EBL-induced optical depth is calculated using a template EBL model. The likelihood of the observed spectrum is then maximized, including a normalization factor for the EBL opacity among the free parameters. Results. The collected data allowed us to describe the night-wise flux changes and also to produce differential energy spectra for all nights in the observed period. The estimated intrinsic spectra of all the nights could be fitted by power-law functions. Evaluating the changes in the fit parameters, we conclude that the spectral shape for most of the nights were compatible, regardless of the flux level, which enabled us to produce an average spectrum from which the EBL imprint could be constrained. The likelihood ratio test shows that the model with an EBL density 1.07 (-0.20, +0.24)stat+sys, relative to the one in the tested EBL template, is preferred at the 4.6σ level to the no-EBL hypothesis, with the assumption that the intrinsic source spectrum can be modeled as a log-parabola. This would translate into a constraint of the EBL density in the wavelength range [0.24 μm, 4.25 μm], with a peak value at 1.4 μm of λFλ = 12.27+2.75 -2:29 nWm-2 sr-1, including systematics. © 2016 ESO.

Adrian-Martinez S.,Polytechnic University of Valencia | Albert A.,Upper Alsace University | Andre M.,University of Barcelona | Anton G.,Friedrich - Alexander - University, Erlangen - Nuremberg | And 193 more authors.
Astronomy and Astrophysics | Year: 2015

Context. The source(s) of the neutrino excess reported by the IceCube Collaboration is unknown. The TANAMI Collaboration recently reported on the multiwavelength emission of six bright, variable blazars which are positionally coincident with two of the most energetic IceCube events. Objects like these are prime candidates to be the source of the highest-energy cosmic rays, and thus of associated neutrino emission. Aims. We present an analysis of neutrino emission from the six blazars using observations with the ANTARES neutrino telescope. Methods. The standard methods of the ANTARES candidate list search are applied to six years of data to search for an excess of muons - and hence their neutrino progenitors - from the directions of the six blazars described by the TANAMI Collaboration, and which are possibly associated with two IceCube events. Monte Carlo simulations of the detector response to both signal and background particle fluxes are used to estimate the sensitivity of this analysis for different possible source neutrino spectra. A maximum-likelihood approach, using the reconstructed energies and arrival directions of through-going muons, is used to identify events with properties consistent with a blazar origin. Results. Both blazars predicted to be the most neutrino-bright in the TANAMI sample (1653-329 and 1714-336) have a signal flux fitted by the likelihood analysis corresponding to approximately one event. This observation is consistent with the blazar-origin hypothesis of the IceCube event IC14 for a broad range of blazar spectra, although an atmospheric origin cannot be excluded. No ANTARES events are observed from any of the other four blazars, including the three associated with IceCube event IC20. This excludes at a 90% confidence level the possibility that this event was produced by these blazars unless the neutrino spectrum is flatter than -2.4. © ESO, 2015.

King M.A.,Northumbria University | Altamimi Z.,CNRS Pascal Institute | Boehm J.,Vienna University of Technology | Bos M.,University of Porto | And 14 more authors.
Surveys in Geophysics | Year: 2010

The provision of accurate models of Glacial Isostatic Adjustment (GIA) is presently a priority need in climate studies, largely due to the potential of the Gravity Recovery and Climate Experiment (GRACE) data to be used to determine accurate and continent-wide assessments of ice mass change and hydrology. However, modelled GIA is uncertain due to insufficient constraints on our knowledge of past glacial changes and to large simplifications in the underlying Earth models. Consequently, we show differences between models that exceed several mm/year in terms of surface displacement for the two major ice sheets: Greenland and Antarctica. Geodetic measurements of surface displacement offer the potential for new constraints to be made on GIA models, especially when they are used to improve structural features of the Earth's interior as to allow for a more realistic reconstruction of the glaciation history. We present the distribution of presently available campaign and continuous geodetic measurements in Greenland and Antarctica and summarise surface velocities published to date, showing substantial disagreement between techniques and GIA models alike. We review the current state-of-the-art in ground-based geodesy (GPS, VLBI, DORIS, SLR) in determining accurate and precise surface velocities. In particular, we focus on known areas of need in GPS observation level models and the terrestrial reference frame in order to advance geodetic observation precision/accuracy toward 0.1 mm/year and therefore further constrain models of GIA and subsequent present-day ice mass change estimates. © 2010 The Author(s).

Juan J.D.,Institute for Space science | Elosegui P.,Institute for Space science | Nettles M.,Lamont Doherty Earth Observatory | Larsen T.B.,Geological Survey of Denmark | And 9 more authors.
Geophysical Research Letters | Year: 2010

Large calving events at Greenlands largest outlet glaciers are associated with glacial earthquakes and nearinstantaneous increases in glacier flow speed. At some glaciers and ice streams, flow is also modulated in a regular way by ocean tidal forcing at the terminus. At Helheim Glacier, analysis of geodetic data shows decimeter-level periodic position variations in response to tidal forcing. However, we also observe transient increases of more than 100% in the glaciers responsiveness to such tidal forcing following glacial-earthquake calving events. The timing and amplitude of the changes correlate strongly with the step-like increases in glacier speed and longitudinal strain rate associated with glacial earthquakes. The enhanced response to the ocean tides may be explained by a temporary disruption of the subglacial drainage system and a concomitant reduction of the friction at the ice-bedrock interface, and suggests a new means by which geodetic data may be used to infer glacier properties. Copyright 2010 by the American Geophysical Union.

Andersen M.L.,Geological Survey of Denmark | Andersen M.L.,Copenhagen University | Larsen T.B.,Geological Survey of Denmark | Nettles M.,Lamont Doherty Earth Observatory | And 12 more authors.
Journal of Geophysical Research: Earth Surface | Year: 2010

Understanding the behavior of large outlet glaciers draining the Greenland Ice Sheet is critical for assessing the impact of climate change on sea level rise. The flow of marine-terminating outlet glaciers is partly governed by calving-related processes taking place at the terminus but is also influenced by the drainage of surface runoff to the bed through moulins, cracks, and other pathways. To investigate the extent of the latter effect, we develop a distributed surface-energy-balance model for Helheim Glacier, East Greenland, to calculate surface melt and thereby estimate runoff. The model is driven by data from an automatic weather station operated on the glacier during the summers of 2007 and 2008, and calibrated with independent measurements of ablation. Modeled melt varies over the deployment period by as much as 68% relative to the mean, with melt rates approximately 77% higher on the lower reaches of the glacier trunk than on the upper glacier. We compare melt variations during the summer season to estimates of surface velocity derived from global positioning system surveys. Near the front of the glacier, there is a significant correlation (on >95% levels) between variations in runoff (estimated from surface melt) and variations in velocity, with a 1 day delay in velocity relative to melt. Although the velocity changes are small compared to accelerations previously observed following some calving events, our findings suggest that the flow speed of Helheim Glacier is sensitive to changes in runoff. The response is most significant in the heavily crevassed, fast-moving region near the calving front. The delay in the peak of the cross-correlation function implies a transit time of 12-36 h for surface runoff to reach the bed. Copyright 2010 by the American Geophysical Union.

Perea L.,Institute Destudis Espacials Of Catalonia | Perea L.,Institute for Space science | Elosegui P.,German Aerospace Center | Elosegui P.,Institute for Space science | And 2 more authors.
Journal of Guidance, Control, and Dynamics | Year: 2010

The design and validation of a relative control law of a virtual telescope using global positioning system (GPS) and optical metrology for formation flying (FF) acquisition mission is studied. The relative control is used for formation acquisition, whose accuracy requirements are more relaxed than those of formation keeping, and a slight modification is introduced to improve the accuracy. The results show that between epoch and 0.024 orbits-time units, the quality of the satellite geometry significantly reduces, and the Kalman navigation filter responds by implementing only the time update step. The position error of the coronagraph relative to its reference position vary depending on maneuvers. The control action during the formation keeping phase contains high-frequency variations. Simulations reveal that the high-frequency oscillations arise from errors in relative velocity estimates, and that the oscillations vanished in the absence of velocity errors.

Elizalde E.,Institute for Space Science | Skalozub V.,Dnipropetrovsk National University
Problems of Atomic Science and Technology | Year: 2012

It is assumed that the intergalactic magnetic fields were spontaneously generated in the early Universe due to vacuum polarization of non-Abelian gauge fields at high temperature T. Here, a procedure to estimate the field strengths B(T) at different T is developed and the value of B(T ew)∼10 14 G, at the electroweak phase transition temperature, is derived by taking into consideration the present intergalactic magnetic field strength B o ∼ 10 -15 G.

Babkin V.A.,Joint Institute for Nuclear Research | Cata-Danil G.,Polytechnic University of Bucharest | Cata-Danil G.,National Institute for Physics and Nuclear Engineering | Golovatyuk V.M.,Joint Institute for Nuclear Research | And 4 more authors.
Central European Journal of Physics | Year: 2010

A ridge carbon beam filter for Spreading Out the Bragg Peak (SOBP) up to 20 mm in the targeted area has been designed and experimentally tested. The instrument was constructed of Plexiglas with a density of 1. 165 g/cm3 and has a "teeth" structure with a period of 2. 5 mm. The design is based on a simple analytical model and detailed GEANT4 Monte Carlo simulations. In-beam test measurements were performed with a 500-MeV/u 12C beam at the Nuclotron facility of the Joint Institute for Nuclear Research (JINR) in Dubna. The filter can be used as a static device positioned in front of patients undergoing carbon therapy. © 2009 Versita Warsaw and Springer-Verlag Berlin Heidelberg.

Elizalde E.,Institute for Space Science | Skalozub V.,Dnipropetrovsk National University
European Physical Journal C | Year: 2012

Intergalactic magnetic fields are assumed to have been spontaneously generated at the reheating stage of the early Universe, due to vacuum polarization of non-Abelian gauge fields at high temperature. The fact that the screening mass of this type of fields has zero value was discovered recently. A procedure to estimate their field strengths, B(T), at different temperatures is here developed, and the value B(T ew)~10 14 G at the electroweak phase transition temperature is derived by taking into consideration the present value of the intergalactic magnetic field strength, B 0~10 -15 G, coherent on the ~1 Mpc scale. As a particular case, the standard model is considered and the field scale at high temperature is estimated in this case. Model-dependent properties of the phenomena under investigation are briefly discussed, too. © 2012 Springer-Verlag / Società Italiana di Fisica.

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