1,721,157 research outputs found
Living in a loft
No full textLOFT (Large area Observatory For x-ray Timing) is an innovative mission concept for the next generation of X-ray experiments, submitted to the ESA Call for Medium size missions “M3”. Recent developments in the field of Silicon detectors allowed us to design a realistic observatory devoted to X-ray timing studies with an effective area above 10 m2, operating in the energy range 2-30 keV with an energy resolution of ∼250 eV. Such an exceedingly large area (20 times that of RXTE/PCA), with a time resolution better than 10 μs, will enable unprecedently fast and accurate time variability studies related to accreting collapsed objects (e.g. fast coherent pulsations and QPOs). The scientific payload is complemented by a coded-mask wide field monitor based on similar detectors. In this paper we present the mission concept, the payload design and the expected performances. © Copyright owned by the author(s)
AGILE observation of gamma-ray variability of the bright blazar 3C 273 during the MWL campaign of Dec 2007 - Jan 2008
No full textEight years after the last EGRET observations in gamma-ray, the GRID (30 MeV - 50 GeV) and SuperAGILE (18 - 60 keV) experiments on board the AGILE mission observed the Virgo sky from December 16, 2007 and January 6, 2008. We organized a multi-wavelength campaign for the FSRQ 3C 273 with joint observations from the REM observatory (covering the near-IR and optical), Swift (near-UV and 0.2-10 keV). We report the preliminary results from the campaign. The first week the source was not detectable in Gamma Rays. Then we detected gamma-ray activity, with a flux comparable to the EGRET detection in June 1991. The hard x light curve show a decrease in flux of ∼ 20% during the period of activity in Gamma-ray. İ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence
High resolution X-ray Timing from a LOFT
No full textLOFT (Large area Observatory For X-ray Timing) is an innovative mission concept for a next generation of X-ray timing experiments. Recent developments in the field of Silicon detectors allow to design a realistic observatory devoted to X-ray timing studies with an effective area around 13 m2, in the energy range 2-30 keV. In this paper we present the mission concept together with the key properties of the detectors, as demonstrated in our laboratory, including an energy resolution better than 500 eV at room temperature. The overall power and weight budgets fit to a standard mission design. Such an exceedingly large area (>20 times RXTE/PCA), with a time resolution better than 10 ms, will allow unprecedently fast and accurate time variability studies related to accreting compact objects (e.g. fast coherent pulsations and QPOs). We describe here the scientific performance of a timing observatory in the 10 m2 class for open problems in fundamental physics, such as strong gravity effects, the measurement of the mass of black holes and neutron stars, the equation of state of ultradense matter. Finally, we present the results of simulated LOFT observations
A scientific approach to Plato's Atlantis
No full textThe myth of Atlantis is hard to die. This attempt to use scientific
evidence to give it the final smash ends up with the doubt that it might not be
totally unsubstantiated. The time of the supposed existence of Atlantis (around
twelve thousand years ago) was, in fact, characterized by technological revolutions,
acknowledged by archaeology, and abrupt climate changes, documented by geology.
In principle, it cannot therefore be ruled out that some of those dramatic events
left a memory, later used by Plato as a basis for its tale. The climate changes
involved the majority of the northern hemisphere, thus all the ancient civilizations
(Egyptian, Mesopotamian, Indian and Chinese) could have preserved reminiscence,
but it is clear that the events occurring closer to Greece would have been more
accessible to Plato. Among the Mediterranean sites that experienced the cataclysms
of the beginning of the Holocene, a good candidate to host a primordial civilization
might have been the archipelago then existing in the Strait of Sicily, a natural
maritime link between Tunisia and Italy, prized by the presence of an obsidian
source at Pantelleria. Eleven thousand five hundred years ago, a sudden sea level
rise erased the archipelago, submerging the possible settlements, but Pantelleria
obsidian ores are still there and could provide a significant clue. In fact, the
potential discovery of artefacts, originating from a source now submerged by
the sea level rise, would imply that the collection of the mineral took place when
it was still emerged, namely at the time of Atlantis. Even if such discovery would
not be sufficient to prove the existence of the mythical island, it would be enough
to shake up the timeline of the human occupation in the region
Properties of terrestrial gamma ray flashes detected by AGILE MCAL below 30 MeV
No full textWe present the characteristics of 308 terrestrial gamma ray flashes (TGFs) detected by the Minicalorimeter (MCAL) instrument on board the AGILE satellite during the period March 2009-July 2012 in the ±2.5° latitude band and selected to have the maximum photon energy up to 30 MeV. The characteristics of the AGILE events are analyzed and compared to the observational framework established by the two other currently active missions capable of detecting TGFs from space, RHESSI and Fermi. A detailed model of the MCAL dead time is presented, which is fundamental to properly interpret our observations. The most significant contribution to dead time is due to the anticoincidence shield in its current configuration and not to the MCAL detector itself. Longitude and local time distributions are compatible with previous observations, while the duration distribution is biased toward longer values because of dead time. The intensity distribution is compatible with previous observations, when dead time is taken into account. The TGFs cumulative spectrum supports a low production altitude, in agreement with previous measurements. We also compare our sample to lightning sferics detected by the World Wide Lightning Location Network and suggest a new method to assess quantitatively the consistency of two TGF populations based on the comparison of the associated lightning activity. According to this method, AGILE and RHESSI samples are compatible with the same parent population. The AGILE TGF catalog below 30 MeV is accessible online at the website of the ASI Science Data Center http://www.asdc.asi.it/mcaltgfcat
AGILE mini-calorimeter gamma-ray burst catalog
No full textThe mini-calorimeter of the AGILE satellite can observe the high-energy part of gamma-ray bursts with good timing-capability. We present the data of the 85 hard gamma-ray bursts observed by the mini-calorimeter since the launch (April 2007) until October 2009. We report the timing data for 84 and spectral data for 21 bursts. © 2013 ESO
Calibration of AGILE-GRID with in-flight data and Monte Carlo simulations
No full textContext. AGILE is a γ-ray astrophysics mission which has been in orbit since 23 April 2007 and continues to operate reliably. The γ-ray detector, AGILE-GRID, has observed Galactic and extragalactic sources, many of which were collected in the first AGILE Catalog. Aims. We present the calibration of the AGILE-GRID using in-flight data and Monte Carlo simulations, producing instrument response functions (IRFs) for the effective area (A eff), energy dispersion probability (EDP), and point spread function (PSF), each as a function of incident direction in instrument coordinates and energy. Methods. We performed Monte Carlo simulations at different γ-ray energies and incident angles, including background rejection filters and Kalman filter-based γ-ray reconstruction. Long integrations of in-flight observations of the Vela, Crab and Geminga sources in broad and narrow energy bands were used to validate and improve the accuracy of the instrument response functions. Results. The weighted average PSFs as a function of spectra correspond well to the data for all sources and energy bands. Conclusions. Changes in the interpolation of the PSF from Monte Carlo data and in the procedure for construction of the energy-weighted effective areas have improved the correspondence between predicted and observed fluxes and spectra of celestial calibration sources, reducing false positives and obviating the need for post-hoc energy-dependent scaling factors. The new IRFs have been publicly available from the AGILE Science Data Center since November 25, 2011, while the changes in the analysis software will be distributed in an upcoming release. © ESO, 2013
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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