13,025 research outputs found
Method to Look for Imprints of Ultrahigh Energy Nuclei Sources
We propose a new method to search for heavy nuclei sources, on top of
background, in the Ultra-High Energy Cosmic Ray data. We apply this method to
the 69 events recently published by the Pierre Auger Collaboration and find a
tail of events for which it reconstructs the source at a few degrees from the
Virgo galaxy cluster. The reconstructed source is located at ~ 8.5 degrees from
M87. The probability to have such a cluster of events in some random background
and reconstruct the source position in any direction of the sky is about 7 x
10^(-3). The probability to reconstruct the source at less than 10 degrees from
M87 in a data set already containing such a cluster of events is about 4 x
10^(-3). This may be a hint at the Virgo cluster as a bright ultra-high energy
nuclei source. We investigate the ability of current and future experiments to
validate or rule out this possibility, and discuss several alternative
solutions which could explain the existing anisotropy in the Auger data.Comment: 12 pages (2 columns), 10 figures. Published in Physical Review
CFD Analysis of Helicopter Wakes in Ground Effect
The paper presents CFD results for the wake of a helicopter flying a low altitude at different advance ratios.
The wakes are assessed in terms of topology and velocity magnitudes. The structure of the wake near ground
changes rapidly with the advance ratio and its decay appears to be faster than what is suggested by theoretical
analyses. The results show clear the potential of modern CFD for use in helicopter safety and highlights the
need for detailed surveys of helicopter wakes using full-scale physical experiments
Legge e diritto naturale in Alasdair MacIntyre
This paper focuses on the theme of natural rights, as it emerges from the works of Alasdair MacIntyre. In "After Virtue" he argues that «there are no such rights, and belief in them is one with belief in witches and in unicorns», but in later works he endorsed a thomistic view on natural
law, which is compatible with the acknowledgment
of universal human rights. MacIntyre’s writings contain the premises for an ontological foundation of natural rights, despite his rejection of any formulation of them
A Compact Fireball Model of Gamma Ray Bursts
It is proposed that the gamma ray burst photons near the peak of the spectrum
at several hundred KeV are produced on very compact scales, where photon
production is limited by blackbody effects and/or the requirement of energetic
quanta () for efficient further production. The fast variation of
order milliseconds in the time profile is then a natural expectation, given the
other observed GRB parameters. Analytic calculations are presented to show that
the escape of non-thermal, energetic gamma rays can emerge within a second of
the thermal photons from a gammasphere of below cm. The minimum
asymptotic bulk Lorentz factor in this model is found to be of order several
hundred if the photosphere is of order cm and greater for
larger or smaller photospheric radii. It is suggested that prompt UHE gamma
rays might provide a new constraint on the asymptotic Lorentz factor of the
outflow.Comment: To appear in ApJ, revisions requested by the refere
Genealogical Inquiry and Universal Moral Values
Inspired by american pragmatism and Hans Joas' proposal of an affirmative genealogy, I argue in this paper that a genealogical inquiry (both on the biografical and on the historical level) can explain what motivates individuals to moral agency better than Kantian moral philosophy, without renouncing an historically-informed conception of universal moral values
Swift Pointing and the Association Between Gamma-Ray Bursts and Gravitational-Wave Bursts
The currently accepted model for gamma-ray burst phenomena involves the
violent formation of a rapidly rotating solar mass black hole. Gravitational
waves should be associated with the black-hole formation, and their detection
would permit this model to be tested, the black hole progenitor (e.g.,
coalescing binary or collapsing stellar core) identified, and the origin of the
gamma rays (within the expanding relativistic fireball or at the point of
impact on the interstellar medium) located. Even upper limits on the
gravitational-wave strength associated with gamma-ray bursts could constrain
the gamma-ray burst model. To do any of these requires joint observations of
gamma-ray burst events with gravitational and gamma-ray detectors. Here we
examine how the quality of an upper limit on the gravitational-wave strength
associated with gamma-ray burst observations depends on the relative
orientation of the gamma-ray-burst and gravitational-wave detectors, and apply
our results to the particular case of the Swift Burst-Alert Telescope (BAT) and
the LIGO gravitational-wave detectors. A result of this investigation is a
science-based ``figure of merit'' that can be used, together with other mission
constraints, to optimize the pointing of the Swift telescope for the detection
of gravitational waves associated with gamma-ray bursts.Comment: aastex, 14 pages, 2 figure
Can past gamma-ray bursts explain both INTEGRAL and ATIC/PAMELA/Fermi anomalies simultaneously?
Gamma-ray bursts (GRBs) have been invoked to explain both the 511 keV
emission from the galactic bulge and the high-energy positron excess inferred
from the ATIC, PAMELA, and Fermi data. While independent explanations can be
responsible for these phenomena, we explore the possibility of their common
GRB-related origin by modeling the GRB distribution and estimating the rates.
For an expected Milky Way long GRB rate, neither of the two signals is generic;
the local excess requires a 2% coincidence, while the signal from the galactic
center requires a 20% coincidence with respect to the timing of the latest GRB.
The simultaneous explanation requires a 0.4% coincidence. Considering the large
number of statistical "trials" created by multiple searches for new physics,
the coincidences of a few per cent cannot be dismissed as unlikely.
Alternatively, both phenomena can be explained by GRBs if the galactic rate is
higher than expected. We also show that a similar result is difficult to obtain
assuming a simplified short GRB distribution.Comment: 4 pages; version accepted for publicatio
The Expected Duration of Gamma-Ray Bursts in the Impulsive Hydrodynamic Models
Depending upon the various models and assumptions, the existing literature on
Gamma Ray Bursts (GRBs) mentions that the gross theoretical value of the
duration of the burst in the hydrodynamical models is tau~r^2/(eta^2 c), where
r is the radius at which the blastwave associated with the fireball (FB)
becomes radiative and sufficiently strong. Here eta = E/Mc^2, c is the speed of
light, E is initial lab frame energy of the FB, and M is the baryonic mass of
the same (Rees and Meszaros 1992). However, within the same basic framework,
some authors (like Katz and Piran) have given tau ~ r^2 /(eta c). We intend to
remove this confusion by considering this problem at a level deeper than what
has been considered so far. Our analysis shows that none of the previously
quoted expressions are exactly correct and in case the FB is produced
impulsively and the radiative processes responsible for the generation of the
GRB are sufficiently fast, its expected duration would be tau ~ar^2/(eta^2 c),
where a~O(10^1). We further discuss the probable change, if any, of this
expression, in case the FB propagates in an anisotropic fashion. We also
discuss some associated points in the context of the Meszaros and Rees
scenario.Comment: 21 pages, LATEX (AAMS4.STY -enclosed), 1 ps. Fig. Accepted in
Astrophysical Journa
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