1,602 research outputs found
Hints for families of GRBs improving the Hubble diagram
As soon as their extragalactic origins were established, the hope to make
Gamma - Ray Bursts (GRBs) standardizeable candles to probe the very high - z
universe has opened the search for scaling relations between redshift
independent observable quantities and distance dependent ones. Although some
remarkable success has been achieved, the empirical correlations thus found are
still affected by a significant intrinsic scatter which downgrades the
precision in the inferred GRBs Hubble diagram. We investigate here whether this
scatter may come from fitting together objects belonging to intrinsically
different classes. To this end, we rely on a cladistics analysis to partition
GRBs in homogenous families according to their rest frame properties. Although
the poor statistics prevent us from drawing a definitive answer, we find that
both the intrinsic scatter and the coefficients of the \,-\,
and \,-\, correlations significantly change depending on which
subsample is fitted. It turns out that the fit to the full sample leads to a
scaling relation which approximately follows the diagonal of the region
delimited by the fits to each homogenous class. We therefore argue that a
preliminary identification of the class a GRB belongs to is necessary in order
to select the right scaling relation to be used in order to not bias the
distance determination and hence the Hubble diagram.Comment: 10 pages, 6 figures, 4 tables, accepted for publication on MNRA
Cloud optical thickness and liquid water path – does the <i>k</i> coefficient vary with droplet concentration?
Cloud radiative transfer calculations in general circulation models involve a link between cloud microphysical and optical properties. Indeed, the liquid water content expresses as a function of the mean volume droplet radius, while the light extinction is a function of their mean surface radius. There is a small difference between these two parameters because of the droplet spectrum width. This issue has been addressed by introducing an empirical multiplying correction factor to the droplet concentration. Analysis of in situ sampled data, however, revealed that the correction factor decreases when the concentration increases, hence partially mitigating the aerosol indirect effect. <br><br> Five field experiments are reanalyzed here, in which standard and upgraded versions of the droplet spectrometer were used to document shallow cumulus and stratocumulus topped boundary layers. They suggest that the standard probe noticeably underestimates the correction factor compared to the upgraded versions. The analysis is further refined to demonstrate that the value of the correction factor derived by averaging values calculated locally along the flight path overestimates the value derived from liquid water path and optical thickness of a cloudy column, and that there is no detectable relationship between the correction factor and the droplet concentration. It is also shown that the droplet concentration dilution by entrainment-mixing after CCN activation is significantly stronger in shallow cumuli than in stratocumulus layers. These various effects are finally combined to produce the today best estimate of the correction factor to use in general circulation models
An Optical Study of 3C 31, 3C 66B, 3C 120, and Their Jets
Paper freely available at http://cdsads.u-strasbg.fr/cgi-bin/nph-iarticle_query?1991AJ....102..562F&data_type=PDF_HIGH&type=PRINTERWe present the results of BVRI CCD photometry of the radiogalaxies 3C 31, 3C 66B, and 3C 120, and V polarimetry of 3C 120. The photometry of the jet of 3C 66B definitively establishes the synchrotron nature of the optical emission. No optical counterpart of the radio counterjet in 3C 66B and of the radio jets in 3C 31 and 3C 120 is found. A rotating ring and an ionized region are present respectively in 3C 31 (NGC 383) and its companion galaxy NGC 382, but we find no isophotal distortions which could have revealed a gravitational interaction between the two galaxies as it is the case in 3C 66B. The elliptical isophotes of 3C 120 shows a slight off-centering, roughly in the direction of the radio jet, very much like 3C 66B. We find an upper limit of 20% for the polarization level of the condensations in 3C 120
The Self Model and the Conception of Biological Identity in Immunology
The self/non-self model, first proposed by F.M. Burnet, has dominated immunology for sixty years now. According to this model, any foreign element will trigger an immune reaction in an organism, whereas endogenous elements will not, in normal circumstances, induce an immune reaction. In this paper we show that the self/non-self model is no longer an appropriate explanation of experimental data in immunology, and that this inadequacy may be rooted in an excessively strong metaphysical conception of biological identity. We suggest that another hypothesis, one based on the notion of continuity, gives a better account of immune phenomena. Finally, we underscore the mapping between this metaphysical deflation from self to continuity in immunology and the philosophical debate between substantialism and empiricism about identity
Optical and Radio Polarimetry of the M87 Jet at 0.2" Resolution
We discuss optical (HST/WFPC2 F555W) and radio (15 GHz VLA) polarimetry
observations of the M87 jet taken during 1994-1995. Many knot regions are very
highly polarized (, approaching the theoretical maximum for
optically thin synchrotron radiation), suggesting highly ordered magnetic
fields. High degrees of polarization are also observed in interknot regions.
While the optical and radio polarization maps share many similarities, we
observe significant differences between the radio and optical polarized
structures, particularly for bright knots in the inner jet, giving us important
insight into the jet's radial structure. Unlike in the radio, the optical
magnetic field position angle becomes perpendicular to the jet at the upstream
ends of knots HST-1, D, E and F. Moreover, the optical polarization decreases
markedly at the position of the flux maxima in these knots. In contrast, the
magnetic field position angle observed in the radio remains parallel to the jet
in most of these regions, and the decreases in radio polarization are smaller.
More minor differences are seen in other jet regions. Many of the differences
between optical and radio polarimetry results can be explained in terms of a
model whereby shocks occur in the jet interior, where higher-energy electrons
are concentrated and dominate both polarized and unpolarized emissions in the
optical, while the radio maps show strong contributions from lower-energy
electrons in regions with {\bf B} parallel, near the jet surface.Comment: 28 pages, 7 figures; accepted for publication in AJ (May 1999
31 GEANT4 simulation of dose deposition in patients from TomoTherapy Hi-Art Megavoltage computed tomography (MVCT) imaging
A statistical mechanics approach to autopoietic immune networks
The aim of this work is to try to bridge over theoretical immunology and
disordered statistical mechanics. Our long term hope is to contribute to the
development of a quantitative theoretical immunology from which practical
applications may stem. In order to make theoretical immunology appealing to the
statistical physicist audience we are going to work out a research article
which, from one side, may hopefully act as a benchmark for future improvements
and developments, from the other side, it is written in a very pedagogical way
both from a theoretical physics viewpoint as well as from the theoretical
immunology one.
Furthermore, we have chosen to test our model describing a wide range of
features of the adaptive immune response in only a paper: this has been
necessary in order to emphasize the benefit available when using disordered
statistical mechanics as a tool for the investigation. However, as a
consequence, each section is not at all exhaustive and would deserve deep
investigation: for the sake of completeness, we restricted details in the
analysis of each feature with the aim of introducing a self-consistent model.Comment: 22 pages, 14 figur
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