16,949 research outputs found
How immigration might alleviate the effects of an influenza pandemic: a freshwater microbiology story
Thermal photon to dilepton ratio in ultra-relativistic heavy ion collisions
The ratio of transverse momentum distribution of thermal photons to dilepton
has been evaluated. It is observed that this ratio reaches a plateau beyond a
certain value of transverse momentum. We argue that this ratio can be used to
estimate the initial temperature of the system by selecting the transverse
momentum and invariance mass windows judiciously. It is demonstrated that if
the radial flow is large then the plateau disappear and hence a deviation from
the plateau can be used as an indicator of large radial flow. The sensitivity
of the results on various input parameters has been studied.Comment: 9 pages with 11 eps figure
Protention and retention in biological systems
This paper proposes an abstract mathematical frame for describing some
features of cognitive and biological time. We focus here on the so called
"extended present" as a result of protentional and retentional activities
(memory and anticipation). Memory, as retention, is treated in some physical
theories (relaxation phenomena, which will inspire our approach), while
protention (or anticipation) seems outside the scope of physics. We then
suggest a simple functional representation of biological protention. This
allows us to introduce the abstract notion of "biological inertia".Comment: This paper was made possible only as part of an extended
collaboration with Francis Bailly (see references), a dear friend and
"ma\^itre \'a penser", who contributed to the key ideas. Francis passed away
in february 2008: we continue here our inspiring discussions and joint wor
Near-Infrared Spectroscopy of Molecular Hydrogen Emission in Four Reflection Nebulae: NGC 1333, NGC 2023, NGC 2068, and NGC 7023
We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2)
emission from NGC 1333, NGC 2023, NGC 2068, and NGC 7023 and derive the
physical properties of the molecular material in these reflection nebulae. Our
observations of NGC 2023 and NGC 7023 and the physical parameters we derive for
these nebulae are in good agreement with previous studies. Both NGC 1333 and
NGC 2068 have no previously-published analysis of near-infrared spectra. Our
study reveals that the rotational-vibrational states of molecular hydrogen in
NGC 1333 are populated quite differently from NGC 2023 and NGC 7023. We
determine that the relatively weak UV field illuminating NGC 1333 is the
primary cause of the difference. Further, we find that the density of the
emitting material in NGC 1333 is of much lower density, with n ~ 10^2 - 10^4
cm^-3. NGC 2068 has molecular hydrogen line ratios more similar to those of NGC
7023 and NGC 2023. Our model fits to this nebula show that the bright,
H_2-emitting material may have a density as high as n ~ 10^5 cm^-3, similar to
what we find for NGC 2023 and NGC 7023. Our spectra of NGC 2023 and NGC 7023
show significant changes in both the near-infrared continuum and H_2 intensity
along the slit and offsets between the peaks of the H_2 and continuum emission.
We find that these brightness changes may correspond to real changes in the
density and temperatures of the emitting region, although uncertainties in the
total column of emitting material along a given line of sight complicates the
interpretation. The spatial difference in the peak of the H_2 and near-infrared
continuum peaks in NGC 2023 and NGC 7023 shows that the near-infrared continuum
is due to a material which can survive closer to the star than H_2 can.Comment: Submitted for publication in ApJ. 34 pages including 12 embedded
postscript figures. Also available at
http://www.astronomy.ohio-state.edu/~martini/pub
Cosmological Magnetic Fields from Primordial Helical Seeds
Most early Universe scenarios predict negligible magnetic fields on
cosmological scales if they are unprocessed during subsequent expansion of the
Universe. We present a new numerical treatment of the evolution of primordial
fields and apply it to weakly helical seeds as they occur in certain early
Universe scenarios. We find that initial helicities not much larger than the
baryon to photon number can lead to fields of about 10^{-13} Gauss with
coherence scales slightly below a kilo-parsec today.Comment: 4 revtex pages, 2 postscript figures include
Rain: Relaxations in the sky
We demonstrate how, from the point of view of energy flow through an open
system, rain is analogous to many other relaxational processes in Nature such
as earthquakes. By identifying rain events as the basic entities of the
phenomenon, we show that the number density of rain events per year is
inversely proportional to the released water column raised to the power 1.4.
This is the rain-equivalent of the Gutenberg-Richter law for earthquakes. The
event durations and the waiting times between events are also characterised by
scaling regions, where no typical time scale exists. The Hurst exponent of the
rain intensity signal . It is valid in the temporal range from
minutes up to the full duration of the signal of half a year. All of our
findings are consistent with the concept of self-organised criticality, which
refers to the tendency of slowly driven non-equilibrium systems towards a state
of scale free behaviour.Comment: 9 pages, 8 figures, submitted to PR
Time-Domain Measurement of Broadband Coherent Cherenkov Radiation
We report on further analysis of coherent microwave Cherenkov impulses
emitted via the Askaryan mechanism from high-energy electromagnetic showers
produced at the Stanford Linear Accelerator Center (SLAC). In this report, the
time-domain based analysis of the measurements made with a broadband (nominally
1-18 GHz) log periodic dipole array antenna is described. The theory of a
transmit-receive antenna system based on time-dependent effective height
operator is summarized and applied to fully characterize the measurement
antenna system and to reconstruct the electric field induced via the Askaryan
process. The observed radiation intensity and phase as functions of frequency
were found to agree with expectations from 0.75-11.5 GHz within experimental
errors on the normalized electric field magnitude and the relative phase; 0.039
microV/MHz/TeV and 17 deg, respectively. This is the first time this agreement
has been observed over such a broad bandwidth, and the first measurement of the
relative phase variation of an Askaryan pulse. The importance of validation of
the Askaryan mechanism is significant since it is viewed as the most promising
way to detect cosmogenic neutrino fluxes at E > 10^15 eV.Comment: 10 pages, 9 figures, accepted by Phys. Rev.
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