6,458 research outputs found
Cumulative light curves of gamma-ray bursts and relaxation systems
The cumulative light curves of a large sample of gamma-ray bursts (GRBs) were
obtained by summing the BATSE counts. The smoothed profiles are much simpler
than the complex and erratic running light curves that are normally used. For
most GRBs the slope of the cumulative light curve (S) is approximately constant
over a large fraction of the burst. The bursts are modelled as relaxation
systems that continuously accumulate energy in the reservoir and
discontinuously release it. The slope is a measure of the cumulative power
output of the central engine. A plot of S versus peak flux in 64ms (P64ms)
shows a very good correlation over a wide range for both short and long GRBs.
No relationship was found between S and GRBs with known redshift. The standard
slope (S'), which is representative of the power output per unit time, is
correlated separately with P64ms for both sub-classes indicating more powerful
outbursts for the short GRBs. S' is also anticorrelated with GRB duration.
These results imply that GRBs are powered by accretion into a black hole.Comment: 4 pages, 2 figures. Accepted for publication in Astronomy and
Astrophysics Letter
On a generalization of Lie(): a CataLAnKe theorem
We define a generalization of the free Lie algebra based on an -ary
commutator and call it the free LAnKe. We show that the action of the symmetric
group on the multilinear component with generators is given
by the representation , whose dimension is the th Catalan
number. An application involving Specht modules of staircase shape is
presented. We also introduce a conjecture that extends the relation between the
Whitehouse representation and Lie().Comment: 14 page
Temporal properties of short and long gamma-ray bursts
A temporal analysis was performed on a sample of 100 bright short GRBs with
T90 < 2s from the BATSE Current Catalog along with a similar analysis on 319
long bright GRBs with T90 > 2s from the same catalog. The short GRBs were
denoised using a median filter and the long GRBs were denoised using a wavelet
method. Both samples were subjected to an automated pulse selection algorithm
to objectively determine the effects of neighbouring pulses. The rise times,
fall times, FWHM, pulse amplitudes and areas were measured and their frequency
distributions are presented. The time intervals between pulses were also
measured. The frequency distributions of the pulse properties were found to be
similar and consistent with lognormal distributions for both the short and long
GRBs. The time intervals between the pulses and the pulse amplitudes of
neighbouring pulses were found to be correlated with each other. The same
emission mechanism can account for the two sub-classes of GRBs.Comment: 3 pages, 8 figures; Proceedings of "Gamma-Ray Burst and Afterglow
Astronomy 2001", Woods Hol
Transport policy and health inequalities: a health impact assessment of Edinburgh's transport policy
Health impact assessment (HIA) can be used to examine the relationships between inequalities and health. This HIA of Edinburgh's transport policy demonstrates how HIA can examine how different transport policies can affect different population groupings to varying degrees.
In this case, Edinburgh's economy is based on tourism, financial services and Government bodies. These need a good transport infrastructure, which maintains a vibrant city centre. A transport policy that promotes walking, cycling and public transport supports this and is also good for health.
The HIA suggested that greater spend on public transport and supporting sustainable modes of transport was beneficial to health, and offered scope to reduce inequalities. This message was understood by the City Council and influenced the development of the city's transport and land-use strategies. The paper discusses how HIA can influence public policy
Gamma-ray bursts and X-ray melting of material as a potential source of chondrules and planets
The intense radiation from a gamma-ray burst (GRB) is shown to be capable of
melting stony material at distances up to 300 light years which subsequently
cool to form chondrules. These conditions were created in the laboratory for
the first time when millimeter sized pellets were placed in a vacuum chamber in
the white synchrotron beam at the European Synchrotron Radiation Facility
(ESRF). The pellets were rapidly heated in the X-ray and gamma-ray furnace to
above 1400 C melted and cooled. This process heats from the inside unlike
normal furnaces. The melted spherical samples were examined with a range of
techniques and found to have microstructural properties similar to the
chondrules that come from meteorites. This experiment demonstrates that GRBs
can melt precursor material to form chondrules that may subsequently influence
the formation of planets. This work extends the field of laboratory
astrophysics to include high power synchrotron sources.Comment: 8 pages, 10 figures. Proceedings of the 5th INTEGRAL Workshop, Munich
16-20 February 2004. High resolution figures available at
http://bermuda.ucd.ie/%7Esmcbreen/papers/duggan_01.pd
Influence of Sediment Nutrients on Growth of Emergent Hygrophila
Hygrophila (
Hygrophila
polysperma
(Roxb.) T. Anderson) is
a plants which forms serious aquatic weed problems. Both
submerged and emergent growth forms occur. Nutritional
studies with a controlled release fertilizer and sediments collected
from hygrophila-infested areas were conducted with
the emergent growth habit to provide insights into growth of
this introduced plant. Plant dry weights for experimental 16-
week culture periods with low average temperatures were associated
with low amounts of hygrophila biomass as compared
to culture periods with high average temperatures.
Hygrophila cultured in sand rooting media with the controlled
release fertilizer produced as much as 20 times more
dry weight than plants cultured in sediments only. First-degree
linear regression statistics showed hygrophila dry
weights were highly related to ammonia nitrogen, magnesium,
sodium, and pH values in the sediments. These findings
show the close relationship of the emergent growth
habit of hygrophila to sediment nutrients. Analyses for certain
sediment characteristics may provide an indication of
the potential growth that may be expected for weed infestations
of this plant. Hygrophila grows year round in south
Florida; however, visual observations of canals and other bodies
of water indicate that lower amounts of hygrophila plants
occur during the cooler months of year than during the summer
season. These findings show the seasonal growth of
emergent hygrophila occurs with biomass dependent on
both sediment nutrients and temperature
Emission Spectra from Internal Shocks in Gamma-Ray-Burst Sources
Unsteady activity of gamma-ray burst sources leads to internal shocks in
their emergent relativistic wind. We study the emission spectra from such
shocks, assuming that they produce a power-law distribution of relativistic
electrons and posses strong magnetic fields. The synchrotron radiation emitted
by the accelerated electrons is Compton up-scattered multiple times by the same
electrons. A substantial component of the scattered photons acquires high
energies and produces e+e- pairs. The pairs transfer back their kinetic energy
to the radiation through Compton scattering. The generic spectral signature
from pair creation and multiple Compton scattering is highly sensitive to the
radius at which the shock dissipation takes place and to the Lorentz factor of
the wind. The entire emission spectrum extends over a wide range of photon
energies, from the optical regime up to TeV energies. For reasonable values of
the wind parameters, the calculated spectrum is found to be in good agreement
with the burst spectra observed by BATSE.Comment: 12 pages, latex, 2 figures, submitted to ApJ
The Host Galaxy of GRB980703 at Radio Wavelengths - a Nuclear Starburst in a ULIRG
We present radio observations of GRB980703 at 1.43, 4.86, and 8.46 GHz for
the period of 350 to 1000 days after the burst. These radio data clearly
indicate that there is a persistent source at the position of GRB980703 with a
flux density of approximately 70 Jy at 1.43 GHz, and a spectral index,
, where . We show that emission
from the afterglow of GRB980703 is expected to be one to two orders of
magnitude fainter, and therefore cannot account for these observations. We
interpret this persistent emission as coming from the host galaxy --- the first
example of a gamma-ray burst (GRB) host detection at radio wavelengths. We show
that emission from an AGN is unlikely, and find that it can be explained as a
result of a star-formation rate (SFR) of massive stars (M>5M) of 90
M/yr, which gives a total SFR of M/yr. Using the
correlation between the radio and far-IR (FIR) luminosities of star-forming
galaxies, we find that the host of GRB980703 is at the faint end of the class
of Ultra Luminous Infrared Galaxies (ULIRGs), with L_{FIR}\sim few\times
10^{12} L. From the radio measurements of the offset between the burst
and the host, and the size of the host, we conclude that GRB980703 occurred
near the center of the galaxy in a region of maximum star formation. A
comparison of the properties of this galaxy with radio and optical surveys at a
similar redshift () reveals that the host of GRB980703 is an
average star-forming galaxy. This result has significant implications for the
potential use of a GRB-selected galaxy sample for the study of galaxies and the
IGM at high redshifts.Comment: Submitted to Ap
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