24,057 research outputs found
Does The Addition of a Duration Improve the L_iso - E_peak Relation For Gamma-Ray Bursts?
Firmani et al. proposed a new Gamma Ray Burst (GRB) luminosity relation that
showed a significant improvement over the L_iso-E_peak relation. The new
proposed relation simply modifies the E_peak value by multiplying it by a power
of T_0.45, where T_0.45 is a particular measure of the GRB duration. We begin
by reproducing the results of Firmani for his 19 bursts. We then test the
Firmani relation for the same 19 bursts except that we use independently
measured values for L_iso, T_0.45, and E_peak, and we find that the relation
deteriorates substantially. We further test the relation by using 60 GRBs with
measured spectroscopic redshifts, and find a relation that has a comparable
scatter as the original L_iso-E_peak relation. That is, a much larger sample of
bursts does not reproduce the small scatter as reported by Firmani et al.
Finally, we investigate whether the Firmani relation is improved by the use of
any of 32 measures of duration in place of T_0.45. The quality of each
alternative duration measure is evaluated with the root mean square of the
scatter between the observed and fitted logarithmic Liso values. Although we
find some durations yield slightly better results than T_0.45, the differences
between the duration measures are minimal. We find that the addition of a
duration does not add any significant improvement to the L_iso-E_peak relation.
We also present a simple and direct derivation of the Firmani relation from
both the L_iso-E_peak and Amati relations. In all we conclude that the Firmani
relation neither has an independent existence nor does it provide any
significant improvement on previously known relations that are simpler.Comment: ApJ in press, 17 pages, 3 figures, 3 table
Contractor decision making and incentive fee contracts
Contractor decision making and incentive fee contracts in procurement of complex weapons and space system
Environmental influences on galaxy evolution
We investigate the role of mergers and interactions in the evolution of galaxies by studying galaxies in compact groups. Compact groups of galaxies have high spatial densities and low velocity dispersions making these regions ideal laboratories in which to study the effect of interactions and mergers. Based on a detailed spectroscopic and multi-color imaging study, we find that both the isophotal shapes and the stellar kinematics indicate that many of the elliptical galaxies in compact groups have been affected by tidal interactions. At the same time, however, we find that only a few elliptical galaxies in compact groups have evidence for the young stellar populations that would be expected if they are the result of recent merger of two spiral galaxies. Therefore, we conclude that tidal interactions affect galaxy properties at the current epoch, but the bulk of basic galaxy formation and transformation must have occurred at much higher redshift
A Gamma Ray Burst with a 220 Microsecond Rise Time and a Sharp Spectral Cutoff
The Gamma Ray Burst GRB920229 has four extreme and unprecedented properties;
a rise in brightness with an e-folding time scale of , a fall
in brightness with an e-folding time scale of , a large
change in spectral shape over a time of , and a sharp spectral
cutoff to high energies with . The rapid changes occur
during a spike in the light curve which was seen 0.164 s after the start of the
burst. The spectrum has a peak at 200 keV with no significant
flux above 239 keV, although the cutoff energy shifts to less than 100 keV
during the decay of the spike. These numbers can be used to place severe limits
on fireball models of bursts. The thickness of the energy production region
must be smaller than , ejected shells must have a dispersion of the
Lorentz factor of less than roughly 1% along a particular radius, and the
angular size of the radiation emission region is of order 1 arc-minute as
viewed from the burst center. The physical mechanism that caused the sharp
spectral cutoff has not been determined.Comment: 20 pages, 3 figures, Submitted to ApJ
The VWFA: It\u27s not just for words anymore
Reading is an important but phylogenetically new skill. While neuroimaging studies have identified brain regions used in reading, it is unclear to what extent these regions become specialized for use predominantly in reading vs. other tasks. Over the past several years, our group has published three studies addressing this question, particularly focusing on whether the putative visual word form area (VWFA) is used predominantly in reading, or whether it is used more generally in a number of tasks. Our three studies utilize a range of neuroimaging techniques, including task based fMRI experiments, a seed based resting state functional connectivity (RSFC) experiment, and a network based RSFC experiment. Overall, our studies indicate that the VWFA is not used specifically or even predominantly for reading. Rather the VWFA is a general use region that has processing properties making it particularly useful for reading, though it continues to be used in any task that requires its general processing properties. Our network based RSFC analysis extends this finding to other regions typically thought to be used predominantly for reading. Here, we review these findings and describe how the three studies complement each other. Then, we argue that conceptualizing the VWFA as a brain region with specific processing characteristics rather than a brain region devoted to a specific stimulus class, allows us to better explain the activity seen in this region during a variety of tasks. Having this type of conceptualization not only provides a better understanding of the VWFA but also provides a framework for understanding other brain regions, as it affords an explanation of function that is in keeping with the long history of studying the brain in terms of the type of information processing performed (Posner, 1978)
A -adic RanSaC algorithm for stereo vision using Hensel lifting
A -adic variation of the Ran(dom) Sa(mple) C(onsensus) method for solving
the relative pose problem in stereo vision is developped. From two 2-adically
encoded images a random sample of five pairs of corresponding points is taken,
and the equations for the essential matrix are solved by lifting solutions
modulo 2 to the 2-adic integers. A recently devised -adic hierarchical
classification algorithm imitating the known LBG quantisation method classifies
the solutions for all the samples after having determined the number of
clusters using the known intra-inter validity of clusterings. In the successful
case, a cluster ranking will determine the cluster containing a 2-adic
approximation to the "true" solution of the problem.Comment: 15 pages; typos removed, abstract changed, computation error remove
Are There Any Redshift >8 Gamma-Ray Bursts in the BATSE Catalog?
Several luminosity indicators have been found for Gamma-Ray Bursts (GRBs)
wherein measurable light curve and spectral properties are well-correlated with
the peak luminosity. Several papers have each applied one different luminosity
relation to find redshifts for BATSE GRBs and claim to identify specific bursts
with z>8. The existence of such high redshift events is not surprising, as
BATSE has enough sensitivity to see them and GRBs are expected out to the
redshift of the first star formation. To improve results we used five
luminosity relations with updated calibrations to determine redshifts with
error bars. Combining these relations, we calculated the redshifts of 36 BATSE
GRBs with claimed z>8. Our results include 13 bursts with our derived best
redshift z_best>8, which looks promising at first. But the calculated redshift
uncertainties are significantly large in these selected cases. With only one
exception, all of our bursts have z_1siglow<9. The one exception (BATSE trigger
2035) is likely a short duration burst at z>~4. Our best case for a very high
redshift event is BATSE trigger 3142 with z_best>20 and z_1siglow=8.9, however
we can only say z>4.1 at the two-sigma confidence level. In all, we cannot
point toward any one BATSE burst as confidently having z>8. One implication is
to greatly weaken prior claims that GRBs have a steeply rising rate-density out
to high redshifts.Comment: ApJ in press, 18 page
Theory of the Ramsey spectroscopy and anomalous segregation in ultra-cold rubidium
The recent anomalous segregation experiment of Lewandowski et al. (PRL, 88,
070403, 2002) shows dramatic, rapid internal state segregation for two
hyperfine levels of rubidium. We simulate an effective one dimensional model of
the system for experimental parameters and find reasonable agreement with the
data. The Ramsey frequency is found to be insensitive to the decoherence of the
superposition, and is only equivalent to the interaction energy shift for a
pure superposition. A Quantum Boltzmann equation describing collisions is
derived using Quantum Kinetic Theory, taking into account the different
scattering lengths of the internal states. As spin-wave experiments are likely
to be attempted at lower temperatures we examine the effect of degeneracy on
decoherence by considering the recent experiment of Lewandowski et al. where
degeneracy is around 10%. We also find that the segregation effect is only
possible when transport terms are included in the equations of motion, and that
the interactions only directly alter the momentum distributions of the states.
The segregation or spin wave effect is thus entirely due to coherent atomic
motion as foreseen in the experimental reportComment: 26 pages, 4 figures, to be published in J. Phys.
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