3,091 research outputs found
Simulating a faint gamma-ray burst population
There have now been three supernova-associated gamma-ray bursts (GRBs) at
redshift z < 0.17, namely 980425, 030329, and 031203, but the nearby and
under-luminous GRBs 980425 and 031203 are distinctly different from the
`classical' or standard GRBs. It has been suggested that they could be
classical GRBs observed away from their jet axes, or they might belong to a
population of under-energetic GRBs. Recent radio observations of the afterglow
of GRB 980425 suggest that different engines may be responsible for the
observed diversity of cosmic explosions. Given this assumption, a crude
constraint on a luminosity function for faint GRBs with a mean luminosity
similar to that of GRB 980425 and an upper limit on the rate density of
980425-type events, we simulate the redshift distribution of under-luminous
GRBs assuming BATSE and Swift sensitivities. A local rate density of about 0.6%
of the local supernova Type Ib/c rate yields simulated probabilities for
under-luminous events to occur at rates comparable to the BATSE GRB
low-redshift distribution. In this scenario the probability of BATSE/HETE
detecting at least one GRB at z<0.05 is 0.78 over 4.5 years, a result that is
comparable with observation. Swift has the potential to detect 1--5
under-luminous GRBs during one year of observation.Comment: 5 pages, 3 figures, MNRAS Letter, Accepte
Using temporal distributions of transient events to characterize cosmological source populations
The brightest events in a time series of cosmological transients obey an
observation time dependence which is often overlooked. This dependence can be
exploited to probe the global properties of electromagnetic and gravitational
wave transients (Howell et al. 2007a, Coward & Burman 2005). We describe a new
relation based on a peak flux--observation time distribution and show that it
is invariant to the luminosity distribution of the sources (Howell et al.
2007b). Applying this relation, in combination with a new data analysis filter,
to \emph{Swift} gamma-ray burst data, we demonstrate that it can constrain
their rate density.Comment: published in proceedings of FRONTIERS OF FUNDAMENTAL AND
COMPUTATIONAL PHYSICS: 10th International Symposium, AIP,1246,203, (2010
An improved method for estimating source densities using the temporal distribution of Cosmological Transients
It has been shown that the observed temporal distribution of transient events
in the cosmos can be used to constrain their rate density. Here we show that
the peak flux--observation time relation takes the form of a power law that is
invariant to the luminosity distribution of the sources, and that the method
can be greatly improved by invoking time reversal invariance and the temporal
cosmological principle. We demonstrate how the method can be used to constrain
distributions of transient events, by applying it to Swift gamma-ray burst data
and show that the peak flux--observation time relation is in good agreement
with recent estimates of source parameters. We additionally show that the
intrinsic time dependence allows the method to be used as a predictive tool.
Within the next year of Swift observation, we find a 50% chance of obtaining a
peak flux greater than that of GRB 060017 -- the highest Swift peak flux to
date -- and the same probability of detecting a burst with peak flux > 100
photons s^{-1} cm^{-2} within 6 years.Comment: Submitted to ApJ Letter
Quiet, Clean, Short-Haul Experimental Engine (QCSEE) Over-The-Wing (OTW) engine acoustic design
The acoustic considerations involved in the low source noise basic engine design and the design procedures followed in the development of the over-the-wing (OTW) nacelle acoustic treatment design are presented. Laboratory experiments, component tests, and scale model and engine tests supporting the OTW engine acoustic design are referenced. Acoustic design features include a near-sonic inlet, low fan and core pressure ratios, low fan tip speed, high and low frequency stacked core treatment, multiple thickness treatment, and fan frame and stator vane treatment
Quiet Clean Short-haul Experimental Engine (QCSEE). Core engine noise measurements
Noise measurements were taken on a turbofan engine which uses the same core, with minor modifications, employed on the quiet clean short-haul experimental engine (QCSEE) propulsion systems. Both nearfield and farfield noise measurements were taken in order to determine the core internally generated noise levels. The resulting noise measurements were compared to predicted combustor and turbine noise levels, to verify or improve the predicted QCSEE combustor and turbine noise levels
Welding embrittlement of the parent plate outside the visible heat affected zone region
Enbrittlement of steels outside the visible heat affected zone region
due to plastic straining from the weldment and subsequent ageing has been
reported by several authors. The problem is reviewed in this Note and an
attempt made to detect this region in a semi-killed mild steel by means of
a micro hardness testing survey taken at regular intervals in a direction
away from the veld fusion boundary. Recommendations for future work in
understanding the magnitude of this Problem particularly in multi stressed
welded joints are given
GRB optical afterglow and redshift selection effects: The learning curve effect at work
We show how the observed gamma ray burst (GRB) optical afterglow (OA) and
redshift distributions are changing in time from selection effects. For a
subset of {\it Swift} triggered long duration bursts, we show that the mean
time taken to acquire spectroscopic redshifts for a GRB OA has evolved to
shorter times. We identify a strong correlation between the mean time taken to
acquire a spectroscopic redshift and the measured redshift. This correlation
reveals that shorter response times favour smaller redshift bursts. This is
compelling evidence for a selection effect that biases longer response times
with relatively brighter high redshift bursts. Conversely, for shorter response
times, optically fainter bursts that are relatively closer are bright enough
for spectroscopic redshifts to be acquired. This selection effect could explain
why the average redshift, measured in 2005, has evolved to
, by mid 2008. Understanding these selection effects provides an
important tool for separating the contributions of intrinsically faint bursts,
those obscured by host galaxy dust and bursts not seen in the optical because
their OAs are observed at late times. The study highlights the importance of
rapid response telescopes capable of spectroscopy, and identifies a new
redshift selection effect that has not been considered previously, namely the
response time to measure the redshift.Comment: 5 pages, 4 figures, MNRAS Letter (accepted
An optimal Mars Trojan asteroid search strategy
Trojan asteroids are minor planets that share the orbit of a planet about the
Sun and librate around the L4 or L5 Lagrangian points of stability. Although
only three Mars Trojans have been discovered, models suggest that at least ten
times this number should exist with diameters >= 1 km. We derive a model that
constrains optimal sky search areas and present a strategy for the most
efficient use of telescope survey time that maximizes the probability of
detecting Mars Trojans. We show that the Gaia space mission could detect any
Mars Trojans larger than 1 km in diameter, provided the relative motion
perpendicular to Gaia's CCD array is less than 0.40 arcsec per second.Comment: 6 pages, 6 figures, 3 tables, accepted for publication in MNRAS.
arXiv admin note: substantial text overlap with arXiv:1111.112
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