2,171 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
Detection of inner Solar System Trojan Asteroids by Gaia
The Gaia satellite, planned for launch by the European Space Agency (ESA) in
2013, is the next generation astrometry mission following Hipparcos. While
mapping the whole sky, the Gaia space mission is expected to discover thousands
of Solar System Objects. These will include Near-Earth Asteroids and objects at
Solar elongations as low as 45 degrees, which are difficult to observe with
ground-based telescopes. We present the results of simulations for the
detection of Trojan asteroids in the orbits of Earth and Mars by Gaia.Comment: 4 pages, 3 figures, based on a talk presented at the Gaia-FUN-SSO-2
International Workshop, Paris Observatory, 19-21 September 2012. Part of the
proceedings of that worksho
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
Where are the missing gamma ray burst redshifts?
In the redshift range z = 0-1, the gamma ray burst (GRB) redshift
distribution should increase rapidly because of increasing differential volume
sizes and strong evolution in the star formation rate. This feature is not
observed in the Swift redshift distribution and to account for this
discrepancy, a dominant bias, independent of the Swift sensitivity, is
required. Furthermore, despite rapid localization, about 40-50% of Swift and
pre-Swift GRBs do not have a measured redshift. We employ a heuristic technique
to extract this redshift bias using 66 GRBs localized by Swift with redshifts
determined from absorption or emission spectroscopy. For the Swift and
HETE+BeppoSAX redshift distributions, the best model fit to the bias in z < 1
implies that if GRB rate evolution follows the SFR, the bias cancels this rate
increase. We find that the same bias is affecting both Swift and HETE+BeppoSAX
measurements similarly in z < 1. Using a bias model constrained at a 98% KS
probability, we find that 72% of GRBs in z < 2 will not have measurable
redshifts and about 55% in z > 2. To achieve this high KS probability requires
increasing the GRB rate density in small z compared to the high-z rate. This
provides further evidence for a low-luminosity population of GRBs that are
observed in only a small volume because of their faintness.Comment: 5 pages, submitted to MNRA
Search strategies for Trojan asteroids in the inner Solar System
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. They are
important because they carry information on early Solar System formation, when
collisions between bodies were more frequent. Discovery and study of
terrestrial planet Trojans will help constrain models for the distribution of
bodies and interactions in the inner Solar System.
We present models that constrain optimal search areas, and strategies for
survey telescopes to maximize the probability of detecting inner planet
Trojans. We also consider implications for detection with respect to the Gaia
satellite, and limitations of Gaia's observing geometry.Comment: 14 pages, 6 figures, 2 tables, accepted for publication in Planetary
and Space Scienc
Measurement of thermal cycles in the weld heat affected zone of mild steel
The thermal cycles in the mild steel parent plate adjacent to a bead
on plate weld have been measured for heat inputs of 108, 54 and 42 kJ/inch,
by means of embedded thermocouples connected to high response automatic
recorders. The results show that decreasing the heat input increases the
cooling rate and decreases the width of the heat affected zone.
For thermal cycles in which the peak temperatures reached 900°C or
above, two points of inflection have been noted in the temperature ranges
400° - 600°C and 950° - 1200°C. The inflection in the lower temperature
range, which has been observed by other workers, has been attributed to
latent heat from the exothermic transformation of austenite to ferrite.
The higher inflection point, not previously reported, has been tentatively
related to the solidification in the weld pool and the release of the
latent heat of fusion
Fall back accretion and energy injections in gamma-ray bursts
Intense flares that occur at late times relative to the prompt phase have
been observed by the satellite in the X-ray afterglows of gamma-ray
bursts (GRBs). Here, we present a detailed analysis on the fall back accretion
process to explain the intense flare phase in the very early X-ray afterglow
light curves. To reproduce the afterglow at late times, we resort to the
external shock by engaging energy injections. By applying our model to GRBs
080810, 081028 and 091029, we show that their X-ray afterglow light curves can
be reproduced well. We then apply our model to the ultra-long GRB
111209A, which is the longest burst ever observed. The very early X-ray
afterglow of GRB 111209A showed many interesting features, such as a
significant bump observed at around 2000 s after the /BAT trigger. We
assume two constant energy injection processes in our model. These can explain
the observed plateau at X-ray wavelength in the relatively early stage
( s) and a second X-ray plateau and optical rebrightening at
about s. Our analysis supports the scenario that a significant amount
of material may fall back toward the central engine after the prompt phase,
causing an enhanced and long lived mass accretion rate powering a
Poynting-flux-dominated outflow.Comment: 2 tables, 3 figures, accepted by MNRA
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