2,724 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
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
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
Nuclear Equation of State from Observations of Short Gamma-Ray Burst Remnants
The favoured progenitor model for short -ray bursts (SGRBs) is the
merger of two neutron stars that triggers an explosion with a burst of
collimated -rays. Following the initial prompt emission, some SGRBs
exhibit a plateau phase in their -ray light curves that indicates additional
energy injection from a central engine, believed to be a rapidly rotating,
highly magnetised neutron star. The collapse of this `protomagnetar' to a black
hole is likely to be responsible for a steep decay in -ray flux observed at
the end of the plateau. In this letter, we show that these observations can be
used to effectively constrain the equation of state of dense matter. In
particular, we show that the known distribution of masses in binary neutron
star systems, together with fits to the -ray light curves, provide
constraints that exclude the softest and stiffest plausible equations of state.
We further illustrate how a future gravitational wave observation with Advanced
LIGO/Virgo can place tight constraints on the equation of state, by adding into
the picture a measurement of the chirp mass of the SGRB progenitor.Comment: accepted for publication in Phys. Rev.
Observation time to first detection of double neutron star mergers by gravitational wave observatories
We constrain the uncertainty in waiting times for detecting the first
double-neutron-star (DNS) mergers by gravitational wave observatories. By
accounting for the Poisson fluctuations in the rate density of DNS mergers and
galaxy space density inhomogeneity in the local Universe, we define a detection
`zone' as a region in a parameter space constrained by the double neutron star
merger rate and two LIGO operations parameters: an observation horizon distance
and science run duration. Assuming a mean rate of about 80 DNS mergers per
Milky Way galaxy Myr^{-1}, we find a 1/20 chance of observing a merger by
Enhanced LIGO in only 1 yr of observation. The minimum waiting time and
temporal zone width for an Advanced LIGO sensitivity are much shorter and imply
that there is a 95% probability of detecting a DNS merger in less than 60 days
and a 1/20 chance of a first detection in about 1 day. At the 5% probability
threshold for a first detection, we find that the effect of galaxy clusters on
detection is smoothed out and may only influence detection rates after 5-10
years observation time.Comment: 4 pages, 4 figures, accepted by MNRAS with minor revision
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