7,554 research outputs found
Burst Populations and Detector Sensitivity
The F_T (peak bolometric photon flux) vs. E_p (peak energy) plane is a
powerful tool to compare the burst populations detected by different detectors.
Detector sensitivity curves in this plane demonstrate which burst populations
the detectors will detect. For example, future CZT-based detectors will show
the largest increase in sensitivity for soft bursts, and will be particularly
well-suited to study X-ray rich bursts and X-ray Flashes. Identical bursts at
different redshifts describe a track in the F_T-E_p plane.Comment: 4 pages, 4 figures. To appear in the proceedings of the 2003 GRB
Conference (Santa Fe, NM, 2003 Sep 8-12
Burst Detector Sensitivity: Past, Present & Future
I compare the burst detection sensitivity of CGRO's BATSE, Swift's BAT, the
GLAST Burst Monitor (GBM) and EXIST as a function of a burst's spectrum and
duration. A detector's overall burst sensitivity depends on its energy
sensitivity and set of accumulations times Delta t; these two factors shape the
detected burst population. For example, relative to BATSE, the BAT's softer
energy band decreases the detection rate of short, hard bursts, while the BAT's
longer accumulation times increase the detection rate of long, soft bursts.
Consequently, Swift is detecting long, low fluence bursts (2-3x fainter than
BATSE).Comment: To appear in the proceedings of the 16th Annual October Astrophysics
Conference in Maryland, "Gamma Ray Bursts in the Swift Era", Washington DC.,
November 29-December 2, 2005, 4 pages, 3 figure
Sky Coverage and Burst Repetition
To investigate the repeater content of gamma ray burst samples I develop two
models where sources burst at a constant average rate. I find that the sky
coverage affects the number of repeaters in a sample predominantly through the
detector livetime, and that the number of bursts in the sample is the primary
parameter. Thus the repeater content of burst samples should be compared within
the context of a repetition model; a direct comparison between two samples is
possible only if the samples have similar sizes. The observed repeater fraction
may not be the actual fraction if the sources burst on average less than once
during the detector livetime. Sources which burst repeatedly during active
phases separated by more than the observation period must be treated
separately.Comment: 12 pages AASTeX 4.0, 3 figures on request from [email protected]
Intensity Distributions of Gamma-Ray Bursts
Observations of individual bursts chosen by the vagaries of telescope
availability demonstrated that bursts are not standard candles and that their
apparent energy can be as great as 10^54 erg. However, determining the
distribution of their apparent energy (and of other burst properties) requires
the statistical analysis of a well-defined burst sample; the sample definition
includes the threshold for including a burst in the sample. Thus optical groups
need to the criteria behind the decision to search for a spectroscopic
redshift. Currently the burst samples are insufficient to choose between
lognormal and power law functional forms of the distribution, and the parameter
values for these functional forms differ between burst samples. Similarly, the
actual intensity distribution may be broader than observed, with a low energy
tail extending below the detection threshold.Comment: For the proceedings of GAMMA 2001 (Baltimore, April 4-6, 2001), 5
page
Properties of GRB Host Galaxies
The transients following GRB970228 and GRB970508 showed that these (and
probably all) GRBs are cosmological. However, the host galaxies expected to be
associated with these and other bursts are largely absent, indicating that
either bursts are further than expected or the host galaxies are underluminous.
This apparent discrepancy does not invalidate the cosmological hypothesis, but
instead host galaxy observations can test more sophisticated models.Comment: 5 pages, AIPPROC LaTeX, to appear in "Gamma-Ray Bursts, 4th
Huntsville Symposium," eds. C. Meegan, R. Preece and T. Koshu
Modelling crystal aggregation and deposition\ud in the catheterised lower urinary tract
Urethral catheters often become encrusted with crystals of magnesium struvite and calcium phosphate. The encrustation can block the catheter, which can cause urine retention in the bladder and reflux into the kidneys. We develop a mathematical model to investigate crystal deposition on the catheter surface, modelling the bladder as a reservoir of fluid and the urethral catheter as a rigid channel. At a constant rate, fluid containing crystal particles of unit size enters the reservoir, and flows from the reservoir through the channel and out of the system. The crystal particles aggregate, which we model using Becker–Döring coagulation theory, and are advected through the channel, where they continue to aggregate and are deposited on the channel’s walls. Inhibitor particles also enter the reservoir, and can bind to the crystals, preventing further aggregation and deposition. The crystal concentrations are spatially homogeneous in the reservoir, whereas the channel concentrations vary spatially as a result of advection, diffusion and deposition. We investigate the effect of inhibitor particles on the amount of deposition. For all parameter values, we find that crystals deposit along the full length of the channel, with maximum deposition close to the channel’s entrance
The role of Sox9 in mouse mammary gland development and maintenance of mammary stem and luminal progenitor cells.
BackgroundIdentification and characterization of molecular controls that regulate mammary stem and progenitor cell homeostasis are critical to our understanding of normal mammary gland development and its pathology.ResultsWe demonstrate that conditional knockout of Sox9 in the mouse mammary gland results in impaired postnatal development. In short-term lineage tracing in the postnatal mouse mammary gland using Sox9-CreER driven reporters, Sox9 marked primarily the luminal progenitors and bipotent stem/progenitor cells within the basal mammary epithelial compartment. In contrast, long-term lineage tracing studies demonstrate that Sox9+ precursors gave rise to both luminal and myoepithelial cell lineages. Finally, fate mapping of Sox9 deleted cells demonstrates that Sox9 is essential for luminal, but not myoepithelial, lineage commitment and proliferation.ConclusionsThese studies identify Sox9 as a key regulator of mammary gland development and stem/progenitor maintenance
A Gamma-Ray Burst Trigger Toolkit
(Abbreviated) The detection rate of a GRB detector can be increased by using
a count rate trigger with many accumulation times Dt and energy bands DE, but
the nominal sensitivity is less important than how much fainter a burst could
be at the detection threshold as Dt and DE are changed. Predictions of the
detection rate depend on the assumed burst population, which can be wildly in
error. I base the fiducial rate on the BATSE observations: 550 bursts per sky
above a peak flux of 0.3 ph/cm^2/s averaged over Dt=1.024s and DE=50-300 keV. I
find that triggering on any value of Dt decreases the average threshold peak
flux on the 1.024s timescale by a factor of 0.6. Extending DE to lower energies
includes the large flux of the X-ray background, increasing the background
count rate; a low energy DE is advantageous only for very soft bursts. Whether
a large fraction of the population of bright bursts is soft is disputed; the
new population of X-ray Flashes is soft but relatively faint.Comment: To appear in the Ap.J., v.578. 12 pages text, 4 figures. Minor
changes including discussion of burst statistic
Post-Launch Analysis of Swift's Gamma-Ray Burst Detection Sensitivity
The dependence of Swift's detection sensitivity on a burst's temporal and
spectral properties shapes the detected burst population. Using simplified
models of the detector hardware and the burst trigger system I find that Swift
is more sensitive to long, soft bursts than CGRO's BATSE, a reference detector
because of the large burst database it accumulated. Thus Swift has increased
sensitivity in the parameter space region into which time dilation and spectral
redshifting shift high redshift bursts.Comment: 20 pages, 8 figures, Ap. J. in pres
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