12,674 research outputs found
Gamma-Ray Bursts and Cosmology
The unrivalled, extreme luminosities of gamma-ray bursts (GRBs) make them the
favored beacons for sampling the high redshift Universe. To employ GRBs to
study the cosmic terrain -- e.g., star and galaxy formation history -- GRB
luminosities must be calibrated, and the luminosity function versus redshift
must be measured or inferred. Several nascent relationships between gamma-ray
temporal or spectral indicators and luminosity or total energy have been
reported. These measures promise to further our understanding of GRBs once the
connections between the luminosity indicators and GRB jets and emission
mechanisms are better elucidated. The current distribution of 33 redshifts
determined from host galaxies and afterglows peaks near z ~ 1, whereas for the
full BATSE sample of long bursts, the lag-luminosity relation predicts a broad
peak z ~ 1-4 with a tail to z ~ 20, in rough agreement with theoretical models
based on star formation considerations. For some GRB subclasses and apparently
related phenomena -- short bursts, long-lag bursts, and X-ray flashes -- the
present information on their redshift distributions is sparse or entirely
lacking, and progress is expected in Swift era when prompt alerts become
numerous.Comment: Invited talk given at the JENAM 2003 Minisymposium on Physics of
Gamma-Ray Bursts, August 29-30, 2003, Budapest, Hungary; 6 pages, 1 figur
The Life and Times of the Parkes-Tidbinbilla Interferometer
The Parkes-Tidbinbilla took advantage of a real-time radio-link connecting
the Parkes and Tidbinbilla antennas to form the world's longest real-time
interferometer. Built on a minuscule budget, it was an extraordinarily
successful instrument, generating some 24 journal papers including 3 Nature
papers, as well as facilitating the early development of the Australia
Telescope Compact Array. Here we describe its origins, construction, successes,
and life cycle, and discuss the future use of single-baseline interferometers
in the era of SKA and its pathfinders.Comment: Accepted by Journal of Astronomical History & Heritage. arXiv admin
note: substantial text overlap with arXiv:1210.098
A high pressure, high temperature combustor and turbine-cooling test facility
A new test facility is being constructed for developing turbine-cooling and combustor technology for future generation aircraft gas turbine engines. Prototype engine hardware will be investigated in this new facility at gas stream conditions up to 2480 K average turbine inlet temperature and 4.14 x 10 to the 6th power n sq m turbine inlet pressure. The facility will have the unique feature of fully automated control and data acquisition through the use of an integrated system of mini-computers and programmable controllers which will result in more effective use of operating time, will limit the number of operators required, and will provide built in self protection safety systems. The facility and the planning and design considerations are described
Neural network classification of questionable EGRET events
High energy gamma rays (greater than 20 MeV) pair producing in the spark chamber of the Energetic Gamma Ray Telescope Experiment (EGRET) give rise to a characteristic but highly variable 3-D locus of spark sites, which must be processed to decide whether the event is to be included in the database. A significant fraction (about 15 percent or 10(exp 4) events/day) of the candidate events cannot be categorized (accept/reject) by an automated rule-based procedure; they are therefore tagged, and must be examined and classified manually by a team of expert analysts. We describe a feedforward, back-propagation neural network approach to the classification of the questionable events. The algorithm computes a set of coefficients using representative exemplars drawn from the preclassified set of questionable events. These coefficients map a given input event into a decision vector that, ideally, describes the correct disposition of the event. The net's accuracy is then tested using a different subset of preclassified events. Preliminary results demonstrate the net's ability to correctly classify a large proportion of the events for some categories of questionables. Current work includes the use of much larger training sets to improve the accuracy of the net
Experimental performance of a conical pressure probe at Mach numbers of 3.0, 4.5, and 6.0
Wind tunnel investigation of performance of conical pressure probe at hypersonic speed
Calibration of Tests for Time Dilation in GRB Pulse Structures
Two tests for cosmological time dilation in -ray bursts -- the peak
alignment and auto-correlation statistics -- involve averaging information near
the times of peak intensity. Both tests require width corrections, assuming
cosmological origin for bursts, since narrower temporal structure from higher
energy would be redshifted into the band of observation, and since intervals
between pulse structures are included in the averaging procedures. We analyze
long ( 2 s) BATSE bursts and estimate total width corrections for trial
time-dilation factors (TDF = [1+]/[1+]) by
time-dilating and redshifting bright bursts. Both tests reveal significant
trends of increasing TDF with decreasing peak flux, but neither provides
sufficient discriminatory power to distinguish between actual TDFs in the range
2--3.Comment: 5 pages in LATeX, REVTEX style, 2 embedded figures. To appear in
Third Huntsville GRB Workshop Proceeding
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