4,800 research outputs found
Hard X-ray astrophysics
Past hard X-ray and lower energy satellite instruments are reviewed and it is shown that observation above 20 keV and up to hundreds of keV can provide much valuable information on the astrophysics of cosmic sources. To calculate possible sensitivities of future arrays, the efficiencies of a one-atmosphere inch gas counter (the HEAO-1 A-2 xenon filled HED3) and a 3 mm phoswich scintillator (the HEAO-1 A-4 Na1 LED1) were compared. Above 15 keV, the scintillator was more efficient. In a similar comparison, the sensitivity of germanium detectors did not differ much from that of the scintillators, except at high energies where the sensitivity would remain flat and not rise with loss of efficiency. Questions to be addressed concerning the physics of active galaxies and the diffuse radiation background, black holes, radio pulsars, X-ray pulsars, and galactic clusters are examined
Magnetic Field Limits on SGRs
We measure the period and spin-down rate for SGR 1900+14 during the
quiescient period two years before the recent interval of renewed burst
activity. We find that the spin-down rate doubled during the burst activity
which is inconsistent with both mangetic dipole driven spin down and a magnetic
field energy source for the bursts. We also show that SGRs 1900+14 and 1806-20
have braking indices of 1 which indicate that the spin-down is due to
wind torques and not magnetic dipole radiation. We further show that a
combination of dipole radiation, and wind luminosity, coupled with estimated
ages and present spin parameters, imply that the magnetic fields of SGRs
1900+14 and 1806-20 are less than the critical field of 4 G and
that the efficiency for conversion of wind luminosity to x-ray luminosity is
<2%.Comment: 5 pages, 2 figures submitted to 5th Huntsville GRB Symposium
proceeding
Parameterization of the set of positive- definite matrices and an algorithm for its generation
Numerical analysis on set elements for positive definite symmetric matrices by parameterizatio
Is SGR 1900+14 a Magnetar?
We present RXTE observations of the soft gamma--ray repeater SGR 1900+14
taken September 4-18, 1996, nearly 2 years before the 1998 active period of the
source. The pulsar period (P) of 5.1558199 +/- 0.0000029 s and period
derivative (Pdot) of (6.0 +/- 1.0) X 10^-11 s/s measured during the 2-week
observation are consistent with the mean Pdot of (6.126 +/- 0.006) X 10^-11 s/s
over the time up to the commencement of the active period. This Pdot is less
than half that of (12.77 +/- 0.01) X 10^-11 s/s observed during and after the
active period. If magnetic dipole radiation were the primary cause of the
pulsar spindown, the implied pulsar magnetic field would exceed the critical
field of 4.4 X 10^13 G by more than an order of magnitude, and such field
estimates for this and other SGRs have been offered as evidence that the SGRs
are magnetars, in which the neutron star magnetic energy exceeds the rotational
energy. The observed doubling of Pdot, however, would suggest that the pulsar
magnetic field energy increased by more than 100% as the source entered an
active phase, which seems very hard to reconcile with models in which the SGR
bursts are powered by the release of magnetic energy. Because of this, we
suggest that the spindown of SGR pulsars is not driven by magnetic dipole
radiation, but by some other process, most likely a relativistic wind. The
Pdot, therefore, does not provide a measure of the pulsar magnetic field
strength, nor evidence for a magnetar.Comment: 14 pages, aasms4 latex, figures 1 & 2 changed, accepted by ApJ
letter
Algorithm for Liapunov stability analysis
Development of algorithm provides automatic computation of quadratic estimate of domain of stability for stable equilibrium states of nonlinear systems of ordinary differential equations
Paleobiomarkers and defining exobiology experiments for future Mars experiments
Mars is a cold, dry planet with an oxidizing surface bombarded by ultraviolet and ionizing radiation, making prospects for an extant Mars biota bleak. Yet, it is suggested that early Earth and early Mars were similar enought that life may have evoled on Mars. If life did evolve on Mars, what evidence for its existence might we find? What constitutes a Martian paleobiomarker, and how can we distinguish such a marker from abiotically produced substances? The topics studied to answer this question include carbon and nitrogen cycling, as well as the stability and relative abundance of their intermediates in microbially dominated ecosystems. The microbially dominated ecosystems studied are the crytoendolithic microbial community living within sand rocks, the endoevaporite microbial community living inside salt crystals, and the microbial communities living beneath perennially ice-covered lakes and hypersaline ponds. The nitrogen cycle of these communities ranges from simple, where only assimilation occurs, to the more complex, where a complete cycle occurs. The carbon cycle of these communities appears to be complete
Renewable Energy Resources Impact on Clean Electrical Power by developing the North-West England Hydro Resource Model.
This paper describes the development of a sequential decision support system to promote hydroelectric power in North-West England. The system, composed of integrated models, addresses barriers to the installation of hydroelectric power schemes. Information is linked through an economic assessment which identifies different turbine options, assesses their suitability for location and demand; and combines the different types of information in a way that supports decision making. The system is structured into five components: the hydrological resource is modelled using Low Flows 2000, the turbine options are identified from hydrological, environmental and demand requirements; and the consequences of different solutions will be fed into other components so that the environmental impacts and public acceptability can be assessed and valued. A preliminary case study is presented on an old gunpowder works to illustrate how the resource model may be employed. Historical architectural structures, power uptake and educational instruction of hydro power technology are considered
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