9,058 research outputs found
The X-ray Spectrum of Soft Gamma Repeater 1806-20
Soft Gamma Repeaters (SGRs) are a class of rare, high-energy galactic
transients that have episodes of short (~0.1 sec), soft (~30 keV), intense
(~100 Crab), gamma-ray bursts. We report an analysis of the x-ray emission from
95 SGR1806-20 events observed by the International Cometary Explorer. The
spectral shape remains remarkably constant for bursts that differ in intensity
by a range of 50. Below 15 keV the number spectrum falls off rapidly such that
we can estimate the total intensity of the events. Assuming that SGR1806-20 is
associated with the supernova remnant G10.0-0.3 (Kulkarni and Frail, Murakami
\etal), the brightest events had a total luminosity of ~1.8 x 10^42 erg sec^-1,
a factor of 2 x 10^4 above the Eddington limit. A third of the emission was
above 30 keV. There are at least three processes that are consistent with the
spectral rollover below 15 keV. (1)The rollover is consistent with some forms
of self absorption. Typical thermal temperatures are ~20 keV and require an
emitting surface with a radius between 10 and 50 km. The lack of spectral
variability implies that only the size of the emitting surface varies between
events. If the process is thermal synchrotron the required magnetic field might
be too small to confine the plasma against the super Eddington flux. (2)The low
energy rollover could be due to photoelectric absorption by ~10^24 Hydrogen
atoms cm^-2 of neutral material with a cosmic abundance assuming a continuum
similar to TB with T= ~22 keV. (3) Emission in the two lowest harmonics from a
1.3 x 10^12 Gauss field would appear as Doppler broadened lines and fall off
rapidly below 15 keV.Comment: TeX: 32 pg+ 8 appended postscript figures, in press ApJ(9/94
Constraints on the Gamma-ray Burst Luminosity Function from PVO and BATSE
We examine the width of the gamma-ray burst luminosity function through the
distribution of GRB peak fluxes as detected by the Pioneer Venus Orbiter (PVO)
and the Burst and Transient Source Experiment (BATSE). The strength of the
analysis is greatly enhanced by using a merged catalog of peak fluxes from both
instruments with good cross-calibration of their sensitivities. The range of
peak fluxes is increased by approximately a factor of 20 relative to the BATSE
catalog. Thus, more sensitive investigations of the
distribution are possible. We place constraints on the width of the luminosity
function of gamma-ray bursts brighter than the BATSE completeness limit by
comparing the intensity distribution in the merged catalog with those produced
by a variety of spatial density and luminosity functions. For the models
examined, of the {\em detectable\/} bursts have peak luminosities within
a range of 10, indicating that the peak luminosities of gamma-ray bursts span a
markedly less wide range of values than many other of their measurable
properties. We also discuss for which slopes of a power-law luminosity function
the observed width is at the upper end of the constrained range. This is
important in determining the power-law slopes for which luminosity-duration
correlations could be important.Comment: 10 pages latex + 2 uuencoded figures; APJL accepte
Market assessment of photovoltaic power systems for agricultural applications worldwide
Agricultural sector PV market assessments conducted in the Phillippines, Nigeria, Mexico, Morocco, and Colombia are extrapolated worldwide. The types of applications evaluated are those requiring less than 15 kW of power and operate in a stand alone mode. The major conclusions were as follows: PV will be competitive in applications requiring 2 to 3 kW of power prior to 1983; by 1986 PV system competitiveness will extend to applications requiring 4 to 6 kW of power, due to capital constraints, the private sector market may be restricted to applications requiring less than about 2 kW of power; the ultimate purchase of larger systems will be governments, either through direct purchase or loans from development banks. Though fragmented, a significant agriculture sector market for PV exists; however, the market for PV in telecommunications, signalling, rural services, and TV will be larger. Major market related factors influencing the potential for U.S. PV Sales are: lack of awareness; high first costs; shortage of long term capital; competition from German, French and Japanese companies who have government support; and low fuel prices in capital surplus countries. Strategies that may aid in overcoming some of these problems are: setting up of a trade association aimed at overcoming problems due to lack of awareness, innovative financing schemes such as lease arrangements, and designing products to match current user needs as opposed to attempting to change consumer behavior
Femtolensing: Beyond the Semi-Classical Approximation
Femtolensing is a gravitational lensing effect in which the magnification is
a function not only of the positions and sizes of the source and lens, but also
of the wavelength of light. Femtolensing is the only known effect of
(10^{-13}-10^{-16} M_{\sun}) dark-matter objects and may possibly be
detectable in cosmological gamma-ray burst spectra. We present a new and
efficient algorithm for femtolensing calculations in general potentials. The
physical-optics results presented here differ at low frequencies from the
semi-classical approximation, in which the flux is attributed to a finite
number of mutually coherent images. At higher frequencies, our results agree
well with the semi-classical predictions. Applying our method to a point-mass
lens with external shear, we find complex events that have structure at both
large and small spectral resolution. In this way, we show that femtolensing may
be observable for lenses up to solar masses, much larger than
previously believed. Additionally, we discuss the possibility of a search for
femtolensing of white dwarfs in the LMC at optical wavelengths.Comment: 21 LATEX(AASMS) pg + 5 append. ps figures, ApJ (sub
Tidal scattering of stars on supermassive black holes in galactic centers
Some of the mass that feeds the growth of a massive black hole (BH) in a
galactic center is supplied by tidal disruption of stars that approach it on
unbound, low angular momentum orbits. For each star that is disrupted, others
narrowly escape after being subjected to extreme tidal distortion, spin-up,
mixing and mass-loss, which may affect their evolution and appearance. We show
that it is likely that a significant fraction of the stars around massive BHs
in galactic centers have undergone such extreme tidal interactions and survived
subsequent total disruption, either by being deflected off their orbit or by
missing the BH due to its Brownian motion. We discuss possible long-term
observable consequences of this process, which may be relevant for
understanding the nature of stars in galactic centers, and may provide a
signature of the existence of massive BHs there.Comment: 5 pages 4 figures. ApJL in press, minor changes to reflect journal
version including redifinition of unbound tidally disturbed stars and
additional reference
Effects of Continued Selfing on Hymenolepis nano (Cestoda)
The effects of selfing on Hymenolepis nana were investigated by establishing single infections in mice, thus ensuring self-fertilization. Selfing appears to increase the frequency of cysticercoid abnormalities. The proportion of eggs developing into cysticercoids in Tribolium confusum appears to decrease with continued selfing. Similarly, the proportion of cysticercoids developing into adults in mice decreases with continued selfing. Selfing causes no observable variation in the size of either adults or cysticercoids, nor does it cause changes in the frequency of appearance of certain morphological variants of the adult worm. The variant of reversal of genital pore appears to cause sterility of those proglottids demonstrating this modification. No selfed strain could be maintained beyond the fifth selfed generation. A population of twenty adult worms, the number employed in control mice, does not appear sufficient to prevent the decrease in viability caused by selfing. Cercomer damage is of little importance in the infectivity of the cysticercoid
Observation of Spin Flips with a Single Trapped Proton
Radio-frequency induced spin transitions of one individual proton are
observed for the first time. The spin quantum jumps are detected via the
continuous Stern-Gerlach effect, which is used in an experiment with a single
proton stored in a cryogenic Penning trap. This is an important milestone
towards a direct high-precision measurement of the magnetic moment of the
proton and a new test of the matter-antimatter symmetry in the baryon sector
Direct high-precision measurement of the magnetic moment of the proton
The spin-magnetic moment of the proton is a fundamental property of
this particle. So far has only been measured indirectly, analysing the
spectrum of an atomic hydrogen maser in a magnetic field. Here, we report the
direct high-precision measurement of the magnetic moment of a single proton
using the double Penning-trap technique. We drive proton-spin quantum jumps by
a magnetic radio-frequency field in a Penning trap with a homogeneous magnetic
field. The induced spin-transitions are detected in a second trap with a strong
superimposed magnetic inhomogeneity. This enables the measurement of the
spin-flip probability as a function of the drive frequency. In each measurement
the proton's cyclotron frequency is used to determine the magnetic field of the
trap. From the normalized resonance curve, we extract the particle's magnetic
moment in units of the nuclear magneton . This
measurement outperforms previous Penning trap measurements in terms of
precision by a factor of about 760. It improves the precision of the forty year
old indirect measurement, in which significant theoretical bound state
corrections were required to obtain , by a factor of 3. By application
of this method to the antiproton magnetic moment the fractional
precision of the recently reported value can be improved by a factor of at
least 1000. Combined with the present result, this will provide a stringent
test of matter/antimatter symmetry with baryons.Comment: published in Natur
- …