15,556 research outputs found
Reference frequency transmission over optical fiber
A 100-MHz reference frequency from a hydrogen maser frequency standard has been transmitted via optical fiber over a 14-km distance with a measured stability of 1.5 X 10 to the-15 power for 1000 seconds averaging time. This capability was demonstrated in a frequency distribution experiment performed in April, 1986. The reference frequency was transmitted over a single-mode fiber-optic link from Deep Space Station (DSS) 13 to DSS 12 and back. The background leading up to the experiment and the significance of stable reference frequency distribution in the Deep Space Network (DSN) is discussed. Also described are the experiment, including the fiber-optic link, the measurement method and equipment, and finally the results of the experiment
Particle acceleration at ultrarelativistic shocks: an eigenfunction method
We extend the eigenfunction method of computing the power-law spectrum of
particles accelerated at a relativistic shock fronts to apply to shocks of
arbitrarily high Lorentz factor. In agreement with the findings of Monte-Carlo
simulations, we find the index of the power-law distribution of accelerated
particles which undergo isotropic diffusion in angle at an ultrarelativistic,
unmagnetized shock is s=4.23 (where s=-d(ln f)/dp with f the Lorentz invariant
phase-space density and p the momentum). This corresponds to a synchrotron
index for uncooled electrons of a=0.62 (taking cooling into account a=1.12),
where a=-d(ln F)/dn, F is the radiation flux and n the frequency. We also
present an approximate analytic expression for the angular distribution of
accelerated particles, which displays the effect of particle trapping by the
shock: compared with the non-relativistic case the angular distribution is
weighted more towards the plane of the shock and away from its normal. We
investigate the sensitivity of our results to the transport properties of the
particles and the presence of a magnetic field. Shocks in which the ratio of
Poynting to kinetic energy flux upstream is not small are less compressive and
lead to larger values of .Comment: Minor additions on publicatio
External Compton emission from relativistic jets in Galactic black hole candidates and ultraluminous X-ray sources
Galactic binary systems that contain a black hole candidate emit hard X-rays
in their low luminosity mode. We show that this emission can be understood as
due to the Compton scattering of photons from the companion star and/or the
accretion disk by relativistic electrons in a jet. The same electrons are also
responsible for the radio emission. Two sources -- XTE J1118+480 and Cygnus X-1
-- are modelled as representatives of black holes with low and high luminosity
companion stars respectively. We further show that the ultraluminous compact
X-ray sources observed in nearby galaxies have the properties expected of
stellar mass black holes with high luminosity companions in which the jet is
oriented close to our line of sight.Comment: Submitted to A&A letters, Oct 16, 200
Spectral and temporal signatures of ultrarelativistic protons in compact sources
We present calculations of the spectral and temporal radiative signatures
expected from ultrarelativistic protons in compact sources. The coupling
between the protons and the leptonic component is assumed to occur via
Bethe-Heitler pair production. This process is treated by modeling the results
of Monte-Carlo simulations and incorporating them in a time-dependent kinetic
equation, that we subsequently solve numerically. Thus, the present work is, in
many respects, an extension of the leptonic `one-zone' models to include
hadrons. Several examples of astrophysical importance are presented, such as
the signature resulting from the cooling of relativistic protons on an external
black-body field and that of their cooling in the presence of radiation from
injected electrons. We also investigate and refine the threshold conditions for
the 'Pair Production/Synchrotron' feedback loop which operates when
relativistic protons cool efficiently on the synchrotron radiation of the
internally produced Bethe-Heitler pairs. We demonstrate that an additional
component of injected electrons lowers the threshold for this instability.Comment: 12 pages, 14 figures, accepted for publication in A&
A wind model for high energy pulses
A solution to the sigma problem - that of finding a mechanism capable of
converting Poynting energy flux to particle-borne energy flux in a pulsar wind
- was proposed several years ago by Coroniti and Michel who considered a
particular prescription for magnetic reconnection in a striped wind. This
prescription was later shown to be ineffective. In this paper, we discuss the
basic microphysics of the reconnection process and conclude that a more rapid
prescription is permissible. Assuming dissipation to set in at some distance
outside the light-cylinder, we compute the resulting radiation signature and
find that the synchrotron emission of heated particles appears periodic, in
general showing both a pulse and an interpulse. The predicted spacing of these
agrees well with observation in the case of the Crab and Vela pulsars. Using
parameters appropriate for the Crab pulsar - magnetization parameter at the
light cylinder sigma_L = 6 x 10^4, Lorentz factor Gamma=250 - reasonable
agreement is found with the observed total pulsed luminosity. This suggest that
the high-energy pulses from young pulsars originate not in the co-rotating
magnetosphere within the light cylinder (as in all other models) but from the
radially directed wind well outside it.Comment: 6 pages, 2 figures. To appear in the Proceedings of the 270.
WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants, Jan.
21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J. Truemper.
Proceedings are available as MPE-Report 27
Pair plasma cushions in the hole-boring scenario
Pulses from a 10 PW laser are predicted to produce large numbers of
gamma-rays and electron-positron pairs on hitting a solid target. However, a
pair plasma, if it accumulates in front of the target, may partially shield it
from the pulse. Using stationary, one-dimensional solutions of the two-fluid
(electron-positron) and Maxwell equations, including a classical radiation
reaction term, we examine this effect in the hole-boring scenario. We find the
collective effects of a pair plasma "cushion" substantially reduce the
reflectivity, converting the absorbed flux into high-energy gamma-rays. There
is also a modest increase in the laser intensity needed to achieve threshold
for a non-linear pair cascade.Comment: 17 pages, 5 figures. Accepted for publication in Plasma Physics and
Controlled Fusion. Typos corrected, reference update
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