1,194 research outputs found
Alignment Timescale of the Microquasar GRO J1655-40
The microquasar GRO J1655-40 has a black hole with spin angular momentum
apparently misaligned to the orbital plane of its companion star. We
analytically model the system with a steady state disc warped by Lense-Thirring
precession and find the timescale for the alignment of the black hole with the
binary orbit. We make detailed stellar evolution models so as to estimate the
accretion rate and the lifetime of the system in this state. The secondary can
be evolving at the end of the main sequence or across the Hertzsprung gap. The
mass-transfer rate is typically fifty times higher in the latter case but we
find that, in both cases, the lifetime of the mass transfer state is at most a
few times the alignment timescale. The fact that the black hole has not yet
aligned with the orbital plane is therefore consistent with either model. We
conclude that the system may or may not have been counter-aligned after its
supernova kick but that it is most likely to be close to alignment rather than
counteralignment now.Comment: Accepted for publication in MNRA
The Evolutionary Status of SS433
We consider possible evolutionary models for SS 433. We assume that
common-envelope evolution is avoided if radiation pressure is able to expel
most of a super-Eddington accretion flow from a region smaller than the
accretor's Roche lobe. This condition is satisfied, at least initially, for
largely radiative donors with masses in the range 4-12 solar masses. For donors
more massive than about 5 solar masses, moderate mass ratios q = M_2/M_1 > 1
are indicated, thus tending to favor black-hole accretors. For lower mass
donors, evolutionary considerations do not distinguish between a neutron star
or black hole accretor. In all cases the mass transfer (and mass loss) rates
are much larger than the likely mass-loss rate in the precessing jets. Almost
all of the transferred mass is expelled at radii considerably larger than the
jet acceleration region, producing the "stationary" H-alpha line, the infrared
luminosity, and accounting for the low X-ray luminosity.Comment: 13 pages, Astrophysical Journal Letters, accepte
Effect of low-Raman window position on correlated photon-pair generation in a chalcogenide Ge11.5As24Se64.5 nanowire
We investigated correlated photon-pair generation via spontaneous four-wave mixing in an integrated chalcogenideGe11.5As24Se64.5photonicnanowire. The coincidence to accidental ratio, a key measurement for the quality of correlated photon-pair sources, was measured to be only 0.4 when the photon pairs were generated at 1.9 THz detuning from the pump frequency due to high spontaneous Raman noise in this regime. However, the existence of a characteristic low-Raman window at around 5.1 THz in this material's Raman spectrum and dispersion engineering of the nanowire allowed us to generate photon pairs with a coincidence to accidental ratio of 4.5, more than 10 times higher than the 1.9 THz case. Through comparing the results with those achieved in chalcogenide As2S3waveguides which also exhibit a low Raman-window but at a larger detuning of 7.4 THz, we find that the position of the characteristic low-Raman window plays an important role on reducing spontaneous Raman noise because the phonon population is higher at smaller detuning. Therefore the ultimate solution for Raman noise reduction in Ge11.5As24Se64.5 is to generate photon pairs outside the Raman gain band at more than 10 THz detuning
An evolutionary study of the pulsating subdwarf B eclipsing binary PG1336-018 (NY Vir)
The formation of subdwarf B (sdB) stars is not well understood within the
current framework of stellar single and binary evolution. In this study, we
focus on the formation and evolution of the pulsating sdB star in the very
short-period eclipsing binary PG1336-018. We aim at refining the formation
scenario of this unique system, so that it can be confronted with observations.
We probe the stellar structure of the progenitors of sdB stars in short-period
binaries using detailed stellar evolution calculations. Applying this to
PG1336-018 we reconstruct the common-envelope phase during which the sdB star
was formed. The results are interpreted in terms of the standard
common-envelope formalism (the alpha-formalism) based on the energy equation,
and an alternative description (the gamma-formalism) using the angular momentum
equation. We find that if the common-envelope evolution is described by the
alpha-formalism, the sdB progenitor most likely experienced a helium flash. We
then expect the sdB mass to be between 0.39 and 0.48 Msun, and the sdB
progenitor initial mass to be below ~2 Msun. However, the results for the
gamma-formalism are less restrictive, and a broader sdB mass range (0.3 - 0.8
Msun) is possible in this case. Future seismic mass determination will give
strong constraints on the formation of PG1336-018 and, in particular, on the CE
phase.Comment: 9 pages, 7 figures, 2 tables, accepted for publication in A&
Hybrid photonic circuit for multiplexed heralded single photons
A key resource for quantum optics experiments is an on-demand source of
single and multiple photon states at telecommunication wavelengths. This letter
presents a heralded single photon source based on a hybrid technology approach,
combining high efficiency periodically poled lithium niobate waveguides,
low-loss laser inscribed circuits, and fast (>1 MHz) fibre coupled
electro-optic switches. Hybrid interfacing different platforms is a promising
route to exploiting the advantages of existing technology and has permitted the
demonstration of the multiplexing of four identical sources of single photons
to one output. Since this is an integrated technology, it provides scalability
and can immediately leverage any improvements in transmission, detection and
photon production efficiencies.Comment: 5 pages, double column, 3 figure
Determining solar abundances using helioseismology
The recent downward revision of solar photospheric abundances of Oxygen and
other heavy elements has resulted in serious discrepancies between solar models
and solar structure as determined through helioseismology. In this work we
investigate the possibility of determining the solar heavy-element abundance
without reference to spectroscopy by using helioseismic data. Using the
dimensionless sound-speed derivative in the solar convection zone, we find that
the heavy element abundance, Z, of 0.0172 +/- 0.002, which is closer to the
older, higher value of the abundances.Comment: To appear in Ap
Slow-light optical bullets in arrays of nonlinear Bragg-grating waveguides
We demonstrate how to control independently both spatial and temporal
dynamics of slow light. We reveal that specially designed nonlinear waveguide
arrays with phase-shifted Bragg gratings demonstrate the frequency-independent
spatial diffraction near the edge of the photonic bandgap, where the group
velocity of light can be strongly reduced. We show in numerical simulations
that such structures allow a great flexibility in designing and controlling
dispersion characteristics, and open a way for efficient spatiotemporal
self-trapping and the formation of slow-light optical bullets.Comment: 4 pages, 4 figures; available from
http://link.aps.org/abstract/PRL/v97/e23390
Integrated spatial multiplexing of heralded single photon sources
The non-deterministic nature of photon sources is a key limitation for single
photon quantum processors. Spatial multiplexing overcomes this by enhancing the
heralded single photon yield without enhancing the output noise. Here the
intrinsic statistical limit of an individual source is surpassed by spatially
multiplexing two monolithic silicon correlated photon pair sources,
demonstrating a 62.4% increase in the heralded single photon output without an
increase in unwanted multi-pair generation. We further demonstrate the
scalability of this scheme by multiplexing photons generated in two waveguides
pumped via an integrated coupler with a 63.1% increase in the heralded photon
rate. This demonstration paves the way for a scalable architecture for
multiplexing many photon sources in a compact integrated platform and achieving
efficient two photon interference, required at the core of optical quantum
computing and quantum communication protocols.Comment: 10 pages, 3 figures, comments welcom
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