685 research outputs found
Turbulence transport throughout the heliosphere
We employ a turbulence transport model to compute distributions of turbulence throughout the heliosphere. The model determines the radial dependence of three (coupled) quantities that characterize interplanetary turbulence, the energy per unit mass, the cross helicity or Alfvénicity, and a similarity length scale. A fourth integrated quantity, the plasma temperature, is modified by heat deposition due to turbulent dissipation. The model includes advection, expansion, and reflection effects as well as the tendency toward dynamic alignment, and a von Kármán type dissipation function that represents decay of turbulence due to cascade to small scales. Two types of forcing are also featured, one a simple model of stream shear, and the other a driving in the outer heliosphere associated with wave energy injection due to pickup protons of interstellar origin. Parameters for the model have been tuned using observation data from Voyager and Ulysses. We analyze the constraining observations to provide boundary conditions and parameters that vary with heliocentric latitude, with some extrapolations. The fully assembled model permits the computation of the distribution of turbulence throughout the entire heliosphere, and we present solutions for several appropriate parameter sets
Period derivative of the M15 X-ray Binary AC211/X2127+119
We have combined Rossi X-ray Timing Explorer observations of X2127+119, the
low-mass X-ray binary in the globular cluster M15, with archival X-ray
lightcurves to study the stability of the 17.1 hr orbital period. We find that
the data cannot be fit by the Ilovaisky (1993) ephemeris, and requires either a
7sigma change to the period or a period derivative Pdot/P~9x10e-7 per year.
Given its remarkably low L_X/L_opt such a Pdot lends support to models that
require super-Eddington mass transfer in a q~1 binary.Comment: 11 pages, 3 figures, to be published in New Astronom
Long-Term X-ray Variability in GX 354-0
We report for the first time the detection of long-term X-ray variability in
the bright bulge source GX 354-0 (=4U 1728-34) observed with the All Sky
Monitor (ASM) on board the Rossi X-Ray Timing Explorer (RXTE). The 2-year RXTE
ASM database reveals significant power at ~72 days. Similar behaviour was seen
in the 6-year Ariel 5 ASM database, but at a period of ~63 days. The timescales
and light curves resemble the ~78 days modulation seen in Cyg X-2 and we
therefore interpret this modulation in GX 354-0 as a super-orbital effect.Comment: 9 pages, 3 figures, accepted for publication in New Astronom
Using optical spectroscopy to map the geometry and structure of the irradiated accretion discs in low-mass X-ray binaries:the pilot study of MAXI J0637-430
The recurring transient outbursts in low-mass X-ray binaries (LMXBs) provide
us with strong test-beds for constraining the poorly understood accretion
process. While impossible to image directly, phase-resolved spectroscopy can
provide a powerful diagnostic to study their highly complex, time-dependent
accretion discs. We present an 8-month long multi-wavelength (UV, optical,
X-ray) monitoring campaign of the new candidate black hole LMXB MAXI
J0637430 throughout its 2019/2020 outburst, using the {\em Neil Gehrels
Swift Observatory}, as well as three quasi-simultaneous epochs of Gemini/GMOS
optical spectroscopy. We find evidence for the existence of a correlation
between the X-ray irradiation heating the accretion disc and the evolution of
the He {\sc ii} 4686 \AA emission line profiles detected in the optical
spectra. Our results demonstrate a connection between the line emitting regions
and physical properties of the X-ray irradiation heating the discs during
outburst cycles of LMXBs. Further, we are able to show that changes in the
physical properties of the irradiation heating the disc in outburst can be
imprinted within the H/He emission line profiles themselves in these systems.Comment: 23 pages (including 3 appendices), 10 figures, supplementary figures
included in the appendices, accepted for publication in MNRA
Families of Quintic Calabi-Yau 3-Folds with Discrete Symmetries
At special loci in their moduli spaces, Calabi-Yau manifolds are endowed with
discrete symmetries. Over the years, such spaces have been intensely studied
and have found a variety of important applications. As string compactifications
they are phenomenologically favored, and considerably simplify many important
calculations. Mathematically, they provided the framework for the first
construction of mirror manifolds, and the resulting rational curve counts.
Thus, it is of significant interest to investigate such manifolds further. In
this paper, we consider several unexplored loci within familiar families of
Calabi-Yau hypersurfaces that have large but unexpected discrete symmetry
groups. By deriving, correcting, and generalizing a technique similar to that
of Candelas, de la Ossa and Rodriguez-Villegas, we find a calculationally
tractable means of finding the Picard-Fuchs equations satisfied by the periods
of all 3-forms in these families. To provide a modest point of comparison, we
then briefly investigate the relation between the size of the symmetry group
along these loci and the number of nonzero Yukawa couplings. We include an
introductory exposition of the mathematics involved, intended to be accessible
to physicists, in order to make the discussion self-contained.Comment: 54 pages, 3 figure
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