526 research outputs found
The doubling of the superorbital period of Cyg X-1
We study properties of the superorbital modulation of the X-ray emission of
Cyg X-1. We find that it has had a stable period of about 300 d in soft and
hard X-rays and in radio since 2005 until at least 2010, which is about double
the previously seen period. This new period, seen in the hard spectral state
only, is detected not only in the light curves but also in soft X-ray hardness
ratios and in the amplitude of the orbital modulation. On the other hand, the
spectral slope in hard X-rays, >20 keV, averaged over superorbital bins is
constant, and the soft and hard X-rays and the radio emission change in phase.
This shows that the superorbital variability consists of changing the
normalization of an intrinsic spectrum of a constant shape and of changes of
the absorbing column density with the phase. The maximum column density is
achieved at the superorbital minimum. The amplitude changes are likely to be
caused by a changing viewing angle of an anisotropic emitter, most likely a
precessing accretion disc. The constant shape of the intrinsic spectrum shows
that this modulation is not caused by a changing accretion rate. The modulated
absorbing column density shows the presence of a bulge around the disc centre,
as proposed previously. We also find the change of the superorbital period from
about 150 d to about 300 d to be associated with almost unchanged average X-ray
fluxes, making the period change difficult to explain in the framework of
disc-irradiation models. Finally, we find no correlation of the X-ray and radio
properties with the reported detections in the GeV and TeV gamma-ray range.Comment: MNRAS, in press, 8 page
Photoelastically induced light modulation in gradient index lenses
A new photoelastic light modulator is demonstrated based on the modulation of the birefringence and of the index profile in graded index lenses. Using the birefringence modulation we obtained 35% modulation depth in a quarter-pitch lens and 65% using a half pitch lens at acoustic frequencies up to the MHz range. Using the index profile modulation in a half-pitch lens as a fibre-to-fibre connector we obtained 15% modulation without the incorporation of any polarizer
Compositional marker in vivo reveals intramyocellular lipid turnover during fasting-induced lipolysis
Intramyocellular lipid (IMCL) is of particular metabolic interest, but despite many proton magnetic resonance spectroscopy (¹H MRS) studies reporting IMCL content measured by the methylene (CH₂) resonance signal, little is known about its composition. Here we validated IMCL CH₃:CH₂ ratio as a compositional marker using ¹H MRS at short echo time, and investigated IMCL content and composition during a 28-hour fast in 24 healthy males. Increases in IMCL CH₂ relative to the creatine and phosphocreatine resonance (Cr) at 3.0 ppm (an internal standard) correlated with circulating free fatty acid (FA) concentrations, supporting the concept of increased FA influx into IMCL. Significant decreases in IMCL CH₃:CH₂ ratio indicated a less unsaturated IMCL pool after fasting, and this compositional change related inversely to IMCL baseline composition, suggesting a selective efflux of unsaturated shorter-chain FA from the IMCL pool. This novel in vivo evidence reveals IMCL turnover during extended fasting, consistent with the concept of a flexible, responsive myocellular lipid store. There were also differences between soleus and tibialis anterior in basal IMCL composition and in response to fasting. We discuss the potential of this marker for providing insights into normal physiology and mechanisms of disease.We thank the participants, staff at the Cambridge NIHR/Wellcome Trust Clinical Research Facility and the Wolfson Brain Imaging Centre, Sarah Nutland (NIHR Cambridge BioResource, Cambridge, UK) for facilitating participant recruitment and Edwina French (MRC Laboratory of Molecular Biology, Cambridge, UK) for help with phantoms. We acknowledge grants from Addenbrooke’s Charitable Trust and the British Society for Pediatric Endocrinology and Diabetes. LH is a British Heart Foundation Senior Fellow in Basic Science. DBS is supported by the Wellcome Trust (107064). AT, AK and DBD are funded by the UK NIHR Cambridge Biomedical Research Centre and Medical Research Council (UD99999906), and AS by the NIHR via the NIHR Cambridge Clinical Research Facility
Outbursts of EX Hydrae: mass-transfer events or disc instabilities?
We present the 45-yr record of EX Hya's lightcurve and discuss the
characteristics of its 15 observed outbursts. We then concentrate on the 1998
outburst, reporting the first outburst X-ray observations. We discover an X-ray
beat-cycle modulation, indicating that an enhanced accretion stream couples
directly with the magnetosphere in outburst, confirming our previous
prediction. Optical eclipse profiles late in outburst show that the visible
light is dominated by an enhanced mass-transfer stream overflowing the
accretion disc. We are uncertain whether the enhanced mass transfer is
triggered by a disc instability, or by some other cause. While in outburst, EX
Hya shows some of the characteristics of SW Sex stars.Comment: To appear in MNRAS (8 pages; 9 figs
Spatial and Temporal Variations in Interstellar Absorption toward HD 72127AB
New optical spectra of Ca II and Na I toward HD 72127AB provide additional
evidence for both spatial and temporal variations in the complex interstellar
absorption along the two sight lines; archival UV spectra yield information on
the abundances, depletions, and physical conditions in the gas toward HD
72127A. Similarities in the strengths of various tracers of interstellar
material in the two lines of sight suggest that the total hydrogen column
densities [N(H) ~ 2.5 x 10^{20} cm^{-2}] and the depletions and ionization in
the main components at low LSR velocities also are similar. Toward HD 72127A,
the main components are relatively cool (T < 900 K), but with depletions
resembling those found in warm, diffuse disc clouds; the generally weaker
components at higher velocities have much milder depletions, more like those
found in halo clouds. Several trace neutral species -- Ca I, Cr I, and Fe I --
are much stronger toward HD 72127B, however. The column density of Cr I, for
example, is about 30 times the value determined toward zeta Oph (the only
previous detection of that species in the ISM). Dielectronic recombination in
warmer gas (T > 5000 K) may be largely responsible for the enhanced abundances
of those trace neutral species toward HD 72127B. If the main components toward
HD 72127AB are associated with material in the Vela SNR, the differences in
abundances and physical conditions occur on scales of about 1100 AU.Comment: 13 pages, 4 figures, accepted to MNRA
Orbital and spin physics in LiNiO2 and NaNiO2
We derive a spin-orbital Hamiltonian for a triangular lattice of e_g orbital
degenerate (Ni^{3+}) transition metal ions interacting via 90 degree
superexchange involving (O^{2-}) anions, taking into account the on-site
Coulomb interactions on both the anions and the transition metal ions. The
derived interactions in the spin-orbital model are strongly frustrated, with
the strongest orbital interactions selecting different orbitals for pairs of Ni
ions along the three different lattice directions. In the orbital ordered
phase, favoured in mean field theory, the spin-orbital interaction can play an
important role by breaking the U(1) symmetry generated by the much stronger
orbital interaction and restoring the threefold symmetry of the lattice. As a
result the effective magnetic exchange is non-uniform and includes both
ferromagnetic and antiferromagnetic spin interactions. Since ferromagnetic
interactions still dominate, this offers yet insufficient explanation for the
absence of magnetic order and the low-temperature behaviour of the magnetic
susceptibility of stoichiometric LiNiO_2. The scenario proposed to explain the
observed difference in the physical properties of LiNiO_2 and NaNiO_2 includes
small covalency of Ni-O-Li-O-Ni bonds inducing weaker interplane superexchange
in LiNiO_2, insufficient to stabilize orbital long-range order in the presence
of stronger intraplane competition between superexchange and Jahn-Teller
coupling.Comment: 33 pages, 12 postscript figures, uses iopams.sty . This article
features in New Journal of Physics as part of a Focus Issue on Orbital
Physics - all contributions may be freely accessed at
(http://stacks.iop.org/1367-2630/6/i=1/a=E05). The published version of this
article may be found at http://stacks.iop.org/1367-2630/7/12
Blended learning environments to foster self-directed learning
This book on blended learning environments to foster self-directed learning highlights the focus on research conducted in several teaching and learning contexts where blended learning had been implemented and focused on the fostering of self-directed learning. Several authors have contributed to the book, and each chapter provides a unique perspective on blended learning and self-directed learning research. From each chapter, it becomes evident that coherence on the topics mentioned is established. One of the main aspects drawn in this book, and addressed by several authors in the book, is the use of the Community of Inquiry (CoI) framework when implementing teaching and learning strategies in blended learning environments to foster self-directed learning. This notion of focusing on the CoI framework is particularly evident in both theoretical and empirical dissemination presented in this book. What makes this book unique is the fact that researchers and peers in varied fields would benefit from the findings presented by each chapter, albeit theoretical, methodological or empirical in nature – this, in turn, provides opportunities for future research endeavours to further the narrative of how blended learning environments can be used to foster self-directed learning
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