4,217 research outputs found
Apsidal advance in SS 433?
Context. The Galactic microquasar SS 433 launches oppositely directed jets at
speeds approximately a quarter of the speed of light. Both the speed and
direction of the jets exhibit small fluctuations. A component of the speed
variation has 13 day periodicity and the orbital phase at which its maximum
speed occurs has advanced approximately 90 degrees in 25 years. Aims. To
examine the possibility that these variations are associated with a mildly
eccentric orbit and conditions necessary to achieve this apsidal advance.
Methods. The advance of the orbital phase for maximum speed is taken to be
advance of the apses of the putative elliptical orbit. It is compared with
calculations of the effects of tides induced in the companion and also with
gravitational perturbations from the circumbinary disc. These calculations are
made in the light of recent results on the SS 433 system. Results. The 13 day
periodicity in the speed of the jets of SS 433 might be attributed to a mildly
elliptical orbit, through periodic approaches of the donor and the compact
object. Advance of the apses of such an elliptical orbit due to tidal effects
induced in a normal companion looks to be to small; if caused by the
circumbinary disc the mass of the inner regions of that disc is ~ 0.15 solar
masses.Comment: 2 pages, no figures To appear in A & A. No significant changes from
original version poste
SS 433: The wiggle of the wind
The Balmer H alpha emission line in the stationary spectrum of SS 433 has a
component originating in the wind above the accretion disk. The Doppler motion
of this line is a blurred representation of the motion of the compact object
accreting. I show how this may be understood in terms of emission lasting over
a few days, like radiation from the jet bolides.Comment: 4 pages, 3 figures. A coda to my circumbinary disk studies. To appear
in A&
Interpretation of observations of the circumbinary disk of SS 433
Context. The Galactic microquasar SS 433 is possessed of a circumbinary disk
most clearly seen in the brilliant Balmer H alpha emission line. The orbital
speed of the glowing material is an important determinant of the mass of the
binary system. The circumbinary disk may be fed through the L2 point and in
turn may feed a very extended radio feature known as the ruff. Aims. To present
an analysis of spectroscopic optical data from H alpha and He I spectral lines
which reveal the circumbinary disk. To use comparisons of the rather different
signals to better understand the disk and improve estimates of the rotational
speed of the inner rim. To present a simple model which naturally explains some
apparently bizarre spectral variations with orbital phase. Methods. Published
spectra, taken almost nightly over two orbital periods of the binary system,
are analysed. H alpha and He I lines are analysed as superpositions of Gaussian
components and a simple model constructed. Results. The data are understood in
terms of a hot spot, generated by proximity of the compact object, rotating
round the inner circumbinary disk with a period of 13 days. The glowing
material fades with time, quite slowly for the H alpha source but more rapidly
for the He I spectral lines. The orbital speed of the inner rim is
approximately 250 km/s. Conclusions. The mass of the binary system must exceed
40 solar masses and the compact object must be a rather massive stellar black
hole. The corollary is that the orbital speed of the companion must exceed 130
km/s.Comment: Article; 6 pages, 8 figures. The new version of 28 July 2010, to
appear in A&A, is 8 pages and 8 figures. The principal addition is some
discussion of the behaviour of a gas stream from the L2 point. I also have
added a note about the possibility that absorption lines taken as indicating
an orbital speed of 60 km/s for the companion might have their origin in the
circumbinary dis
More on the circumbinary disk of SS 433
Certain lines in spectra of the Galactic microquasar SS 433, in particular
the brilliant H alpha line, have been interpreted as emission from a
circumbinary disk. In this interpretation the orbital speed of the glowing
material is in excess of 200 km/s and the mass of the binary system in excess
of 40 solar masses. A very simple model of excitation of disk material is in
remarkable agreement with the observations, yet it seems that the very
existence of a circumbinary disk is regarded as controversial.
Published spectra, taken almost nightly over two orbital periods of the
binary system, show H alpha and He I lines; these were analysed as
superpositions of Gaussian components. A model in which the excitation of any
given patch of putative circumbinary material is proportional to the inverse
square of its instantaneous distance from the compact object was constructed
and compared with observations.
The new model provides an excellent description of the observations. The
variation of the H alpha and He I spectra with orbital phase are described
quantitatively provided the radius of the emitting ring is not much greater
than the radius of the closest stable circumbinary orbit.
The new analysis has greatly strengthened the case for a circumbinary disk
orbiting the SS 433 system with a speed of over 200 km/s and presents supposed
alternative explanations with major difficulties. If the circumbinary disk
scenario is essentially correct, the mass of the binary system must exceed 40
solar masses and the compact object must be a rather massive black hole. The
case is so strong that this possibility should be taken seriously.Comment: 7 pages, 7 figures. The second version has two additional figures and
an extended discussion. To appear in A &
Effect of BMI and Binge Eating on Food Reward and Energy Intake: Further Evidence for a Binge Eating Subtype of Obesity
Background: The psychological characteristics of binge eating have been proposed as a phenotype to further understanding of overconsumption and susceptibility to obesity. This study examined the influence of trait binge eating in lean and overweight or obese women on appetite, food reward and energy intake. Methods: 25 lean and 25 overweight or obese women were categorised as either ‘binge type' or ‘non-binge type' based on their scores on the Binge Eating Scale. Food reward and food intake were assessed in fasted and fed conditions. Results: Overweight or obese binge types (O-B) consumed more energy than overweight or obese non-binge types (O-NB) and lean binge (L-B) and non-binge types (L-NB). Both L-B and O-B exhibited greater preference for sweet foods. In O-NB, L-B and L-NB, lower liking and wanting for sweet foods was exhibited in the fed condition compared to the fasted condition. However, in O-B wanting for sweet foods was greater when they were fed compared to when they were in a fasted state. Conclusions: These findings provide further support for trait binge eating as a hedonic subtype of obesity. Binge types were characterised by greater intake of high-fat sweet foods and increased wanting for these foods when satiated. Additionally, these findings highlight the potential for separation in liking and wanting for food as a marker of susceptibility to overeat
Magnetism in Nb(1-y)Fe(2+y) - composition and magnetic field dependence
We present a systematic study of transport and thermodynamic properties of
the Laves phase system NbFe. Our measurements confirm that
Fe-rich samples, as well as those rich in Nb (for ), show
bulk ferromagnetism at low temperature. For stoichiometric NbFe, on the
other hand, magnetization, magnetic susceptibility and magnetoresistance
results point towards spin-density wave (SDW) order, possibly helical, with a
small ordering wavevector \AA. Our results suggest that on
approaching the stoichiometric composition from the iron-rich side,
ferromagnetism changes into long-wavelength SDW order. In this scenario,
changes continuously from 0 to small, finite values at a Lifshitz point in the
phase diagram, which is located near . Further reducing the Fe content
suppresses the SDW transition temperature, which extrapolates to zero at
. Around this Fe content magnetic fluctuations dominate the
temperature dependence of the resistivity and of the heat capacity which
deviate from their conventional Fermi liquid forms, inferring the presence of a
quantum critical point. Because the critical point is located between the SDW
phase associated with stoichiometric NbFe and the ferromagnetic order which
reemerges for very Nb-rich NbFe, the observed temperature dependences could
be attributed both to proximity to SDW order or to ferromagnetism.Comment: 13 pages, 20 figure
Two-loop self-energy correction in high-Z hydrogen-like ions
A complete evaluation of the two-loop self-energy diagrams to all orders in
Z\alpha is presented for the ground state of H-like ions with Z\ge 40.Comment: RevTeX, 5 figures, 1 tabl
Non-Collinear Ferromagnetic Luttinger Liquids
The presence of electron-electron interactions in one dimension profoundly
changes the properties of a system. The separation of charge and spin degrees
of freedom is just one example. We consider what happens when a system
consisting of a ferromagnetic region of non-collinearity, i.e. a domain wall,
is coupled to interacting electrons in one-dimension (more specifically a
Luttinger liquid). The ferromagnetism breaks spin charge separation and the
presence of the domain wall introduces a spin dependent scatterer into the
problem. The absence of spin charge separation and the effects of the electron
correlations results in very different behaviour for the excitations in the
system and for spin-transfer-torque effects in this model.Comment: 6 pages, submitted to Journal of Physics: Conference Series for JEMS
201
An analytically solvable model of the effect of magnetic breakdown on angle-dependent magnetoresistance in a quasi-two-dimensional metal
We have developed an analytical model of angle-dependent magnetoresistance
oscillations (AMROs) in a quasi-two-dimensional metal in which magnetic
breakdown occurs. The model takes account of all the contributions from
quasiparticles undergoing both magnetic breakdown and Bragg reflection at each
junction and allows extremely efficient simulation of data which can be
compared with recent experimental results on the organic metal
kappa-ET2Cu(NCS)2. AMROs resulting from both closed and open orbits emerge
naturally at low field, and the model enables the transition to breakdown-AMROs
with increasing field to be described in detail.Comment: 4 pages, 3 figure
The biology of appetite control: Do resting metabolic rate and fat-free mass drive energy intake?
The prevailing model of homeostatic appetite control envisages two major inputs; signals from adipose tissue and from peptide hormones in the gastrointestinal tract. This model is based on the presumed major influence of adipose tissue on food intake. However, recent studies have indicated that in obese people fat-free mass (FFM) is strongly positively associated with daily energy intake and with meal size. This effect has been replicated in several independent groups varying in cultural and ethnic backgrounds, and appears to be a robust phenomenon. In contrast fat mass (FM) is weakly, or mildly negatively associated with food intake in obese people. In addition resting metabolic rate (RMR), a major component of total daily energy expenditure, is also associated with food intake. This effect has been replicated in different groups and is robust. This action is consistent with the proposal that energy requirements — reflected in RMR (and other aspects of energy expenditure) constitute a biological drive to eat. Consistent with its storage function, FM has a strong inhibitory effect on food intake in lean subjects, but this effect appears to weaken dramatically as adipose tissue increases. This formulation can account for several features of the development and maintenance of obesity and provides an alternative, and transparent, approach to the biology of appetite control
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