7,078 research outputs found
Superhumps in Low-Mass X-Ray Binaries
We propose a mechanism for the superhump modulations observed in optical
photometry of at least two black hole X-ray transients (SXTs). As in extreme
mass-ratio cataclysmic variables (CVs), superhumps are assumed to result from
the presence of the 3:1 orbital resonance in the accretion disc. This causes
the disc to become non-axisymmetric and precess. However the mechanism for
superhump luminosity variations in low mass X-ray binaries (LMXBs) must differ
from that in CVs, where it is attributed to a tidally-driven modulation of the
disc's viscous dissipation, varying on the beat between the orbital and disc
precession period. By contrast in LMXBs, tidal dissipation in the outer
accretion disc is negligible: the optical emission is overwhelming dominated by
reprocessing of intercepted central X-rays. Thus a different origin for the
superhump modulation is required. Recent observations and numerical simulations
indicate that in an extreme mass-ratio system the disc area changes on the
superhump period. We deduce that the superhumps observed in SXTs arise from a
modulation of the reprocessed flux by the changing area. Therefore, unlike the
situation in CVs, where the superhump amplitude is inclination-independent,
superhumps should be best seen in low-inclination LMXBs, whereas an orbital
modulation from the heated face of the secondary star should be more prominent
at high inclinations. Modulation at the disc precession period (10s of days)
may indicate disc asymmetries such as warping. We comment on the orbital period
determinations of LMXBs, and the possibility and significance of possible
permanent superhump LMXBs.Comment: 6 pages, 1 encapsulated figure. MNRAS in press; replaced to correct
typographical error
Solar wind turbulent heating by interstellar pickup protons: 2-component model
We apply a recently developed 2-component phenomenology to the turbulent heating of the core solar wind protons as seen at the Voyager 2 spacecraft. We find that this new description improves the model predictions of core temperature and correlation scale of the fluctuations, yielding excellent agreement with the Voyager measurements. However, the model fluctuation intensity substantially exceeds the Voyager measurements in the outer heliosphere, indicating that this picture needs further refinement
The Compact UV Nucleus of M33
The most luminous X-ray source in the Local Group is associated with the
nucleus of M33. This source, M33 X-8, appears modulated by ~20% over a ~106 day
period, making it unlikely that the combined emission from unresolved sources
could explain the otherwise persistent ~1e39 erg/s X-ray flux (Dubus et al.
1997, Hernquist et al. 1991). We present here high resolution UV imaging of the
nucleus with the Planetary Camera of the HST undertaken in order to search for
the counterpart to X-8. The nucleus is bluer and more compact than at longer
wavelength images but it is still extended with half of its 3e38 erg/s UV
luminosity coming from the inner 0.14". We cannot distinguish between a
concentrated blue population and emission from a single object.Comment: 3 figures, accepted for publication in ApJ Letter
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Fast and frugal framing effects?
Three experiments examine whether simple pair-wise comparison judgments, involving the “recognition heuristic” (Goldstein & Gigerenzer, 2002), are sensitive to implicit cues to the nature of the comparison required. Experiments 1 & 2 show that participants frequently choose the recognized option of a pair if asked to make “larger” judgments but are significantly less likely to choose the unrecognized option when asked to make “smaller” judgments. Experiment 3 demonstrates that, overall, participants consider recognition to be a more reliable guide to judgments of a magnitude criterion than lack of recognition and that this intuition drives the framing effect. These results support the idea that, when making pair-wise comparison judgments, inferring that the recognized item is large is simpler than inferring that the unrecognized item is small
Practice Reviews in Peak Period Rail Demand Management: Munich & Washington DC
The paper reviews current rail peak demand management approaches in Munich and the Washington DC metropolitan areas, through a practice review approach. Munich and the Washington DC metropolitan area offer two different approaches in the management of rail passenger demand in peak periods and beyond. By reviewing a range of strategies in use and under consideration, a broader picture emerges of the potential options and solutions available. In the Washington DC metropolitan area, the Metro rail system links activity centres in the District of Columbia with suburban jurisdictions in Maryland and Virginia. The Metro system, as a late twentieth-century rail network, is seen as a leader for US transit in terms of scale, the quality of network design and planning, and popularity with riders. Lessons drawn from DC Metro on rail demand management approaches are indicative of best-practice in the USA at present. In Munich, the U-Bahn is a new-generation metro-style urban rail network, which is complemented by the more suburban oriented “S-Bahn” - of a longer-distance, more radial-style configuration. Munich’s transport planners are proactive in their tracking of passenger flows and their actioning of a variety of measures that have produced “smooth” passenger flows that avoid the “excessive peaks” of many other major rail systems. This is perhaps partially a contributor to the strong overall financial outcomes for mass transit in Munich. In addition, the network characteristics of the Munich rail systems are notable, in that there are a substantial number of popular destination/origin stations, and the system is not over-reliant on a small number of major inner city stations. From these case study examples of established and emerging practice, suggestions are drawn for strategic options to assist transport agencies and rail operators in addressing peak demand issues through a more managed and structured approach
Disc precession in Be/X-ray binaries drives superorbital variations of outbursts and colour
Superorbital periods that are observed in the brightness of Be/X-ray binaries
may be driven by a misaligned and precessing Be star disc. We examine how the
precessing disc model explains the superorbital variation of (i) the magnitude
of the observed X-ray outbursts and (ii) the observed colour. With
hydrodynamical simulations we show that the magnitude of the average accretion
rate on to the neutron star, and therefore the X-ray outbursts, can vary by
over an order of magnitude over the superorbital period for Be star spin-orbit
misalignments as a result of weak tidal truncation. Most
Be/X-ray binaries are redder at optical maximum when the disc is viewed closest
to face-on since the disc adds a large red component to the emission. However,
A0538-66 is redder at optical minimum. This opposite behaviour requires an
edge-on disc at optical minimum and a radially narrow disc such that it does
not add a large red signature when viewed face-on. For A0538-66, the
misalignment of the disc to the binary orbit must be about and
the inclination of the binary orbit to the line of sight must be similarly
high, although restricted to by the absence of X-ray eclipses.Comment: Accepted for publication in MNRA
Prospecting Period Measurements with LSST - Low Mass X-ray Binaries as a Test Case
The Large Synoptic Survey Telescope (LSST) will provide for unbiased sampling
of variability properties of objects with mag 24. This should allow for
those objects whose variations reveal their orbital periods (), such
as low mass X-ray binaries (LMXBs) and related objects, to be examined in much
greater detail and with uniform systematic sampling. However, the baseline LSST
observing strategy has temporal sampling that is not optimised for such work in
the Galaxy. Here we assess four candidate observing strategies for measurement
of in the range 10 minutes to 50 days. We simulate multi-filter
quiescent LMXB lightcurves including ellipsoidal modulation and stochastic
flaring, and then sample these using LSST's operations simulator (OpSim) over
the (mag, ) parameter space, and over five sightlines sampling a range
of possible reddening values. The percentage of simulated parameter space with
correctly returned periods ranges from 23 %, for the current baseline
strategy, to 70 % for the two simulated specialist strategies. Convolving
these results with a distribution, a modelled Galactic spatial
distribution and reddening maps, we conservatively estimate that the most
recent version of the LSST baseline strategy will allow determination
for 18 % of the Milky Way's LMXB population, whereas strategies that do
not reduce observations of the Galactic Plane can improve this dramatically to
32 %. This increase would allow characterisation of the full binary
population by breaking degeneracies between suggested distributions
in the literature. Our results can be used in the ongoing assessment of the
effectiveness of various potential cadencing strategies.Comment: Replacement after addressing minor corrections from the referee -
mainly improvements in clarificatio
Heating the outer heliosphere by pickup protons
There is a growing body of literature that demonstrates the ability of a turbulent cascade within the solar wind to heat the thermal protons. Several sources of energy are required to accomplish the observed heating. Wind shear and shocks originating with the multiple source of wind plasma heat the wind inside ∽AU. However, beyond this distance little is left of these sources and all that remains is the energy injected into the plasma by the pickup of newborn protons originating from interstellar neutrals. Recent advances in the theory of wave excitation by the newborn protons allows us to return to the published heating theory and remove a previously unexplained parameterization of the heating due to pickup protons. Furthermore, recent observational evidence suggests that large-scale correlations between the wind speed and the proton temperature exist into the distant outer heliosphere that motivate an attempt to connect the two within the structure of the heating theory
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