4,481 research outputs found
Constraining the neutron star equation of state using XMM-Newton
We have identified three possible ways in which future XMM-Newton
observations can provide significant constraints on the equation of state of
neutron stars. First, using a long observation of the neutron star X-ray
transient CenX-4 in quiescence one can use the RGS spectrum to constrain the
interstellar extinction to the source. This removes this parameter from the
X-ray spectral fitting of the pn and MOS spectra and allows us to investigate
whether the variability observed in the quiescent X-ray spectrum of this source
is due to variations in the soft thermal spectral component or variations in
the power law spectral component coupled with variations in N_H. This will test
whether the soft thermal spectral component can indeed be due to the hot
thermal glow of the neutron star. Potentially such an observation could also
reveal redshifted spectral lines from the neutron star surface. Second,
XMM-Newton observations of radius expansion type I X-ray bursts might reveal
redshifted absorption lines from the surface of the neutron star. Third,
XMM-Newton observations of eclipsing quiescent low-mass X-ray binaries provide
the eclipse duration. With this the system inclination can be determined
accurately. The inclination determined from the X-ray eclipse duration in
quiescence, the rotational velocity of the companion star and the
semi-amplitude of the radial velocity curve determined through optical
spectroscopy, yield the neutron star mass.Comment: 4 pages, 1 figure, proceedings of the XMM-Newton workshop, June 2007,
accepted for publication in A
Detection of a 1258 Hz high-amplitude kilohertz quasi-periodic oscillation in the ultra-compact X-ray binary 1A 1246-588
We have observed the ultra-compact low-mass X-ray binary (LMXB) 1A 1246-588
with the Rossi X-ray Timing Explorer (RXTE). In this manuscript we report the
discovery of a kilohertz quasi-periodic oscillation (QPO) in 1A 1246-588. The
kilohertz QPO was only detected when the source was in a soft high-flux state
reminiscent of the lower banana branch in atoll sources. Only one kilohertz QPO
peak is detected at a relatively high frequency of 1258+-2 Hz and at a single
trial significance of more than 7 sigma. Kilohertz QPOs with a higher frequency
have only been found on two occasions in 4U 0614+09. Furthermore, the frequency
is higher than that found for the lower kilohertz QPO in any source, strongly
suggesting that the QPO is the upper of the kilohertz QPO pair often found in
LMXBs. The full-width at half maximum is 25+-4 Hz, making the coherence the
highest found for an upper kilohertz QPO. From a distance estimate of ~6 kpc
from a radius expansion burst we derive that 1A 1246-588 is at a persistent
flux of ~0.2-0.3 per cent of the Eddington flux, hence 1A 1246-588 is one of
the weakest LMXBs for which a kilohertz QPO has been detected. The
root-mean-square (rms) amplitude in the 5-60 keV band is 27+-3 per cent, this
is the highest for any kilohertz QPO source so far, in line with the general
anti-correlation between source luminosity and rms amplitude of the kilohertz
QPO peak identified before. Using the X-ray spectral information we produce a
colour-colour diagram. The source behaviour in this diagram provides further
evidence for the atoll nature of the source.Comment: 4 pages, 3 figures, accepted for publication in MNRA
The Central Beam and Cycle Management of the CERN Accelerator Complex
The efficient exploitation of the CERN accelerator complex in the future, with new cycles to fill the LHC and possibly a dedicated neutrino cycle in addition to the actual fixed-target program, will require a rapid and coordinated response to adapt to the changing user requests. This paper reviews the general sequencing problem and describes some preliminary concepts and algorithms suitable for managing a network of accelerators. The benefits derived from the architecture that has already been implemented in the PS complex, since its start up in March, are presented. The last accelerator in the injector chain, the SPS, is currently running fixed super-cycles. Its event-based timing system will be integrated into the central control by the year 2001 in a way that is transparent to the SPS equipment
Spectral analysis of boundary layers in Rayleigh-Benard convection
A combined experimental and numerical study of the boundary layer in a 4:1
aspect-ratio Rayleigh-B\'{e}nard cell over a four-decade range of Rayleigh
numbers has been undertaken aimed at gaining a better insight into the
character of the boundary layers. The experiments involved the simultaneous
Laser Doppler Anemometry (LDA) measurements of fluid velocity at two locations,
i.e. in the boundary layer and far away from it in the bulk, for Rayleigh
numbers varying between and . In parallel,
direct numerical simulations (DNS) have been performed for the same
configuration for Rayleigh numbers between and . The temperature and velocity probability density functions and the power
spectra of the horizontal velocity fluctuations measured in the boundary layer
and in the bulk flow are found to be practically identical. Except for the
smallest Rayleigh numbers, the spectra in the boundary layer and in the bulk
central region are continuous and have a wide range of active scales. This
indicates that both the bulk and the boundary layers are turbulent in the
number range considered. However, molecular effects can still be
observed and the boundary layer does not behave like a classical shear-driven
turbulent boundary layer.Comment: 10 pages, 6 figures, Accepted for publication in Phys. Rev.
Time-resolved X-Shooter spectra and RXTE light curves of the ultra-compact X-ray binary candidate 4U 0614+091
In this paper we present X-Shooter time resolved spectroscopy and RXTE PCA
light curves of the ultra-compact X-ray binary candidate 4U 0614+091. The
X-Shooter data are compared to the GMOS data analyzed previously by Nelemans et
al. (2004). We confirm the presence of C III and O II emission features at ~
4650 {\AA} and ~ 5000 {\AA}. The emission lines do not show evident Doppler
shifts that could be attributed to the motion of the donor star/hot spot around
the center of mass of the binary. We note a weak periodic signal in the
red-wing/blue-wing flux ratio of the emission feature at ~ 4650 {\AA}. The
signal occurs at P = 30.23 +/- 0.03 min in the X-Shooter and at P = 30.468 +/-
0.006 min in the GMOS spectra when the source was in the low/hard state. Due to
aliasing effects the period in the GMOS and X-Shooter data could well be the
same. We deem it likely that the orbital period is thus close to 30 min,
however, as several photometric periods have been reported for this source in
the literature already, further confirmation of the 30 min period is warranted.
We compare the surface area of the donor star and the disc of 4U 0614+091 with
the surface area of the donor star and the disc in typical hydrogen-rich
low-mass X-ray binaries and the class of AM Canum Venaticorum stars and argue
that the optical emission in 4U 0614+091 is likely dominated by the disc
emission. Additionally, we search for periodic signals in all the publicly
available RXTE PCA light curves of 4U 0614+091 which could be associated with
the orbital period of this source. A modulation at the orbital period with an
amplitude of ~ 10% such as those that have been found in other ultra-compact
X-ray binaries (4U 0513-40, 4U 1820-30) is not present in 4U 0614+091.Comment: Accepted for publication in MNRAS, 11 pages, 7 figure
Wind and boundary layers in Rayleigh-Benard convection. Part 2: boundary layer character and scaling
The effect of the wind of Rayleigh-Benard convection on the boundary layers
is studied by direct numerical simulation of an L/H=4 aspect-ratio domain with
periodic side boundary conditions for Ra={10^5, 10^6, 10^7} and Pr=1. It is
shown that the kinetic boundary layers on the top- and bottom plate have some
features of both laminar and turbulent boundary layers. A continuous spectrum,
as well as significant forcing due to Reynolds stresses indicates undoubtedly a
turbulent character, whereas the classical integral boundary layer parameters
-- the shape factor and friction factor (the latter is shown to be dominated by
the pressure gradient) -- scale with Reynolds number more akin to laminar
boundary layers. This apparent dual behavior is caused by the large influence
of plumes impinging onto and detaching from the boundary layer. The
plume-generated Reynolds stresses have a negligible effect on the friction
factor at the Rayleigh numbers we consider, which indicates that they are
passive with respect to momentum transfer in the wall-parallel direction.
However, the effect of Reynolds stresses cannot be neglected for the thickness
of the kinetic boundary layer. Using a conceptual wind model, we find that the
friction factor C_f should scale proportional to the thermal boundary layer
thickness as C_f ~ lambda_Theta, while the kinetic boundary layer thickness
lambda_u scales inversely proportional to the thermal boundary layer thickness
and wind Reynolds number lambda_u ~ lambda_Theta^{-1} Re^{-1}. The predicted
trends for C_f and \lambda_u are in agreement with DNS results
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