243 research outputs found
The eclipsing bursting X-ray binary EXO 0748-676 revisited by XMM-Newton
The bright eclipsing and bursting low-mass X-ray binary EXO 0748-676 has been
observed at several occasions by XMM-Newton during the initial calibration and
performance verification (CAL/PV) phase. We present here the results obtained
from observations with the EPIC cameras. Apart from several type-I X-ray
bursts, the source shows a high degree of variability with the presence of soft
flares. The wide energy coverage and high sensitivity of XMM-Newton allows for
the first time a detailed description of the spectral variability.
The source is found to be the superposition of a central (~2 10^8 cm)
Comptonized emission, most probably a corona surrounding the inner edge of an
accretion disk, associated with a more extended (~3 10^10 cm) thermal halo at a
typical temperature of ~0.6 keV with an indication of non-solar abundances.
Most of the variations of the source can be accounted for by a variable
absorption affecting only the central comptonized component and reaching up to
NH ~1.3 10^23 cm^{-2}. The characteristics of the surrounding halo are found
compatible with an irradiated atmosphere of an accretion disc which intercepts
the central emission due to the system high inclination.Comment: 6 pages, 4 figures, accepted for publication in A&A Letters, XMM
special issu
A simulation system for behaviour evaluation of off-rooad mobile robots
Abstract: This study deals with optimization of planetary rover traversability over challenging outdoor terrains. In this aim, it is necessary to simulate rover behaviour on soft soil with a high degree of realism. The paper describes a simulation system for mobile robots evolving on natural and unstructured surfaces. This simulator integrates the whole dynamics of the multibody systems and complex interactions with soft ground as well as control schemes.
A Search for Exozodiacal Dust and Faint Companions Near Sirius, Procyon, and Altair with the NICMOS Coronagraph
We observed Sirius, Altair, and Procyon with the NICMOS Coronagraph on the
Hubble Space Telescope to look for scattered light from exozodiacal dust and
faint companions within 10 AU from these stars. We did not achieve enough
dynamic range to surpass the upper limits set by IRAS on the amount of
exo-zodiacal dust in these systems, but we did set strong upper limits on the
presence of nearby late-type and sub-stellar companions.Comment: 10 pages, 4 figure
An extensive photometric study of the recently discovered intermediate polar V515 And (XSS J00564+4548)
We report results of photometry of the intermediate polar V515 And. The
observations were obtained over 33 nights in 2008 and 2009. The total duration
of the observations was 233 h. We clearly detected two oscillations with
periods of 465.48493\pm0.00007$ and 488.61822\pm0.00009 s, which may be the
white dwarf spin period and the orbital sideband. The semi-amplitudes of the
oscillations are 25 and 20 mmag, accordingly. The oscillation with a period of
465.48493 s has a stable smooth asymmetric pulse profile whereas the pulse
profile of the oscillation with a period of 488.61822 s reveals significant
changes from a quasi-sinusoidal shape to a shape somewhat resembling a
light-curve of an eclipsing binary. Two detected oscillations imply an orbital
period of 2.73 h. V515 And is one of the most rapidly spinning intermediate
polars with orbital periods less than 3 h and may be not in spin equilibrium.
This can be proved by future observations. For this purpose we obtained
oscillation ephemerises with a formal shelf life of about 100 yr. (a 1 sigma
confidence level).Comment: 10 pages, 8 figures, will be published in MNRA
Uranium resources, scenarios, nuclear and energy dynamics
ISBN 978-1-49-51-6286-2International audienceA dynamic simulation of coupled supply and demand of energy, resources and nuclear reactors is done with the global model Prospective Outlook for Long Term Energy Supply (POLES) over this century. In this model, both electricity demand and uranium supply are not independent of the cost of all base load electricity suppliers. Uranium consuming Thermal Neutron Reactors and future generation, free from the uranium market once started, breeder reactors are only one part of the market and are in a global competition, not limited to the other nuclear generation. In this paper we present a new model of the impact of uranium scarcity on the development of nuclear reactors. Many scenarios rely on the subjective definition of ultimate uranium resources. We suggest that when uranium will mainly be extracted together with other resources, its cost should not be simply a function of cumulated uranium mined but also of mine yearly outputs. We describe the sensitivities of our model to breeder reactor physical performance indicators. Used fuels can be seen as a liability or as a source of usable material and a scarce resource limiting fast reactor startups in fast development in India or China. We present the impact of synergetic strategies where countries with opposite strategies share used fuels
Neutronic study of slightly modified water reactors and application to transition scenarios
International audienceIn this paper we have studied slightly modified water reactors and their applications to transition scenarios. The PWR and CANDU reactors have been considered. New fuels based on Thorium have been tested : Thorium/Plutonium and Thorium/Uranium- 233, with different fissile isotope contents. Changes in the geometry of the assemblies were also explored to modify the moderation ratio, and consequently the neutron flux spectrum. A core equivalent assembly methodology was introduced as an exploratory approach and to reduce the computation time. Several basic safety analyses were also performed. We have finally developed a new scenario code, named OSCAR (Optimized Scenario Code for Advanced Reactors), to study the efficiency of these modified reactors in transition to GenIV reactors or in symbiotic fleet
A One-sided, Highly Relativistic Jet from Cygnus X-3
Very Long Baseline Array images of the X-ray binary, Cygnus X-3, were
obtained 2, 4 and 7 days after the peak of a 10 Jy flare on 4 February 1997.
The first two images show a curved one-sided jet, the third a scatter-broadened
disc, presumably at the position of the core. The jet curvature changes from
the first to the second epoch, which strongly suggests a precessing jet. The
ratio of the flux density in the approaching to that in the (undetected)
receding jet is > 330; if this asymmetry is due to Doppler boosting, the
implied jet speed is > 0.81c. Precessing jet model fits, together with the
assumptions that the jet is intrinsically symmetric and was ejected during or
after the major flare, yield the following constraints: the jet inclination to
the line of sight must be < 14 degrees; the cone opening angle must be < 12
degrees; and the precession period must be > 60 days.Comment: 12 pages 7 figures, accepted by Ap
Detection of orbital and superhump periods in Nova V2574 Ophiuchi (2004)
We present the results of 37 nights of CCD unfiltered photometry of nova
V2574 Oph (2004) from 2004 and 2005. We find two periods of 0.14164 d (~3.40 h)
and 0.14773 d (~3.55 h) in the 2005 data. The 2004 data show variability on a
similar timescale, but no coherent periodicity was found. We suggest that the
longer periodicity is the orbital period of the underlying binary system and
that the shorter period represents a negative superhump. The 3.40 h period is
about 4% shorter than the orbital period and obeys the relation between
superhump period deficit and binary period. The detection of superhumps in the
light curve is evidence of the presence of a precessing accretion disk in this
binary system shortly after the nova outburst. From the maximum magnitude -
rate of decline relation, we estimate the decay rate t_2 = 17+/-4 d and a
maximum absolute visual magnitude of M_Vmax = -7.7+/-1.7 mag.Comment: 6 pages, 6 figures, 2 .sty files, AJ accepted, minor change to one of
reference
A 2.4 - 12 microns spectrophotometric study with ISO of Cygnus X-3 in quiescence
We present mid-infrared spectrophotometric results obtained with the ISO on
the peculiar X-ray binary Cygnus X-3 in quiescence, at orbital phases 0.83 to
1.04. The 2.4-12 microns continuum radiation observed with ISOPHOT-S can be
explained by thermal free-free emission in an expanding wind with, above 6.5
microns, a possible additional black-body component with temperature T ~ 250 K
and radius R ~ 5000 solar radii at 10 kpc, likely due to thermal emission by
circumstellar dust. The observed brightness and continuum spectrum closely
match that of the Wolf-Rayet star WR 147, a WN8+B0.5 binary system, when
rescaled at the same 10 kpc distance as Cygnus X-3. A rough mass loss estimate
assuming a WN wind gives ~ 1.2 10^{-4} M(sun)/yr. A line at ~ 4.3 microns with
a more than 4.3 sigma detection level, and with a dereddened flux of 126 mJy,
is interpreted as the expected He I 3p-3s line at 4.295 microns, a prominent
line in the WR 147 spectrum. These results are consistent with a
Wolf-Rayet-like companion to the compact object in Cygnus X-3 of WN8 type, a
later type than suggested by earlier works.Comment: 8 pages, 10 figures ; Accepted in A&
IPHAS J062746.41+014811.3: a deeply eclipsing intermediate polar
We present time-resolved photometry of a cataclysmic variable discovered in
the Isaac Newton Telescope Photometric Halpha Survey of the northern galactic
plane, IPHAS J062746.41+014811.3 and classify the system as the fourth deeply
eclipsing intermediate polar known with an orbital period of Porb=8.16 h, and
spin period of Pspin=2210 s. The system shows mild variations of its
brightness, that appear to be accompanied by a change in the amplitude of the
spin modulation at optical wavelengths, and a change in the morphology of the
eclipse profile. The inferred magnetic moment of the white dwarf is mu_wd = 6-7
x 10^33 Gcm^3, and in this case IPHAS J0627 will either evolve into a
short-period EX Hya-like intermediate polar with a large Pspin\Porb ratio, or,
perhaps more likely, into a synchronised polar. Swift observations show that
the system is an ultraviolet and X-ray source, with a hard X-ray spectrum that
is consistent with those seen in other intermediate polars. The ultraviolet
light curve shows orbital modulation and an eclipse, while the low
signal-to-noise ratio X-ray light curve does not show a significant modulation
on the spin period. The measured X-ray flux is about an order of magnitude
lower than would be expected from scaling by the optical fluxes of well-known
X-ray selected intermediate polars.Comment: 34 pages, 9 figures, accepted for publication in Ap
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