136 research outputs found
From Monterey to Galway: Evolution of the Oceans in National Income Accounts
This paper provides an introduction and overview of selected papers from the 5th International Symposium on the Oceans in National Income Accounts organized by the Socio-Economics Marine Research Unit at the National University of Ireland Galway in March 2021. Eleven papers drawn from the symposium are included in this volume. The papers cover the current status of defining the ocean economy within national accounts, the expansion of ocean accounting to include environmental economic values, and methodological tools to assist the integration
Synthesis from Design Requirements of a Hybrid System for Transport Aircraft Longitudinal Control
Volume I of this report presents a new method for synthesizing hybrid systems directly from desi gn requirements, and applies the method to design of a hybrid system for longitudinal control of transport aircraft. The resulting system satisfies general requirement for safety and effectiveness specified a priori, enabling formal validation to be achieved. Volume II contains seven appendices intended to make the report accessible to readers with backgrounds in human factors, flight dynamics and control, and formal logic. Major design goals are (1) system design integrity based on proof of correctness at the design level, (2) significant simplification and cost reduction in system development and certification, and (3) improved operational efficiency, with significant alleviation of human-factors problems encountered by pilots in current transport aircraft. This report provides for the first time a firm technical basis for criteria governing design and certification of avionic systems for transport aircraft. It should be of primary interest to designers of next-generation avionic systems
ULTRACAM observations of the black hole X-ray transient XTE J1118+480 in quiescence
We present high time-resolution multicolour observations of the quiescent
soft X-ray transient XTE J1118+480 obtained with ULTRACAM. Superimposed on the
double-humped continuum g' and i'-band lightcurves are rapid flare events which
typically last a few minutes. The power density spectrum of the lightcurves can
be described by a broken power-law model with a break frequency at ~2 mHz or a
power-law model plus a broad quasi-periodic oscillation (QPO) at ~2 mHz. In the
context of the cellular-automaton we estimate the size of the quiescent
advection-dominated flow (ADAF) region to be ~10^4 Schwarzschild radii, similar
to that observed in other quiescent black hole X-ray transients, suggesting the
same underlying physics. The similarites between the low/hard and quiescent
state PDS suggest a similar origin for the optical and X-ray variability, most
likely from regions at/near the ADAF.Comment: 9 pages, 6 figures, accepted by MNRA
Optical studies of the X-ray transient XTE J2123-058 -I. Photometry
We present optical photometry of the X-ray transient XTE J2123-058, obtained
in July-October 1998. The light curves are strongly modulated on the 5.95hrs
orbital period, and exhibit dramatic changes in amplitude and form during the
decline. We used synthetic models which include the effect of partial eclipses
and X-ray heating effects, to estimate the system parameters, and we constrain
the binary inclination to be i=73+-4 degrees. The model is successful in
reproducing the light curves at different stages of the decay by requiring the
accretion disc to become smaller and thinner by 30% as the system fades by 1.7
mags in the optical. From Aug 26 the system reaches quiescence with a mean
magnitude of R=21.7+-0.1 and our data are consistent with the optical
variability being dominated by the companion's ellipsoidal modulation.Comment: 6 pages, 6 figure
SS433:the microquasar link with ULXs?
SS433 is the prototype microquasar in the Galaxy and may even be analogous to
the ULX sources if the jets' kinetic energy is taken into account. However, in
spite of 20 years of study, our constraints on the nature of the binary system
are extremely limited as a result of the difficulty of locating spectral
features that can reveal the nature and motion of the mass donor. Newly
acquired, high resolution blue spectra taken when the (precessing) disc is
edge-on suggest that the binary is close to a common-envelope phase, and hence
providing kinematic constraints is extremely difficult. Nevertheless, we do
find evidence for a massive donor, as expected for the inferred very high mass
transfer rate, and we compare SS433's properties with those of Cyg X-3.Comment: 4 pages, 3 figures, to appear in "Compact binaries in the Galaxy and
beyond
Indirect evidence for an active radio pulsar in the quiescent state of the transient ms pulsar SAX J1808.4-3658
Millisecond radio pulsars are neutron stars that have been spun-up by the
transfer of angular momentum during the low-mass X-ray binary phase. The
transition from an accretion-powered to a rotation-powered pulsar takes place
on evolutionary timescales at the end of the accretion process, however it may
also occur sporadically in systems undergoing transient X-ray activity. We have
obtained the first optical spectrum of the low mass transient X-ray pulsar SAX
J1808.4-3658 in quiescence. Similar to the black widow millisecond pulsar
B1957+20, this X-ray pulsar shows a large optical modulation at the orbital
period due to an irradiated companion star. Using the brightness of the
companion star as a bolometer, we conclude that a very high irradiating
luminosity, a factor of ~100 larger than directly observed, must be present in
the system. This most likely derives from a rotation-powered neutron star that
resumes activity during quiescence.Comment: Accepted for publication on ApJL (3 pages +2 color figures,
references added
The massive neutron star or low-mass black hole in 2S0921-630
We report on optical spectroscopy of the eclipsing Halo LMXB 2S0921-630, that
reveals the absorption line radial velocity curve of the K0III secondary star
with a semi-amplitude K_2=92.89 +/- 3.84 km/s, a systemic velocity
=34.9 +/- 3.3 \kms and an orbital period P_orb of 9.0035 +/- 0.0029 day
(1-sigma). Given the quality of the data, we find no evidence for the effects
of X-ray irradiation. Using the previously determined rotational broadening of
the mass donor, and applying conservative limits on the orbital inclination, we
constrain the compact object mass to be 2.0-4.3 Msolar (1-sigma), ruling out a
canonical neutron star at the 99% level. Since the nature of the compact object
is unclear, this mass range implies that the compact object is either a
low-mass black hole with a mass slightly higher than the maximum neutron star
mass (2.9 Msolar) or a massive neutron star. If the compact object is a black
hole, it confirms the prediction of the existence of low-mass black holes,
while if the object is a massive neutron star its high mass severely constrains
the equation of state of nuclear matter.Comment: Accepted by ApJ
Optical spectroscopy of flares from the black hole X-ray transient A0620-00 in quiescence
We present a time-resolved spectrophotometric study of the optical
variability in the quiescent soft X-ray transient A0620-00. Superimposed on the
double-humped continuum lightcurve are the well known flare events which last
tens of minutes. Some of the flare events that appear in the continuum
lightcurve are also present in the emission line lightcurves. From the Balmer
line flux and variations, we find that the persistent emission is optically
thin. During the flare event at phase 1.15 the Balmer decrement dropped
suggesting either a significant increase in temperature or that the flares are
more optically thick than the continuum. The data suggests that there are two
HI emitting regions, the accretion disc and the accretion stream/disc region,
with different Balmer decrements. The orbital modulation of H with the
continuum suggests that the steeper decrement is most likely associated with
the stream/disc impact region. We construct Doppler images of the H and
H emission lines. Apart from showing enhanced blurred emission at the
region where the stream impacts the accretion disc, the maps also show
significant extended structure from the opposite side of the disc. The trailed
spectra show characteristic S-wave features that can be interpreted in the
context of an eccentric accretion disc (abridged).Comment: 13 pages, 10 Figures, accepted my MNRA
Multiwavelength Observations of A0620-00 in Quiescence
[Abridged.] We present multiwavelength observations of the black hole binary
system, A0620-00. Using the Cosmic Origins Spectrograph on the Hubble Space
Telescope, we have obtained the first FUV spectrum of A0620-00. The observed
spectrum is flat in the FUV and very faint (with continuum fluxes \simeq 1e -
17 ergs/cm^2/s/A). We compiled the dereddened, broadband spectral energy
distribution of A0620-00 and compared it to previous SEDs as well as
theoretical models. The SEDs show that the source varies at all wavelengths for
which we have multiple samples. Contrary to previous observations, the
optical-UV spectrum does not continue to drop to shorter wavelengths, but
instead shows a recovery and an increasingly blue spectrum in the FUV. We
created an optical-UV spectrum of A0620-00 with the donor star contribution
removed. The non-stellar spectrum peaks at \simeq3000 {\deg}A. The peak can be
fit with a T=10,000 K blackbody with a small emitting area, probably
originating in the hot spot where the accretion stream impacts the outer disk.
However, one or more components in addition to the blackbody are needed to fit
the FUV upturn and the red optical fluxes in the optical-UV spectrum. By
comparing the mass accretion rate determined from the hot spot luminosity to
the mean accretion rate inferred from the outburst history, we find that the
latter is an order of magnitude smaller than the former, indicating that
\sim90% of the accreted mass must be lost from the system if the predictions of
the disk instability model and the estimated interoutburst interval are
correct. The mass accretion rate at the hot spot is 10^5 the accretion rate at
the black hole inferred from the X-ray luminosity. To reconcile these requires
that outflows carry away virtually all of the accreted mass, a very low rate of
mass transfer from the outer cold disk into the inner hot region, and/or
radiatively inefficient accretion.Comment: ApJ, accepte
Doppler tomography of the transient X-ray binary Centaurus X-4 in quiescence
We present ESO-NTT low resolution spectroscopy of the neutron star X-ray
transient Cen X-4 in quiescence over a complete orbital cycle. Our data reveal
the presence of a K3-7 V companion which contributes 63% to the 5600-6900A flux
and orbits the neutron star with a velocity semi-amplitude of K_2=145.8 +/- 1.0
km s^{-1}. This, combined with a previous determination of the inclination
angle and mass ratio, yields a neutron star and companion mass of M_1=1.5 +/-
1.0 M_Sun and M_2=0.31 +/- 0.27 M_Sun, respectively. The mass donor is thus
undermassive for the inferred spectral type indicating it is probably evolved,
in agreement with previous studies. Doppler tomography of the H_alpha line
shows prominent emission located on the companion and a slightly asymmetric
accretion disc distribution similar to that seen in systems with precessing
eccentric discs. Strong H_alpha emission from the companion can be explained by
X-ray irradiation from the primary. No evidence is found for a hot spot in
H_alpha, whereas one is revealed via Doppler tomography of the HeI lines. This
can be interpreted as the hot spot and outer regions of the disc being at a
higher temperature than in other systems.Comment: 9 pages, 5 figures, accepted for publication in A&
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