12,031 research outputs found
XTRA: The fast X-ray timing detector on XEUS
The Rossi X-ray Timing Explorer (RXTE) has demonstrated that the dynamical
variation of the X-ray emission from accreting neutron stars and stellar mass
black holes is a powerful probe of their strong gravitational fields. At the
same time, the X-ray burst oscillations at the neutron star spin frequency have
been used to set important constraints on the mass and radius of neutron stars,
hence on the equation of state of their high density cores. The X-ray Evolving
Universe Spectroscopy mission (XEUS), the potential follow-on mission to
XMM-Newton, will have a mirror aperture more than ten times larger than the
effective area of the RXTE proportional counter array (PCA). Combined with a
small dedicated fast X-ray timing detector in the focal plane (XTRA: XEUS
Timing for Relativistic Astrophysics), this collecting area will provide a leap
in timing sensitivity by more than one order of magnitude over the PCA for
bright sources, and will open a brand new window on faint X-ray sources, owing
to the negligible detector background. The use of advanced Silicon drift
chambers will further improve the energy resolution by a factor of ~6 over the
PCA, so that spectroscopic diagnostics of the strong field region, such as the
relativistically broadened Iron line, will become exploitable. By combining
fast X-ray timing and spectroscopy, XTRA will thus provide the first real
opportunity to test general relativity in the strong gravity field regime and
to constrain with unprecedented accuracy the equation of state of matter at
supranuclear density.Comment: To appear in X-Ray Timing 2003: Rossi and Beyond, ed. P. Kaaret, F.
K. Lamb, & J. H. Swank (Melville, NY: American Institute of Physics). 8
pages, 10 figures, 1 in colo
X-ray Timing beyond the Rossi X-ray Timing Explorer
With its ability to look at bright galactic X-ray sources with
sub-millisecond time resolution, the Rossi X-ray Timing Explorer (RXTE)
discovered that the X-ray emission from accreting compact stars shows
quasi-periodic oscillations on the dynamical timescales of the strong field
region. RXTE showed also that waveform fitting of the oscillations resulting
from hot spots at the surface of rapidly rotating neutron stars constrain their
masses and radii. These two breakthroughs suddenly opened up a new window on
fundamental physics, by providing new insights on strong gravity and dense
matter. Building upon the RXTE legacy, in the Cosmic Vision exercise, testing
General Relativity in the strong field limit and constraining the equation of
state of dense matter were recognized recently as key goals to be pursued in
the ESA science program for the years 2015-2025. This in turn identified the
need for a large (10 m2 class) aperture X-ray observatory. In recognition of
this need, the XEUS mission concept which has evolved into a single launch L2
formation flying mission will have a fast timing instrument in the focal plane.
In this paper I will outline the unique science that will be addressed with
fast X-ray timing on XEUS.Comment: 12 pages, 6 figures, COSPAR Colloquium "Spectra & Timing of Compact
X-ray Binaries", January 17-20, 2005, Mumbai, India. Advances in Space
Research, 2006, in pres
kHz Quasi-Periodic Oscillations in the low-mass X-ray binary 4U 0614+09
We report on a comprehensive analysis of the kilohertz (above 300 Hz)
quasi-periodic oscillations (kHz QPOs) detected from the neutron star low-mass
X-ray binary 4U0614+09 with the Rossi X-ray Timing Explorer (RXTE). With a much
larger data set than previously analyzed (all archival data from February 1996
up to October 2007), we first investigate the reality of the 1330 Hz QPO
reported by van Straaten et al. (2000). This QPO would be of particular
interest since it has the highest frequency reported for any source. A thorough
analysis of the same observation fails to confirm the detection. On the other
hand, over our extended data set, the highest QPO frequency we measure for the
upper kHz QPO is at about 1224 Hz; a value which is fully consistent with the
maximum values observed in similar systems. Second, we demonstrate that the
frequency dependence of the quality factor and amplitude of the lower and upper
kHz QPOs follow the systematic trends seen in similar systems (Barret et al.,
2006). In particular, 4U0614+09 shows a drop of the quality factor of the lower
kHz QPO above 700 Hz. If this is due to an approach to the innermost stable
circular orbit, it implies a neutron star mass of about 1.9 solar masses.
Finally, when analyzing the data over fixed durations, we have found a gap in
the frequency distribution of the upper QPO, associated with a local minimum of
its amplitude. A similar gap is not present in the distribution of the lower
QPO frequencies, suggesting some cautions when interpreting frequency ratio
distributions, based on the occurrence of the lower QPO only.Comment: 10 pages, 6 color figures, 2 tables, Accepted for publication in
MNRA
Probing X-ray burst -- accretion disk interaction in low mass X-ray binaries through kilohertz quasiperiodic oscillations
The intense radiation flux of Type I X-ray bursts is expected to interact
with the accretion flow around neutron stars. High frequency quasiperiodic
oscillations (kHz QPOs), observed at frequencies matching orbital frequencies
at tens of gravitational radii, offer a unique probe of the innermost disk
regions. In this paper, we follow the lower kHz QPOs, in response to Type I
X-ray bursts, in two prototypical QPO sources, namely 4U 1636-536 and 4U
1608-522, as observed by the Proportional Counter Array of the Rossi X-ray
Timing Explorer. We have selected a sample of 15 bursts for which the kHz QPO
frequency can be tracked on timescales commensurable with the burst durations
(tens of seconds). We find evidence that the QPOs are affected for over ~200 s
during one exceptionally long burst and ~100 s during two others (although at a
less significant level), while the burst emission has already decayed to a
level that would enable the pre-burst QPO to be detected. On the other hand,
for most of our burst-kHz QPO sample, we show that the QPO is detected as soon
as the statistics allow and in the best cases, we are able to set an upper
limit of ~20 s on the recovery time of the QPO. This diversity of behavior
cannot be related to differences in burst peak luminosity. We discuss these
results in the framework of recent findings that accretion onto the neutron
star may be enhanced during Type I X-ray bursts. The subsequent disk depletion
could explain the disappearance of the QPO for ~100 s, as possibly observed in
two events. However, alternative scenarios would have to be invoked for
explaining the short recovery timescales inferred from most bursts. Clearly the
combination of fast timing and spectral information of Type I X-ray bursts
holds great potential in the study of the dynamics of the inner accretion flow
around neutron stars.Comment: 8 pages, 9 figures, appears in Astronomy & Astrophysics, Volume 567,
id.A80, published 07/201
Spacecraft flight control system design selection process for a geostationary communication satellite
The Earth's first artificial satellite, Sputnik 1, slowly tumbled in orbit. The first U.S. satellite, Explorer 1, also tumbled out of control. Now, as we launch the Mars observer and the Cassini spacecraft, stability and control have become higher priorities. The flight control system design selection process is reviewed using as an example a geostationary communication satellite which is to have a life expectancy of 10 to 14 years. Disturbance torques including aerodynamic, magnetic, gravity gradient, solar, micrometeorite, debris, collision, and internal torques are assessed to quantify the disturbance environment so that the required compensating torque can be determined. Then control torque options, including passive versus active, momentum control, bias momentum, spin stabilization, dual spin, gravity gradient, magnetic, reaction wheels, control moment gyros, nutation dampers, inertia augmentation techniques, three-axis control, reactions control system (RCS), and RCS sizing, are considered. A flight control system design is then selected and preliminary stability criteria are met by the control gains selection
Conflict and cooperation in managing international water resources
Water is often not confined within territorial boundaries so conflicts may arise about shared water resources. When such boundaries lie within a federal state, conflicts may be peacefully and efficiently resolved under law, and if the state fail to reach an agreement, the federal government may impose one. Similar international conflicts are more difficult to resolve because no third party has the authority to enforce an agreement among national states, let alone impose one. Such international agreements must be self-enforcing. Efficient outcomes may emerge, but are not guaranteed. International law may emphasize the doctrine of"equitable utilization"of water resources, but there is no clear definition of what this implies. In the Colorado River case, the polluter (the United States) agreed to pay for all the costs of providing the downstream neighbor (Mexico) with clean water. In the Rhine River case, the downstream country (the Netherlands) agreed to pay part - but not all - of the costs of cleanup. In Colombia River Treaty case, both parties agreed to incur construction costs on their side of the border and share evenly the gross (not the net) benefit. This division may well have yielded a smaller net benefit to the United States than unilateral development would have, but the United States ratified the treaty. Negotiated outcomes need not to maximize net benefits for all countries. To some extent, inefficiencies can be traced to the desire to nationalize resources rather than to gain from cooperative development. The Indus Waters Treaty, for example, divided the Indus and its tributaries between India and Pakistan, rather than exploit joint use and development of the basin. Both efficiency and equity should be considered in agreements for managing international water resources. The 1959 Nile Waters Agreement between Egypt and Sudan did not reserve water for upstream riparians - notably, Ethiopia. A basinwide approach could make use of Nile waters more efficient and benefit all three riparians: Egypt, Ethiopia, and Sudan. Construction of dams in Ethiopia would give that country irrigation, would eliminate the annual Nile flood, and would increase the total water available to Ethiopia and Sudan. In negotiations over use of the Nile, the net benefits of basinwide management, and the ways these three riparians could share equitably in gains, should be demonstrated. In the 1980s, Egypt did not run short of water because Sudan did not take its full allocation and because Ethiopia did not withdraw any water from the basin. Increased water demand will inevitably create tension between the states.Water Supply and Sanitation Governance and Institutions,Town Water Supply and Sanitation,Water Law,Water Resources Law,Water and Industry
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