17,956 research outputs found
Beyond XSPEC: Towards Highly Configurable Analysis
We present a quantitative comparison between software features of the defacto
standard X-ray spectral analysis tool, XSPEC, and ISIS, the Interactive
Spectral Interpretation System. Our emphasis is on customized analysis, with
ISIS offered as a strong example of configurable software. While noting that
XSPEC has been of immense value to astronomers, and that its scientific core is
moderately extensible--most commonly via the inclusion of user contributed
"local models"--we identify a series of limitations with its use beyond
conventional spectral modeling. We argue that from the viewpoint of the
astronomical user, the XSPEC internal structure presents a Black Box Problem,
with many of its important features hidden from the top-level interface, thus
discouraging user customization. Drawing from examples in custom modeling,
numerical analysis, parallel computation, visualization, data management, and
automated code generation, we show how a numerically scriptable, modular, and
extensible analysis platform such as ISIS facilitates many forms of advanced
astrophysical inquiry.Comment: Accepted by PASP, for July 2008 (15 pages
SPH Simulations of Direct Impact Accretion in the Ultracompact AM CVn Binaries
The ultracompact binary systems V407 Vul (RX J1914.4+2456) and HM Cnc (RX
J0806.3+1527) - a two-member subclass of the AM CVn stars - continue to pique
interest because they defy unambiguous classification. Three proposed models
remain viable at this time, but none of the three is significantly more
compelling than the remaining two, and all three can satisfy the observational
constraints if parameters in the models are tuned. One of the three proposed
models is the direct impact model of Marsh & Steeghs (2002), in which the
accretion stream impacts the surface of a rapidly-rotating primary white dwarf
directly but at a near-glancing angle. One requirement of this model is that
the accretion stream have a high enough density to advect its specific kinetic
energy below the photosphere for progressively more-thermalized emission
downstream, a constraint that requires an accretion spot size of roughly
1.2x10^5 km^2 or smaller. Having at hand a smoothed particle hydrodynamics code
optimized for cataclysmic variable accretion disk simulations, it was
relatively straightforward for us to adapt it to calculate the footprint of the
accretion stream at the nominal radius of the primary white dwarf, and thus to
test this constraint of the direct impact model. We find that the mass flux at
the impact spot can be approximated by a bivariate Gaussian with standard
deviation \sigma_{\phi} = 164 km in the orbital plane and \sigma_{\theta} = 23
km in the perpendicular direction. The area of the the 2\sigma ellipse into
which 86% of the mass flux occurs is roughly 47,400 km^2, or roughly half the
size estimated by Marsh & Steeghs (2002). We discuss the necessary parameters
of a simple model of the luminosity distribution in the post-impact emission
region.Comment: 24 pages, 5 figures, Accepted for publication in Ap
Advanced detection, isolation, and accommodation of sensor failures in turbofan engines: Real-time microcomputer implementation
The objective of the Advanced Detection, Isolation, and Accommodation Program is to improve the overall demonstrated reliability of digital electronic control systems for turbine engines. For this purpose, an algorithm was developed which detects, isolates, and accommodates sensor failures by using analytical redundancy. The performance of this algorithm was evaluated on a real time engine simulation and was demonstrated on a full scale F100 turbofan engine. The real time implementation of the algorithm is described. The implementation used state-of-the-art microprocessor hardware and software, including parallel processing and high order language programming
Similar phenomena at different scales: Black Holes, the Sun, Gamma-ray Bursts, Supernovae, Galaxies and Galaxy Clusters
Many similar phenomena occur in astrophysical systems with spatial and mass
scales different by many orders of magnitudes. For examples, collimated
outflows are produced from the Sun, proto-stellar systems, gamma-ray bursts,
neutron star and black hole X-ray binaries, and supermassive black holes;
various kinds of flares occur from the Sun, stellar coronae, X-ray binaries and
active galactic nuclei; shocks and particle acceleration exist in supernova
remnants, gamma-ray bursts, clusters of galaxies, etc. In this report I
summarize briefly these phenomena and possible physical mechanisms responsible
for them. I emphasize the importance of using the Sun as an astrophysical
laboratory in studying these physical processes, especially the roles magnetic
fields play in them; it is quite likely that magnetic activities dominate the
fundamental physical processes in all of these systems.
As a case study, I show that X-ray lightcurves from solar flares, black hole
binaries and gamma-ray bursts exhibit a common scaling law of non-linear
dynamical properties, over a dynamical range of several orders of magnitudes in
intensities, implying that many basic X-ray emission nodes or elements are
inter-connected over multi-scales. A future high timing and imaging resolution
solar X-ray instrument, aimed at isolating and resolving the fundamental
elements of solar X-ray lightcurves, may shed new lights onto the fundamental
physical mechanisms, which are common in astrophysical systems with vastly
different mass and spatial scales. Using the Sun as an astrophysical
laboratory, "Applied Solar Astrophysics" will deepen our understanding of many
important astrophysical problems.Comment: 22 pages, 13 figures, invited discourse for the 26th IAU GA, Prague,
Czech Republic, Aug. 2006, to be published in Vol. 14 IAU Highlights of
Astronomy, Ed. K.A. van der Hucht. Revised slightly to match the final
submitted version, after incorporating comments and suggestions from several
colleagues. A full-resolution version is available on request from the author
at [email protected]
Astrophysics of Super-massive Black Hole Mergers
We present here an overview of recent work in the subject of astrophysical
manifestations of super-massive black hole (SMBH) mergers. This is a field that
has been traditionally driven by theoretical work, but in recent years has also
generated a great deal of interest and excitement in the observational
astronomy community. In particular, the electromagnetic (EM) counterparts to
SMBH mergers provide the means to detect and characterize these highly
energetic events at cosmological distances, even in the absence of a
space-based gravitational-wave observatory. In addition to providing a
mechanism for observing SMBH mergers, EM counterparts also give important
information about the environments in which these remarkable events take place,
thus teaching us about the mechanisms through which galaxies form and evolve
symbiotically with their central black holes.Comment: Invited article for the focus issue on astrophysical black holes in
Classical and Quantum Gravity, guest editors: D. Merritt and L. Rezzoll
Stochastic Modeling of Hybrid Cache Systems
In recent years, there is an increasing demand of big memory systems so to
perform large scale data analytics. Since DRAM memories are expensive, some
researchers are suggesting to use other memory systems such as non-volatile
memory (NVM) technology to build large-memory computing systems. However,
whether the NVM technology can be a viable alternative (either economically and
technically) to DRAM remains an open question. To answer this question, it is
important to consider how to design a memory system from a "system
perspective", that is, incorporating different performance characteristics and
price ratios from hybrid memory devices.
This paper presents an analytical model of a "hybrid page cache system" so to
understand the diverse design space and performance impact of a hybrid cache
system. We consider (1) various architectural choices, (2) design strategies,
and (3) configuration of different memory devices. Using this model, we provide
guidelines on how to design hybrid page cache to reach a good trade-off between
high system throughput (in I/O per sec or IOPS) and fast cache reactivity which
is defined by the time to fill the cache. We also show how one can configure
the DRAM capacity and NVM capacity under a fixed budget. We pick PCM as an
example for NVM and conduct numerical analysis. Our analysis indicates that
incorporating PCM in a page cache system significantly improves the system
performance, and it also shows larger benefit to allocate more PCM in page
cache in some cases. Besides, for the common setting of performance-price ratio
of PCM, "flat architecture" offers as a better choice, but "layered
architecture" outperforms if PCM write performance can be significantly
improved in the future.Comment: 14 pages; mascots 201
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