1,650 research outputs found
SS 433: The wiggle of the wind
The Balmer H alpha emission line in the stationary spectrum of SS 433 has a
component originating in the wind above the accretion disk. The Doppler motion
of this line is a blurred representation of the motion of the compact object
accreting. I show how this may be understood in terms of emission lasting over
a few days, like radiation from the jet bolides.Comment: 4 pages, 3 figures. A coda to my circumbinary disk studies. To appear
in A&
Algorithms for the workflow satisfiability problem engineered for counting constraints
The workflow satisfiability problem (WSP) asks whether there exists an
assignment of authorized users to the steps in a workflow specification that
satisfies the constraints in the specification. The problem is NP-hard in
general, but several subclasses of the problem are known to be fixed-parameter
tractable (FPT) when parameterized by the number of steps in the specification.
In this paper, we consider the WSP with user-independent counting constraints,
a large class of constraints for which the WSP is known to be FPT. We describe
an efficient implementation of an FPT algorithm for solving this subclass of
the WSP and an experimental evaluation of this algorithm. The algorithm
iteratively generates all equivalence classes of possible partial solutions
until, whenever possible, it finds a complete solution to the problem. We also
provide a reduction from a WSP instance to a pseudo-Boolean SAT instance. We
apply this reduction to the instances used in our experiments and solve the
resulting PB SAT problems using SAT4J, a PB SAT solver. We compare the
performance of our algorithm with that of SAT4J and discuss which of the two
approaches would be more effective in practice
Interpretation of observations of the circumbinary disk of SS 433
Context. The Galactic microquasar SS 433 is possessed of a circumbinary disk
most clearly seen in the brilliant Balmer H alpha emission line. The orbital
speed of the glowing material is an important determinant of the mass of the
binary system. The circumbinary disk may be fed through the L2 point and in
turn may feed a very extended radio feature known as the ruff. Aims. To present
an analysis of spectroscopic optical data from H alpha and He I spectral lines
which reveal the circumbinary disk. To use comparisons of the rather different
signals to better understand the disk and improve estimates of the rotational
speed of the inner rim. To present a simple model which naturally explains some
apparently bizarre spectral variations with orbital phase. Methods. Published
spectra, taken almost nightly over two orbital periods of the binary system,
are analysed. H alpha and He I lines are analysed as superpositions of Gaussian
components and a simple model constructed. Results. The data are understood in
terms of a hot spot, generated by proximity of the compact object, rotating
round the inner circumbinary disk with a period of 13 days. The glowing
material fades with time, quite slowly for the H alpha source but more rapidly
for the He I spectral lines. The orbital speed of the inner rim is
approximately 250 km/s. Conclusions. The mass of the binary system must exceed
40 solar masses and the compact object must be a rather massive stellar black
hole. The corollary is that the orbital speed of the companion must exceed 130
km/s.Comment: Article; 6 pages, 8 figures. The new version of 28 July 2010, to
appear in A&A, is 8 pages and 8 figures. The principal addition is some
discussion of the behaviour of a gas stream from the L2 point. I also have
added a note about the possibility that absorption lines taken as indicating
an orbital speed of 60 km/s for the companion might have their origin in the
circumbinary dis
Ecology and life history of an Amazon floodplain cichlid: the discus fish Symphysodon (Perciformes: Cichlidae)
The discus fishes of the genus Symphysodon are popular ornamental cichlids that occur in floodplain lakes and flooded forests of the lowland Amazon Basin. These habitats are characterized by extreme seasonal fluctuations in the availability of food, shelter and dissolved oxygen, and also the densities of predators and parasites. Most aspects of discus biology are influenced by these fluctuating conditions. This paper reports an autoecological study of the western Amazonian discus S. haraldi (until recently classified as S. aequifasciatus). This species feeds predominantly on algal periphyton, fine organic detritus, plant matter, and small aquatic invertebrates. At high water it forages alone or in small groups in flooded forests. At low water it forms large aggregations in fallen tree crowns along lake margins. Breeding occurs at the beginning of the flood season, ensuring that the progeny are well grown before the next low water period. Symphysodon haraldi is an iteroparous partial spawner, reaches reproductive maturity within a year, and undertakes parental care of its eggs and larvae. The timing of spawning events, and/or the rate of brood survival, may be influenced by fluctuations in the flood level, resulting in a non-unimodal distribution of size classes for the subsequent 1+ cohort
The Wings of the Remarkable Archaic Mecopteron Notiothauma Reedi Mclachlan With Remarks on Their Protoblattoid Affinities
Automated Certification of Authorisation Policy Resistance
Attribute-based Access Control (ABAC) extends traditional Access Control by
considering an access request as a set of pairs attribute name-value, making it
particularly useful in the context of open and distributed systems, where
security relevant information can be collected from different sources. However,
ABAC enables attribute hiding attacks, allowing an attacker to gain some access
by withholding information. In this paper, we first introduce the notion of
policy resistance to attribute hiding attacks. We then propose the tool ATRAP
(Automatic Term Rewriting for Authorisation Policies), based on the recent
formal ABAC language PTaCL, which first automatically searches for resistance
counter-examples using Maude, and then automatically searches for an Isabelle
proof of resistance. We illustrate our approach with two simple examples of
policies and propose an evaluation of ATRAP performances.Comment: 20 pages, 4 figures, version including proofs of the paper that will
be presented at ESORICS 201
Remarks on the Basic Plan of the Terminal Abdominal Structures of the Males of Winged Insects
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