2,174 research outputs found
Resilience markers for safer systems and organisations
If computer systems are to be designed to foster resilient
performance it is important to be able to identify contributors to resilience. The
emerging practice of Resilience Engineering has identified that people are still a
primary source of resilience, and that the design of distributed systems should
provide ways of helping people and organisations to cope with complexity.
Although resilience has been identified as a desired property, researchers and
practitioners do not have a clear understanding of what manifestations of
resilience look like. This paper discusses some examples of strategies that
people can adopt that improve the resilience of a system. Critically, analysis
reveals that the generation of these strategies is only possible if the system
facilitates them. As an example, this paper discusses practices, such as
reflection, that are known to encourage resilient behavior in people. Reflection
allows systems to better prepare for oncoming demands. We show that
contributors to the practice of reflection manifest themselves at different levels
of abstraction: from individual strategies to practices in, for example, control
room environments. The analysis of interaction at these levels enables resilient
properties of a system to be âseenâ, so that systems can be designed to explicitly
support them. We then present an analysis of resilience at an organisational
level within the nuclear domain. This highlights some of the challenges facing
the Resilience Engineering approach and the need for using a collective
language to articulate knowledge of resilient practices across domains
Mechanical heating by active galaxies
Jets and winds are significant channels for energy loss from accreting black
holes. These outflows mechanically heat their surroundings, through shocks as
well as gentler forms of heating. We discuss recent efforts to understand the
nature and distribution of mechanical heating by central AGNs in clusters of
galaxies, using numerical simulations and analytic models. Specifically, we
will discuss whether the relatively gentle `effervescent heating' mechanism can
compensate for radiative losses in the central regions of clusters, and account
for the excess entropy observed at larger radii.Comment: 10 pages, no figures. Submitted to Philosophical Transactions of the
Royal Society (Series A: Mathematical, Physical and Engineering Sciences),
proceedings of the Poyal Society Discussion Meeting on the Impact of Active
Galaxies on the Universe at Large, London, February 16-17, 200
Radiation Front Sweeping the Ambient Medium of Gamma-Ray Bursts
Gamma-ray bursts (GRBs) are emitted by relativistic ejecta from powerful
cosmic explosions. Their light curves suggest that the gamma-ray emission
occurs at early stages of the ejecta expansion, well before it decelerates in
the ambient medium. If so, the launched gamma-ray front must overtake the
ejecta and sweep the ambient medium outward. As a result a gap is opened
between the ejecta and the medium that surfs the radiation front ahead.
Effectively, the ejecta moves in a cavity until it reaches a radius
R_{gap}=10^{16}E_{54}^{1/2} cm where E is the isotropic energy of the GRB. At
R=R_{gap} the gap is closed, a blast wave forms and collects the medium swept
by radiation. Further development of the blast wave is strongly affected by the
leading radiation front: the front plays the role of a precursor where the
medium is loaded with e+- pairs and preaccelerated just ahead of the blast. It
impacts the emission from the blast at R < R_{load}=5R_{gap} (the early
afterglow). A spectacular observational effect results: GRB afterglows should
start in optical/UV and evolve fast (< min) to a normal X-ray afterglow. The
early optical emission observed in GRB 990123 may be explained in this way. The
impact of the front is especially strong if the ambient medium is a wind from a
massive progenitor of the GRB. In this case three phenomena are predicted: (1)
The ejecta decelerates at R<R_{load} producing a lot of soft radiation. (2) The
light curve of soft emission peaks at
t_{peak}=40(1+z)E_{54}^{1/2}(Gamma_{ej}/100)^{-2} s where Gamma_{ej} is the
Lorentz factor of the ejecta. Given measured redshift z and t_{peak}, one finds
Gamma_{ej}. (3) The GRB acquires a spectral break at 5 - 50 MeV because harder
photons are absorbed by radiation scattered in the wind.Comment: 20 pages, accepted to Ap
Warping of Accretion Disks with Magnetically Driven Outflows: A Possible Origin for Jet Precession
Current theoretical models for the outflows/jets from AGN, X-ray binaries and
young stellar objects involve large-scale magnetic fields threading an
underlying accretion disk. We suggest that such a disk is subjected to warping
instability and retrograde precession driven by magnetic torques associated
with the outflow. The growth timescale for the disk warp and the precession
period are of order the radial infall time of the disk. These effects may be
relevant to jet precession and other variabilities observed in many systems.Comment: 4 pages with 2 figures. ApJL in pres
Theory and astrophysical consequences of a magnetized torus around a rapidly rotating black hole
(Abbrev.) We analyze the topology, lifetime, and emissions of a torus around
a black hole formed in hypernovae and black hole-neutron star coalescence. The
torus is ab initio uniformly magnetized, represented by two counter oriented
current-rings, and develops a state of suspended accretion against a "magnetic
wall" around the black hole. Magnetic stability of the torus gives rise to a
new fundamental limit EB/Ek<0.1 for the ratio of poloidal magnetic field
energy-to-kinetic energy. The lifetime of rapid spin of the black hole is
effectively defined by the timescale of dissipation of black hole-spin energy
in the horizon, and satisfies T= 40s (MH/7MSun)(R/6MH)^4(0.03MH/MT) for a black
hole of mass MH surrounded by a torus of mass MT and radius R. The torus
converts a major fraction Egw/Erot=0.1 into gravitational radiation through a
finite number of multipole mass-moments, and a smaller fraction into MeV
neutrinos and baryon-rich winds. At a source distance of 100Mpc, these
emissions over N=2e4 periods give rise to a characteristic strain amplitude
\sqrt{N}hchar=6e-21. We argue that torus winds create an open magnetic
flux-tube on the black hole, which carries a minor and standard fraction
Ej/Erot=1e-3 in baryon-poor outflows to infinity. We identify this baryon poor
output of tens of seconds with GRBs with contemporaneous and strongly
correlated emissions in gravitational radiation, conceivably at multiple
frequencies. Ultimately, this leaves a black hole binary surrounded by a
supernova remnant.Comment: To appear in ApJ (44p
The AGN Obscuring Torus -- End of the "Doughnut" Paradigm?
Unified schemes of active galactic nuclei (AGN) require an obscuring dusty
torus around the central engine. The compact sizes (only a few pc) determined
in recent high-resolution observations require that the obscuring matter be
clumpy and located inside the region where the black-hole gravity dominates
over the galactic bulge. This location is in line with the scenario depicting
the torus as the region of the clumpy wind coming off the accretion disk in
which the clouds are dusty and optically thick. We study here the outflow
scenario within the framework of hydromagnetic disk winds, incorporating the
cloud properties determined from detailed modeling of the IR emission from
clumpy tori. We find that torus clouds were likely detected in recent water
maser observations of NGC 3079. In the wind scenario, the AGN main dynamic
channel for release of accreted mass seems to be switching at low luminosities
from torus outflow to radio jets. The torus disappears when the bolometric
luminosity decreases below about \E{42} erg/sec because the accretion onto the
central black hole can no longer sustain the required cloud outflow rate. This
disappearance seems to have been observed in both LINERs and radio galaxies.
With further luminosity decrease, suppression of cloud outflow spreads radially
inward from the disk's dusty, molecular region into its atomic, ionized zone,
resulting in disappearance of the broad emission line region at lower
luminosities, yet to be determined.Comment: ApJ Letters, to be publishe
Modeling the Large Scale Structures of Astrophysical Jets in the Magnetically Dominated Limit
We suggest a new approach that could be used for modeling both the large
scale behavior of astrophysical jets and the magnetically dominated explosions
in astrophysics. We describe a method for modeling the injection of magnetic
fields and their subsequent evolution in a regime where the free energy is
magnetically dominated. The injected magnetic fields, along with their
associated currents, have both poloidal and toroidal components, and they are
not force free. The dynamic expansion driven by the Lorentz force of the
injected fields is studied using 3-dimensional ideal magnetohydrodynamic
simulations. The generic behavior of magnetic field expansion, the interactions
with the background medium, and the dependence on various parameters are
investigated.Comment: Accepted to ApJ, May 10, 2006 issue, 12 figures total (3 color
figures
Compton Echoes from Gamma-ray Bursts
Recent observations of gamma-ray bursts (GRBs) have provided growing evidence
for collimated outflows and emission, and strengthened the connection between
GRBs and supernovae. If massive stars are the progenitors of GRBs, the hard
photon pulse will propagate in the pre-burst, dense environment. Circumstellar
material will Compton scatter the prompt GRB radiation and give rise to a
reflection echo. We calculate luminosities, spectra, and light curves of such
Compton echoes in a variety of emission geometries and ambient gas
distributions, and show that the delayed hard X-ray flash from a pulse
propagating into a red supergiant wind could be detectable by Swift out to
z~0.2. Independently of the gamma-ray spectrum of the prompt burst, reflection
echoes will typically show a high-energy cutoff between m_ec^2/2 and m_ec^2
because of Compton downscattering. At fixed burst energy per steradian, the
luminosity of the reflected echo is proportional to the beaming solid angle,
Omega_b, of the prompt pulse, while the number of bright echoes detectable in
the sky above a fixed limiting flux increases as Omega_b^{1/2}, i.e. it is
smaller in the case of more collimated jets. The lack of an X-ray echo at one
month delay from the explosion poses severe constraints on the possible
existence of a lateral GRB jet in SN 1987A. The late r-band afterglow observed
in GRB990123 is fainter than the optical echo expected in a dense red
supergiant environment from a isotropic prompt optical flash. Significant MeV
delayed emission may be produced through the bulk Compton (or Compton drag)
effect resulting from the interaction of the decelerating fireball with the
scattered X-ray radiation.Comment: LaTeX, 18 pages, 4 figures, revised version accepted for publication
in the Ap
A 'Twin-Exhaust' Model for Double Radio Sources
A mechanism is proposed for the formation of radio components in strong double sources such as Cygnus A. Relativistic plasma generated in an active galactic nucleus cannot escape isotropically if the nucleus is surrounded by too much dense thermal gas. There is, however, a possible equilibrium flow in which the plasma escapes along two oppositely-directed channels or âexhaustsâ. At all points on the boundary of these channels, the pressure of the relativistic (possibly magnetized) plasma must balance the pressure of the static thermal gas cloud. The outflow velocity becomes sonic (implying, for ultra-relativistic plasma, a velocity c/ââ 3) where the external pressure is â12 its central value. The channel cross-section reaches a minimum value at this point. The channel then widens again as the external pressure drops still further, and, as in a de Laval nozzle, the flow becomes supersonic. Relativistic plasma can thus be collimated into two relativistic beams.
Hargrave & Ryle's high resolution maps of Cygnus A reveal âhot spotsâ âČ 2 kpc in size at the outer edge of each individual component into which (it is believed) energy is being continuously supplied. We identify these hot spots with the regions where the beams impinge on the intergalactic medium. The dimensions and radio luminosity of Cygnus A imply that the central galactic nucleus must have maintained a power output âŒ10^(46) erg s^(-1) for 10^6 â 10^7 yr. This outflow could have been collimated into two sufficiently narrow beams if the galactic nucleus were surrounded by gas with T â 10^8 K, density âŒ10^3 particles cm^(â3) and scale height ⌠200 pc. Some aspects of the detailed morphology of Cygnus A are also interpreted on the basis of this general model.
The possible role of instabilities in the flow pattern, and the influence of magnetic fields, is discussed. Applications to other sources (which generally require less extreme parameters than Cygnus A) are briefly considered
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