4,594 research outputs found
Super-Extremal Spinning Black Holes via Accretion
A Kerr black hole with mass and angular momentum satisfies the
extremality inequality . In the presence of matter and/or
gravitational radiation, this bound needs to be reformulated in terms of local
measurements of the mass and the angular momentum directly associated with the
black hole. The isolated and dynamical horizon framework provides such
quasi-local characterization of black hole mass and angular momentum. With this
framework, it is possible in axisymmetry to reformulate the extremality limit
as , with the irreducible mass of the black hole
computed from its apparent horizon area and obtained using approximate
rotational Killing vectors on the apparent horizon. The
condition is also equivalent to requiring a non-negative black hole surface
gravity. We present numerical experiments of an accreting black hole that
temporarily violates this extremality inequality. The initial configuration
consists of a single, rotating black hole surrounded by a thick, shell cloud of
negative energy density. For these numerical experiments, we introduce a new
matter-without-matter evolution method.Comment: 11 pages, 10 figure
Robustness of Binary Black Hole Mergers in the Presence of Spurious Radiation
We present an investigation into how sensitive the last orbits and merger of
binary black hole systems are to the presence of spurious radiation in the
initial data. Our numerical experiments consist of a binary black hole system
starting the last couple of orbits before merger with additional spurious
radiation centered at the origin and fixed initial angular momentum. As the
energy in the added spurious radiation increases, the binary is invariably
hardened for the cases we tested, i.e. the merger of the two black holes is
hastened. The change in merger time becomes significant when the additional
energy provided by the spurious radiation increases the Arnowitt-Deser-Misner
(ADM) mass of the spacetime by about 1%. While the final masses of the black
holes increase due to partial absorption of the radiation, the final spins
remain constant to within our numerical accuracy. We conjecture that the
spurious radiation is primarily increasing the eccentricity of the orbit and
secondarily increasing the mass of the black holes while propagating out to
infinity.Comment: 12 pages, 12 figure
A virtual experiment on pedestrian destination choice:the role of schedules, the environment and behavioural categories
Which locations pedestrians decide to visit and in what order drives circulation patterns in pedestrian infrastructure. Destination choice is understood to arise from individuals trading off different factors, such as the proximity and busyness of destinations. Here, a virtual experiment is used to investigate whether this behaviour depends on the layout of buildings, whether planned or imposed destination schedules influence decisions and whether it is possible to distinguish different choice behaviour strategies in pedestrian populations. Findings suggest that virtual experiments can consistently elicit a range of destination choice behaviours indicating the flexibility of this experimental paradigm. The experimental approach facilitates changing the environment layout while controlling for other factors and illustrates this in itself can be important in determining destination choice. Destination schedules are found to be relevant both when imposed or generated by individuals, but adherence to them varies across individuals and depends on prevailing environmental conditions, such as destination busyness. Different destination choice behaviour strategies can be identified, but their properties are sensitive to the detection methods used, and it is suggested such behaviour classification should be informed by specific use-cases. It is suggested that these contributions present useful starting points for future research into pedestrian destination choice
Kramers-Kronig, Bode, and the meaning of zero
The implications of causality, as captured by the Kramers-Kronig relations
between the real and imaginary parts of a linear response function, are
familiar parts of the physics curriculum. In 1937, Bode derived a similar
relation between the magnitude (response gain) and phase. Although the
Kramers-Kronig relations are an equality, Bode's relation is effectively an
inequality. This perhaps-surprising difference is explained using elementary
examples and ultimately traces back to delays in the flow of information within
the system formed by the physical object and measurement apparatus.Comment: 8 pages; American Journal of Physics, to appea
A Spitzer Survey of Novae in M31
We report the results of the first infrared survey of novae in the nearby
spiral galaxy, M31. Both photometric and spectroscopic observations of a sample
of 10 novae (M31N 2006-09c, 2006-10a, 2006-10b, 2006-11a, 2007-07f, 2007-08a,
2007-08d, 2007-10a, 2007-11d, and 2007-11e) were obtained with the Spitzer
Space Telescope. Eight of the novae were observed with the IRAC (all but M31N
2007-11d and 2007-11e) and eight with the IRS (all but 2007-07f and 2007-08a),
resulting in six in common between the two instruments. The observations, which
were obtained between ~3 and ~7 months after discovery, revealed evidence for
dust formation in two of the novae: M31N 2006-10a and (possibly) 2007-07f, and
[Ne II] 12.8 micron line emission in a third (2007-11e). The Spitzer
observations were supplemented with ground-based optical photometric and
spectroscopic data that were used to determine the speed classes and
spectroscopic types of the novae in our survey. After including data for
dust-forming Galactic novae, we show that dust formation timescales are
correlated with nova speed class in that dust typically forms earlier in faster
novae. We conclude that our failure to detect the signature of dust formation
in most of our M31 sample is likely a result of the relatively long delay
between nova eruption and our Spitzer observations. Indeed, the two novae for
which we found evidence of dust formation were the two "slowest" novae in our
sample. Finally, as expected, we found that the majority of the novae in our
sample belong to the Fe II spectroscopic class, with only one clear example of
the He/N class (M31N 2006-10b). Typical of an He/N system, M31N 2006-10b was
the fastest nova in our sample, not detected with the IRS, and just barely
detected in three of the IRAC bands when it was observed ~4 months after
eruption.Comment: 37 pages, 12 figures, accepted for publication in the Astrophysical
Journa
A remarkable recurrent nova in M 31: The predicted 2014 outburst in X-rays with Swift
The M 31 nova M31N 2008-12a was recently found to be a recurrent nova (RN)
with a recurrence time of about 1 year. This is by far the fastest recurrence
time scale of any known RNe. Our optical monitoring programme detected the
predicted 2014 outburst of M31N 2008-12a in early October. We immediately
initiated an X-ray/UV monitoring campaign with Swift to study the
multiwavelength evolution of the outburst. We monitored M31N 2008-12a with
daily Swift observations for 20 days after discovery, covering the entire
supersoft X-ray source (SSS) phase. We detected SSS emission around day six
after outburst. The SSS state lasted for approximately two weeks until about
day 19. M31N 2008-12a was a bright X-ray source with a high blackbody
temperature. The X-ray properties of this outburst were very similar to the
2013 eruption. Combined X-ray spectra show a fast rise and decline of the
effective blackbody temperature. The short-term X-ray light curve showed
strong, aperiodic variability which decreased significantly after about day 14.
Overall, the X-ray properties of M31N 2008-12a are consistent with the average
population properties of M 31 novae. The optical and X-ray light curves can be
scaled uniformly to show similar time scales as those of the Galactic RNe U Sco
or RS Oph. The SSS evolution time scales and effective temperatures are
consistent with a high-mass WD. We predict the next outburst of M31N 2008-12a
to occur in autumn 2015.Comment: 13 pages, 7 figures, 3 tables; accepted for publication in A&
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