2,092 research outputs found
Temporal Relational Reasoning in Videos
Temporal relational reasoning, the ability to link meaningful transformations
of objects or entities over time, is a fundamental property of intelligent
species. In this paper, we introduce an effective and interpretable network
module, the Temporal Relation Network (TRN), designed to learn and reason about
temporal dependencies between video frames at multiple time scales. We evaluate
TRN-equipped networks on activity recognition tasks using three recent video
datasets - Something-Something, Jester, and Charades - which fundamentally
depend on temporal relational reasoning. Our results demonstrate that the
proposed TRN gives convolutional neural networks a remarkable capacity to
discover temporal relations in videos. Through only sparsely sampled video
frames, TRN-equipped networks can accurately predict human-object interactions
in the Something-Something dataset and identify various human gestures on the
Jester dataset with very competitive performance. TRN-equipped networks also
outperform two-stream networks and 3D convolution networks in recognizing daily
activities in the Charades dataset. Further analyses show that the models learn
intuitive and interpretable visual common sense knowledge in videos.Comment: camera-ready version for ECCV'1
Gravitational radiation timescales for extreme mass ratio inspirals
The capture and inspiral of compact stellar masses into massive black holes
is an important source of low-frequency gravitational waves (with frequencies
of ~1-100mHz), such as those that might be detected by the planned Laser
Interferometer Space Antenna (LISA). Simulations of stellar clusters designed
to study this problem typically rely on simple treatments of the black hole
encounter which neglect some important features of orbits around black holes,
such as the minimum radii of stable, non-plunging orbits. Incorporating an
accurate representation of the orbital dynamics near a black hole has been
avoided due to the large computational overhead. This paper provides new, more
accurate, expressions for the energy and angular momentum lost by a compact
object during a parabolic encounter with a non-spinning black hole, and the
subsequent inspiral lifetime. These results improve on the Keplerian
expressions which are now commonly used and will allow efficient computational
simulations to be performed that account for the relativistic nature of the
spacetime around the central black hole in the system.Comment: 19 pages, 4 figures. Changed in response to referee's report.
Accepted for publication in Astrophysical Journa
Cool Customers in the Stellar Graveyard IV: Spitzer Search for Mid-IR excesses Around Five DAs
Hydrogen atmosphere white dwarfs with metal lines, so-called DAZs, require
external accretion of material to explain the presence of weak metal line
absorption in their photospheres. The source of this material is currently
unknown, but could come from the interstellar medium, unseen companions, or
relic planetesimals from asteroid belt or Kuiper belt analogues. Accurate
mid-infrared photometry of these white dwarfs provide additional information to
solve the mystery of this accretion and to look for evidence of planetary
systems that have survived post main sequence evolution. We present {\em
Spitzer} IRAC photometry accurate to 3% for four DAZs and one DA with
circumstellar absorption lines in the UV. We search for excesses due to unseen
companions or circumstellar dust disks. We use {\em Hubble Space Telescope}
NICMOS imaging of these white dwarfs to gauge the level of background
contamination to our targets as well as rule out common proper motion
companions to WD 1620-391. All of our targets show no excesses due to
companions 20 M, ruling out all but very low mass companions to these
white dwarfs at all separations. No excesses due to circumstellar disks are
observed, and we place limits on what types of disks may still be present.Comment: 18 pages, 8 figures, Accepted to A
The Transition from Inspiral to Plunge for a Compact Body in a Circular Equatorial Orbit Around a Massive, Spinning Black Hole
There are three regimes of gravitational-radiation-reaction-induced inspiral
for a compact body with mass mu, in a circular, equatorial orbit around a Kerr
black hole with mass M>>mu: (i) The "adiabatic inspiral regime", in which the
body gradually descends through a sequence of circular, geodesic orbits. (ii) A
"transition regime", near the innermost stable circular orbit (isco). (iii) The
"plunge regime", in which the body travels on a geodesic from slightly below
the isco into the hole's horizon. This paper gives an analytic treatment of the
transition regime and shows that, with some luck, gravitational waves from the
transition might be measurable by the space-based LISA mission.Comment: 8 Pages and 3 Figures; RevTeX; submitted to Physical Review
Stellar Dynamics of Extreme-Mass-Ratio Inspirals
Inspiral of compact stellar remnants into massive black holes (MBHs) is
accompanied by the emission of gravitational waves at frequencies that are
potentially detectable by space-based interferometers. Event rates computed
from statistical (Fokker-Planck, Monte-Carlo) approaches span a wide range due
to uncertaintities about the rate coefficients. Here we present results from
direct integration of the post-Newtonian N-body equations of motion descrbing
dense clusters of compact stars around Schwarzschild MBHs. These simulations
embody an essentially exact (at the post-Newtonian level) treatment of the
interplay between stellar dynamical relaxation, relativistic precession, and
gravitational-wave energy loss. The rate of capture of stars by the MBH is
found to be greatly reduced by relativistic precession, which limits the
ability of torques from the stellar potential to change orbital angular
momenta. Penetration of this "Schwarzschild barrier" does occasionally occur,
resulting in capture of stars onto orbits that gradually inspiral due to
gravitational wave emission; we discuss two mechanisms for barrier penetration
and find evidence for both in the simulations. We derive an approximate formula
for the capture rate, which predicts that captures would be strongly disfavored
from orbits with semi-major axes below a certain value; this prediction, as
well as the predicted rate, are verified in the N-body integrations. We discuss
the implications of our results for the detection of extreme-mass-ratio
inspirals from galactic nuclei with a range of physical properties.Comment: 28 pages, 16 figures. Version 2 is significantly revised to reflect
new insights into J and Q effects, to be published late
Orbital evolution of a test particle around a black hole: higher-order corrections
We study the orbital evolution of a radiation-damped binary in the extreme
mass ratio limit, and the resulting waveforms, to one order beyond what can be
obtained using the conservation laws approach. The equations of motion are
solved perturbatively in the mass ratio (or the corresponding parameter in the
scalar field toy model), using the self force, for quasi-circular orbits around
a Schwarzschild black hole. This approach is applied for the scalar model.
Higher-order corrections yield a phase shift which, if included, may make
gravitational-wave astronomy potentially highly accurate.Comment: 4 pages, 3 Encapsulated PostScript figure
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