1,791 research outputs found
Humpty Dumpty to Moslem art.
High-precision measurements of gravitational-wave-induced length changes in Michelson interferometers, quantum-optical realizations of the Einstein-Bohr dialogue on Young's double-slit experiment using micromascrs, bistable systems, hydrodynamical pattern formation and magically complex phase-space webs with the beauty of Moslem art born out of nonlinear dynamical systems - one could hardly name fields of physics further apart than these
Dissipation of Quasiclassical Turbulence in Superfluid He
We compare the decay of turbulence in superfluid He produced by a moving
grid to the decay of turbulence created by either impulsive spin-down to rest
or by intense ion injection. In all cases the vortex line density decays at
late time as . At temperatures above 0.8 K, all methods
result in the same rate of decay. Below 0.8 K, the spin-down turbulence
maintains initial rotation and decays slower than grid turbulence and ion-jet
turbulence. This may be due to a decoupling of the large-scale superfluid flow
from the normal component at low temperatures, which changes its effective
boundary condition from no-slip to slip.Comment: Main article: 5 pages, 3 figures. Supplemental material: 4 pages, 3
figures. Accepted for publication in Physical Review Letter
Precise Measurement of the Spin Parameter of the Stellar-Mass Black Hole M33 X-7
In prior work, {\it Chandra} and Gemini-North observations of the eclipsing
X-ray binary M33 X-7 have yielded measurements of the mass of its black hole
primary and the system's orbital inclination angle of unprecedented accuracy.
Likewise, the distance to the binary is known to a few percent. In an analysis
based on these precise results, fifteen {\it Chandra} and {\it XMM-Newton}
X-ray spectra, and our fully relativistic accretion disk model, we find that
the dimensionless spin parameter of the black hole primary is . The quoted 1- error includes all sources of observational
uncertainty. Four {\it Chandra} spectra of the highest quality, which were
obtained over a span of several years, all lead to the same estimate of spin to
within statistical errors (2%), and this estimate is confirmed by 11 spectra of
lower quality. There are two remaining uncertainties: (1) the validity of the
relativistic model used to analyze the observations, which is being addressed
in ongoing theoretical work; and (2) our assumption that the black hole spin is
approximately aligned with the angular momentum vector of the binary, which can
be addressed by a future X-ray polarimetry mission.Comment: 14 pages, 3 figures, 1 table, published in ApJ Letters; as explained
in the erratum at the end of the text, the spin parameter has been corrected
upward from a*=0.77 to a*=0.84. Apart from the addition of the erratum, the
paper is unchanged
X-ray reflected spectra from accretion disk models. III. A complete grid of ionized reflection calculations
We present a new and complete library of synthetic spectra for modeling the
component of emission that is reflected from an illuminated accretion disk. The
spectra were computed using an updated version of our code XILLVER that
incorporates new routines and a richer atomic data base. We offer in the form
of a table model an extensive grid of reflection models that cover a wide range
of parameters. Each individual model is characterized by the photon index
\Gamma of the illuminating radiation, the ionization parameter \xi at the
surface of the disk (i.e., the ratio of the X-ray flux to the gas density), and
the iron abundance A_{Fe} relative to the solar value. The ranges of the
parameters covered are: 1.2 \leq \Gamma \leq 3.4, 1 \leq \xi \leq 10^4, and 0.5
\leq A_{Fe} \leq 10. These ranges capture the physical conditions typically
inferred from observations of active galactic nuclei, and also stellar-mass
black holes in the hard state. This library is intended for use when the
thermal disk flux is faint compared to the incident power-law flux. The models
are expected to provide an accurate description of the Fe K emission line,
which is the crucial spectral feature used to measure black hole spin. A total
of 720 reflection spectra are provided in a single FITS
file{\url{http://hea-www.cfa.harvard.edu/~javier/xillver/}} suitable for the
analysis of X-ray observations via the atable model in XSPEC. Detailed
comparisons with previous reflection models illustrate the improvements
incorporated in this version of XILLVER.Comment: 70 pages, 21 figures, submitted to Ap
Detecting the harmonics of oscillations with time-variable frequencies
A method is introduced for the spectral analysis of complex noisy signals containing several frequency components. It enables components that are independent to be distinguished from the harmonics of nonsinusoidal oscillatory processes of lower frequency. The method is based on mutual information and surrogate testing combined with the wavelet transform, and it is applicable to relatively short time series containing frequencies that are time variable. Where the fundamental frequency and harmonics of a process can be identified, the characteristic shape of the corresponding oscillation can be determined, enabling adaptive filtering to remove other components and nonoscillatory noise from the signal. Thus the total bandwidth of the signal can be correctly partitioned and the power associated with each component then can be quantified more accurately. The method is first demonstrated on numerical examples. It is then used to identify the higher harmonics of oscillations in human skin blood flow, both spontaneous and associated with periodic iontophoresis of a vasodilatory agent. The method should be equally relevant to all situations where signals of comparable complexity are encountered, including applications in astrophysics, engineering, and electrical circuits, as well as in other areas of physiology and biology
Advection-Dominated Accretion and Black Hole Event Horizons
The defining characteristic of a black hole is that it possesses an event
horizon through which matter and energy can fall in but from which nothing
escapes. Soft X-ray transients (SXTs), a class of X-ray binaries, appear to
confirm this fundamental property of black holes. SXTs that are thought to
contain accreting black holes display a large variation of luminosity between
their bright and faint states, while SXTs with accreting neutron stars have a
smaller variation. This difference is predicted if the former stars have
horizons and the latter have normal surfaces.Comment: 11 pages, including 2 tables and 2 figures. To appear in The
Astrophysical Journal Letter
The Extreme Spin of the Black Hole in Cygnus X-1
The compact primary in the X-ray binary Cygnus X-1 was the first black hole
to be established via dynamical observations. We have recently determined
accurate values for its mass and distance, and for the orbital inclination
angle of the binary. Building on these results, which are based on our favored
(asynchronous) dynamical model, we have measured the radius of the inner edge
of the black hole's accretion disk by fitting its thermal continuum spectrum to
a fully relativistic model of a thin accretion disk. Assuming that the spin
axis of the black hole is aligned with the orbital angular momentum vector, we
have determined that Cygnus X-1 contains a near-extreme Kerr black hole with a
spin parameter a/M>0.95 (3\sigma). For a less probable (synchronous) dynamical
model, we find a/M>0.92 (3\sigma). In our analysis, we include the
uncertainties in black hole mass, orbital inclination angle and distance, and
we also include the uncertainty in the calibration of the absolute flux via the
Crab. These four sources of uncertainty totally dominate the error budget. The
uncertainties introduced by the thin-disk model we employ are particularly
small in this case given the extreme spin of the black hole and the disk's low
luminosity.Comment: Paper III of three papers on Cygnus X-1; 21 pages including 5 figures
and 12 tables, ApJ in press. The paper is significantly restructured; two
further tests of the robustness of our spin measurement are presented, and
our error analysis has been substantially improved; the conclusions are
unchange
- …