517 research outputs found
Identification of the X-ray pulsar in Hercules: A new optical pulsar
A series of photographic, photoelectric, and spectroscopic observations beginning June 1, 1972 has led to the optical identification of Her X-1 (2U 1705 + 34), a pulsed X-ray source in an eclipsing binary system, with the thirteenth magnitude blue variable star HZ Herculis. The detection of optical pulses at the frequency of the X-ray pulsar on three nights makes the identification conclusive and establishes HZ Her as the second known optical pulsar. The strength of the optical pulses may be correlated with the orbital phase but is not obviously related to the high or low intensity states of the X-ray source
Extreme value statistics and return intervals in long-range correlated uniform deviates
We study extremal statistics and return intervals in stationary long-range
correlated sequences for which the underlying probability density function is
bounded and uniform. The extremal statistics we consider e.g., maximum relative
to minimum are such that the reference point from which the maximum is measured
is itself a random quantity. We analytically calculate the limiting
distributions for independent and identically distributed random variables, and
use these as a reference point for correlated cases. The distributions are
different from that of the maximum itself i.e., a Weibull distribution,
reflecting the fact that the distribution of the reference point either
dominates over or convolves with the distribution of the maximum. The
functional form of the limiting distributions is unaffected by correlations,
although the convergence is slower. We show that our findings can be directly
generalized to a wide class of stochastic processes. We also analyze return
interval distributions, and compare them to recent conjectures of their
functional form
Topological constraints on spiral wave dynamics in spherical geometries with inhomogeneous excitability
We analyze the way topological constraints and inhomogeneity in the
excitability influence the dynamics of spiral waves on spheres and punctured
spheres of excitable media. We generalize the definition of an index such that
it characterizes not only each spiral but also each hole in punctured,
oriented, compact, two-dimensional differentiable manifolds and show that the
sum of the indices is conserved and zero. We also show that heterogeneity and
geometry are responsible for the formation of various spiral wave attractors,
in particular, pairs of spirals in which one spiral acts as a source and a
second as a sink -- the latter similar to an antispiral. The results provide a
basis for the analysis of the propagation of waves in heterogeneous excitable
media in physical and biological systems.Comment: 5 pages, 6 Figures, major revisions, accepted for publication in
Phys. Rev.
Ultraviolet Broad Absorption Features and the Spectral Energy Distribution of the QSO PG 1351+64
We present a moderate-resolution (~20 km/s) spectrum of the mini
broad-absorption-line QSO PG1351+64 between 915-1180 A, obtained with the Far
Ultraviolet Spectroscopic Explorer (FUSE). Additional spectra at longer
wavelengths were also obtained with the HST and ground-based telescopes. Broad
absorption is present on the blue wings of CIII 977, Ly-beta, OVI 1032,1038,
Ly-alpha, NV 1238,1242, SiIV 1393,1402, and CIV 1548,1450. The absorption
profile can be fitted with five components at velocities of ~ -780, -1049,
-1629, -1833, and -3054 km/s with respect to the emission-line redshift of z =
0.088. All the absorption components cover a large fraction of the continuum
source as well as the broad-line region. The OVI emission feature is very weak,
and the OVI/Lyalpha flux ratio is 0.08, one of the lowest among low-redshift
active galaxies and QSOs. The UV continuum shows a significant change in slope
near 1050 A in the restframe. The steeper continuum shortward of the Lyman
limit extrapolates well to the observed weak X-ray flux level. The absorbers'
properties are similar to those of high-redshift broad absorption-line QSOs.
The derived total column density of the UV absorbers is on the order of 10^21
cm^-2, unlikely to produce significant opacity above 1 keV in the X-ray. Unless
there is a separate, high-ionization X-ray absorber, the QSO's weak X-ray flux
may be intrinsic. The ionization level of the absorbing components is
comparable to that anticipated in the broad-line region, therefore the
absorbers may be related to broad-line clouds along the line of sight.Comment: 23 pages, Latex, 5 figure
FUSE Observations of Intrinsic Absorption in the Seyfert 1 Galaxy Mrk 509
We present far-ultraviolet spectra of the Seyfert 1 galaxy Mrk 509 obtained
in 1999 November with the Far Ultraviolet Spectroscopic Explorer (FUSE). Our
data span the observed wavelength range 915-1185 A at a resolution of ~20 km/s.
The spectrum shows a blue continuum, broad OVI 1032,1038 emission, and a broad
CIII 977 emission line. Superposed on these emission components, we resolve
associated absorption lines of OVI 1032,1038, CIII 977, and Lyman lines through
Lzeta. Seven distinct kinematic components are present, spanning a velocity
range of -440 to +170 km/s relative to the systemic velocity. The absorption is
clustered in two groups, one centered at -370m km/s and another at the systemic
velocity. The blue-shifted cluster may be associated with the extended line
emission visible in deep images of Mrk 509 obtained by Phillips et al. Although
several components appear to be saturated, they are not black at their centers.
Partial covering or scattering permits ~7% of the broad-line or continuum flux
to be unaffected by absorption. Of the multiple components, only one has the
same ionization state and column density as highly ionized gas that produces
the OVII and OVIII ionization edges in X-ray spectra of Mrk 509.
This paper will appear in a special issue of Astrophysical Journal Letters
devoted to the first scientific results from the FUSE mission.Comment: To appear in the Astrophysical Journal (Letters). 4 pages, 3 color
PostScript figures. Figures are best viewed and printed in color. Added
acknowledgment that this is one of many papers to be published in a special
issue of ApJL devoted to the first scientific results from the FUSE missio
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