7,600 research outputs found
Flux enhancement in the inner region of a geometrically and optically thick accretion disk
The surface flux (and the corresponding observed flux) of a geometrically
thick ``funnel'' shaped disk is computed taking into account the radiation
impinging on the surface from other parts of the disk. It is found that the
ratio of the maximum apparent luminosity to the real luminosity of the disk is
only a factor even when the opening angle of the disk is small
(). Thus, geometrically beamed emission from ``funnel'' shaped
sub-Eddington disks around stellar mass black holes, cannot explain the
Ultra-Luminous X-ray sources detected in nearby galaxies.Comment: accepted for publication in Ap
Gravitational waves from nonspinning black hole-neutron star binaries: dependence on equations of state
We report results of a numerical-relativity simulation for the merger of a
black hole-neutron star binary with a variety of equations of state (EOSs)
modeled by piecewise polytropes. We focus in particular on the dependence of
the gravitational waveform at the merger stage on the EOSs. The initial
conditions are computed in the moving-puncture framework, assuming that the
black hole is nonspinning and the neutron star has an irrotational velocity
field. For a small mass ratio of the binaries (e.g., MBH/MNS = 2 where MBH and
MNS are the masses of the black hole and neutron star, respectively), the
neutron star is tidally disrupted before it is swallowed by the black hole
irrespective of the EOS. Especially for less-compact neutron stars, the tidal
disruption occurs at a more distant orbit. The tidal disruption is reflected in
a cutoff frequency of the gravitational-wave spectrum, above which the spectrum
amplitude exponentially decreases. A clear relation is found between the cutoff
frequency of the gravitational-wave spectrum and the compactness of the neutron
star. This relation also depends weakly on the stiffness of the EOS in the core
region of the neutron star, suggesting that not only the compactness but also
the EOS at high density is reflected in gravitational waveforms. The mass of
the disk formed after the merger shows a similar correlation with the EOS,
whereas the spin of the remnant black hole depends primarily on the mass ratio
of the binary, and only weakly on the EOS. Properties of the remnant disks are
also analyzed.Comment: 27pages, 21 figures; erratum is added on Aug 5. 201
Exploring binary-neutron-star-merger scenario of short-gamma-ray bursts by gravitational-wave observation
We elucidate the feature of gravitational waves (GWs) from binary neutron
star merger collapsing to a black hole by general relativistic simulation. We
show that GW spectrum imprints the coalescence dynamics, formation process of
disk, equation of state for neutron stars, total masses, and mass ratio. A
formation mechanism of the central engine of short -ray bursts, which
are likely to be composed of a black hole and surrounding disk, therefore could
be constrained by GW observation.Comment: Accepted to PR
Time-oscillating Lyapunov modes and auto-correlation functions for quasi-one-dimensional systems
The time-dependent structure of the Lyapunov vectors corresponding to the
steps of Lyapunov spectra and their basis set representation are discussed for
a quasi-one-dimensional many-hard-disk systems. Time-oscillating behavior is
observed in two types of Lyapunov modes, one associated with the time
translational invariance and another with the spatial translational invariance,
and their phase relation is specified. It is shown that the longest period of
the Lyapunov modes is twice as long as the period of the longitudinal momentum
auto-correlation function. A simple explanation for this relation is proposed.
This result gives the first quantitative connection between the Lyapunov modes
and an experimentally accessible quantity.Comment: 4 pages, 3 figure
Tidal Interaction between a Fluid Star and a Kerr Black Hole in Circular Orbit
We present a semi-analytic study of the equilibrium models of close binary
systems containing a fluid star (mass and radius ) and a Kerr black
hole (mass ) in circular orbit. We consider the limit where
spacetime is described by the Kerr metric. The tidally deformed star is
approximated by an ellipsoid, and satisfies the polytropic equation of state.
The models also include fluid motion in the stellar interior, allowing binary
models with nonsynchronized stellar spin (as expected for coalescing neutron
star-black hole binaries) to be constructed. Tidal disruption occurs at orbital
radius , but the dimensionless ratio depends on the spin parameter of
the black hole as well as on the equation of state and the internal rotation of
the star. We find that the general relativistic tidal field disrupts the star
at a larger than the Newtonian tide; the difference is
particularly prominent if the disruption occurs in the vicinity of the black
hole's horizon. In general, is smaller for a (prograde
rotating) Kerr black hole than for a Schwarzschild black hole. We apply our
results to coalescing black hole-neutron star and black hole-white dwarf
binaries. The tidal disruption limit is important for characterizing the
expected gravitational wave signals and is relevant for determining the
energetics of gamma ray bursts which may result from such disruption.Comment: 29 pages including 8 figures. Minor changes and update. To appear in
ApJ, March 20, 2000 (Vol.532, #1
Observations of Dense Molecular Gas in a Quasar Host Galaxy at z=6.42: Further Evidence for a Non-Linear Dense Gas - Star Formation Relation at Early Cosmic Times
We report a sensitive search for the HCN(J=2-1) emission line towards SDSS
J1148+5251 at z=6.42 with the VLA. HCN emission is a star formation indicator,
tracing dense molecular hydrogen gas (n(H2) >= 10^4 cm^-3) within star-forming
molecular clouds. No emission was detected in the deep interferometer maps of
J1148+5251. We derive a limit for the HCN line luminosity of L'(HCN) < 3.3 x
10^9 K km/s pc^2, corresponding to a HCN/CO luminosity ratio of L'(HCN)/L'(CO)
< 0.13. This limit is consistent with a fraction of dense molecular gas in
J1148+5251 within the range of nearby ultraluminous infrared galaxies (ULIRGs;
median value: L'(HCN)/L'(CO) = 0.17 {+0.05/-0.08}) and HCN-detected z>2
galaxies (0.17 {+0.09/-0.08}). The relationship between L'(HCN) and L(FIR) is
considered to be a measure for the efficiency at which stars form out of dense
gas. In the nearby universe, these quantities show a linear correlation, and
thus, a practically constant average ratio. In J1148+5251, we find
L(FIR)/L'(HCN) > 6600. This is significantly higher than the average ratios for
normal nearby spiral galaxies (L(FIR)/L'(HCN) = 580 {+510/-270}) and ULIRGs
(740 {+505/-50}), but consistent with a rising trend as indicated by other z>2
galaxies (predominantly quasars; 1525 {+1300/-475}). It is unlikely that this
rising trend can be accounted for by a contribution of AGN heating to L(FIR)
alone, and may hint at a higher median gas density and/or elevated
star-formation efficiency toward the more luminous high-redshift systems. There
is marginal evidence that the L(FIR)/L'(HCN) ratio in J1148+5251 may even
exceed the rising trend set by other z>2 galaxies; however, only future
facilities with very large collecting areas such as the SKA will offer the
sensitivity required to further investigate this question.Comment: 5 pages, 2 figures, 2 tables, to appear in ApJL (accepted October 24,
2007
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