204 research outputs found
Neutrino-cooled Accretion Disks around Spinning Black Holes
We calculate the structure of accretion disk around a spinning black hole for
accretion rates 0.01 - 10 M_sun/s. The model is fully relativistic and treats
accurately the disk microphysics including neutrino emissivity, opacity,
electron degeneracy, and nuclear composition. We find that the accretion flow
always regulates itself to a mildly degenerate state with the proton-to-nucleon
ratio Y_e ~ 0.1 and becomes very neutron-rich. The disk has a well defined
"ignition" radius where neutrino flux raises dramatically, cooling becomes
efficient, and Y_e suddenly drops. We also calculate other characteristic radii
of the disk, including the neutrino-opaque and neutrino-trapping radii, and
show their dependence on the accretion rate. Accretion disks around
fast-rotating black holes produce intense neutrino fluxes which may deposit
enough energy above the disk to generate a GRB jet.Comment: 4 pages, 3 figures; to be published in AIP Conference Proceedings
"Gamma Ray Bursts in the Swift Era," Nov. 29 - Dec. 2, 2005, Washington, D
Accretion onto the Companion of Eta Carinae During the Spectroscopic Event. IV. the Disappearance of Highly Ionized Lines
We show that the rapid and large decrease in the intensity of high-ionization
emission lines from the Eta Carinae massive binary system can be explained by
the accretion model. These emission lines are emitted by material in the nebula
around the binary system that is being ionized by radiation from the hot
secondary star. The emission lines suffer three months long deep fading every
5.54 year, assumed to be the orbital period of the binary system. In the
accretion model, for ~70 day the less massive secondary star is accreting mass
from the primary wind instead of blowing its fast wind. The accretion event has
two effects that substantially reduce the high-energy ionizing radiation flux
from the secondary star. (1) The accreted mass absorbs a larger fraction of the
ionizing flux. (2) The accreted mass forms a temporarily blanked around the
secondary star that increases its effective radius, hence lowering its
effective temperature and the flux of high energy photons. This explanation is
compatible with the fading of the emission lines at the same time the X-ray is
declining to its minimum, and with the fading being less pronounced in the
polar directions.Comment: ApJ, in pres
Equation-of-state dependence of the gravitational-wave signal from the ring-down phase of neutron-star mergers
Neutron-star (NS) merger simulations are conducted for 38 representative
microphysical descriptions of high-density matter in order to explore the
equation-of-state dependence of the postmerger ring-down phase. The formation
of a deformed, oscillating, differentially rotating very massive NS is the
typical outcome of the coalescence of two stars with 1.35 for most
candidate EoSs. The oscillations of this object imprint a pronounced peak in
the gravitational-wave (GW) spectra, which is used to characterize the emission
for a given model. The peak frequency of this postmerger GW signal correlates
very well with the radii of nonrotating NSs, and thus allows to constrain the
high-density EoS by a GW detection. In the case of 1.35-1.35
mergers the peak frequency scales particularly well with the radius of a NS
with 1.6 , where the maximum deviation from this correlation is only
60 meters for fully microphysical EoSs which are compatible with NS
observations. Combined with the uncertainty in the determination of the peak
frequency it appears likely that a GW detection can measure the radius of a 1.6
NS with an accuracy of about 100 to 200 meters. We also uncover
relations of the peak frequency with the radii of nonrotating NSs with 1.35
or 1.8 , with the radius or the central energy density
of the maximum-mass Tolman-Oppenheimer-Volkoff configuration, and with the
pressure or sound speed at a fiducial rest-mass density of about twice nuclear
saturation density. Furthermore, it is found that a determination of the
dominant postmerger GW frequency can provide an upper limit for the maximum
mass of nonrotating NSs. The prospects for a detection of the postmerger GW
signal and a determination of the dominant GW frequency are estimated to be in
the range of 0.015 to 1.2 events per year with the upcoming Advanced LIGO
detector.Comment: 29 pages, 28 figures, accepted for publication in Phys. Rev.
Magnetic Braking and Viscous Damping of Differential Rotation in Cylindrical Stars
Differential rotation in stars generates toroidal magnetic fields whenever an
initial seed poloidal field is present. The resulting magnetic stresses, along
with viscosity, drive the star toward uniform rotation. This magnetic braking
has important dynamical consequences in many astrophysical contexts. For
example, merging binary neutron stars can form "hypermassive" remnants
supported against collapse by differential rotation. The removal of this
support by magnetic braking induces radial fluid motion, which can lead to
delayed collapse of the remnant to a black hole. We explore the effects of
magnetic braking and viscosity on the structure of a differentially rotating,
compressible star, generalizing our earlier calculations for incompressible
configurations. The star is idealized as a differentially rotating, infinite
cylinder supported initially by a polytropic equation of state. The gas is
assumed to be infinitely conducting and our calculations are performed in
Newtonian gravitation. Though highly idealized, our model allows for the
incorporation of magnetic fields, viscosity, compressibility, and shocks with
minimal computational resources in a 1+1 dimensional Lagrangian MHD code. Our
evolution calculations show that magnetic braking can lead to significant
structural changes in a star, including quasistatic contraction of the core and
ejection of matter in the outermost regions to form a wind or an ambient disk.
These calculations serve as a prelude and a guide to more realistic MHD
simulations in full 3+1 general relativity.Comment: 20 pages, 19 figures, 3 tables, AASTeX, accepted by Ap
Neutrino Cooled Disk and Its Stability
We investigate the structure and stability of hypercritical accretion flows
around stellar-mass black holes, taking into account neutrino cooling, lepton
conservation, and firstly a realistic equation of state in order to properly
treat the dissociation of nuclei. We obtain the radial distributions of
physical properties, such as density, temperature and electron fraction, for
various mass accretion rates . We find that,
depending on mass accretion rates, different physics affect considerably the
structure of the disk; most important physics is (1) the photodissociation of
nuclei around for relatively low mass accretion rates
(), (2) efficient neutrino cooling
around for moderately high mass accretion rate (), and (3) neutrino trapping () for
very high mass accretion rate (). We
also investigate the stability of hypercritical accretion flows by drawing the
thermal equilibrium curves, and find that efficient neutrino cooling makes the
accretion flows rather stable against both thermal and viscous modes.Comment: 26 pages, 28 figures, Accepted for publication in Ap
Hyper-Accreting Black Holes and Gamma-Ray Bursts
A variety of current models for gamma-ray bursts (GRBs) suggest a common
engine - a black hole of several solar masses accreting matter from a disk at a
rate 0.01 to 10 solar masses per second. Using a numerical model for
relativistic disk accretion, we have studied steady-state accretion at these
high rates. Inside a radius ~ 10**8 cm, for accretion rates greater than about
0.01 solar masses per second, a global state of balanced power comes to exist
between neutrino losses, chiefly pair capture on nucleons, and dissipation.
Energy emitted in neutrinos is less, and in the case of low accretion rates,
very much less, than the maximum efficiency factor for black hole accretion
(0.057 for no rotation; 0.42 for extreme Kerr rotation) times Mdot c**2. The
efficiency for producing a pair fireball along the rotational axis by neutrino
annihilation is calculated and found to be highly variable and very sensitive
to the accretion rate. For some of the higher accretion rates studied, it can
be several per cent or more; for accretion rates less than 0.05 solar masses
per second, it is essentially zero. The efficiency of the Blandford-Znajek
mechanism in extracting rotational energy from the black hole is also
estimated. In light of these results, the viability of various gamma-ray burst
models is discussed and the sensitivity of the results to disk viscosity, black
hole rotation rate, and black hole mass explored. A diverse range of GRB
energies seems unavoidable and neutrino annihilation in hyper-accreting black
hole systems can explain bursts up to 10**52 erg. Larger energies may be
inferred for beaming systems.Comment: 46 pages, includes 9 figures, LaTeX (uses aaspp4.sty), accepted by
The Astrophysical Journal. Additional solutions in Tables and Figs. 4 and 5,
minor revisions to text, references adde
Black Hole Spin Evolution: Implications for Short-hard Gamma Ray Bursts and Gravitational Wave Detection
The evolution of the spin and tilt of black holes in compact black hole -
neutron star and black hole - black hole binary systems is investigated within
the framework of the coalescing compact star binary model for short gamma ray
bursts via the population synthesis method. Based on recent results on
accretion at super critical rates in slim disk models, estimates of natal
kicks, and the results regarding fallback in supernova models, we obtain the
black hole spin and misalignment. It is found that the spin parameter, a_spin},
is less than 0.5 for initially non rotating black holes and the tilt angle,
i_tilt, is less than 45 deg for 50% of the systems in black hole - neutron star
binaries. Upon comparison with the results of black hole - neutron star merger
calculations we estimate that only a small fraction (~ 0.01) of these systems
can lead to the formation of a torus surrounding the coalesced binary
potentially producing a short-hard gamma ray burst. On the other hand, for high
initial black hole spin parameters (a_spin>0.6) this fraction can be
significant (~ 0.4). It is found that the predicted gravitational radiation
signal for our simulated population does not significantly differ from that for
non rotating black holes. Due to the (i) insensitivity of signal detection
techniques to the black hole spin and the (ii) predicted overall low
contribution of black hole binaries to the signal we find that the detection of
gravitational waves are not greatly inhibited by current searches with non
spinning templates. It is pointed out that the detection of a black hole -
black hole binary inspiral system with LIGO or VIRGO may provide a direct
measurement of the initial spin of a black hole.Comment: ApJ accepted: major revision
About the detection of gravitational wave bursts
Several filtering methods for the detection of gravitational wave bursts in
interferometric detectors are presented. These are simple and fast methods
which can act as online triggers. All methods are compared to matched filtering
with the help of a figure of merit based on the detection of supernovae signals
simulated by Zwerger and Muller.Comment: 5 pages, proceedings of GWDAW99 (Roma, Dec. 1999), to appear in Int.
J. Mod. Phys.
Black hole formation via hypercritical accretion during common envelope evolution
Neutron stars inspiralling into a stellar envelope can accrete at rates
vastly exceeding the Eddington limit if the flow develops pressures high enough
to allow neutrinos to radiate the released gravitational energy. It has been
suggested that this hypercritical mode of accretion leads inevitably to the
formation of stellar mass black holes during common envelope evolution. We
study the hydrodynamics of this flow at large radii (R >> R_ns), and show that
for low Mach number flows, in two dimensions, modest density gradients in the
stellar envelope suffice to produce a hot, advection dominated accretion disk
around the accreting object. The formation of outflows from such a disk is
highly probable, and we discuss the impact of the resultant mass loss and
feedback of energy into the envelope for the survival of the neutron star.
Unless outflows are weaker than those inferred for well observed accreting
systems, we argue that in most cases insufficient accretion occurs to force
collapse to a black hole before the envelope has been ejected. This conclusions
is of interest for black hole formation in general, for some models of gamma
ray bursts, and for predictions of the event rate in future LIGO observations.Comment: ApJ, submitte
Neutrino Scattering, Absorption and Annihilation above the accretion disks of Gamma Ray Bursts
The central engine that drives gamma ray burst (GRB) explosions may derive
from the ability of electrons/positrons and nucleons to tap into the momentum
and energy from the large neutrino luminosity emitted by an accretion disk
surrounding a black hole. This transfer of momentum and energy occurs due to
neutrino absorption, scattering, and annihilation and the non-spherical
geometry of the source both increases the annihilation efficiency and, close to
the black hole, directs the momentum transfer towards the disk axis. We present
annihilation efficiencies and the momentum/energy transfers for a number of
accretion disk models and compute the critical densities of infalling material
below which the transfer of neutrino momentum/energy will lead to an explosion.
Models in which the neutrinos and antineutrinos become trapped within the disk
have noticeably different momentum and energy deposition structure compared to
thin disk models that may lead to significant differences in the explosion
dynamics
- âŠ