662 research outputs found
Magnetic Stress at the Marginally Stable Orbit: Altered Disk Structure, Radiation, and Black Hole Spin Evolution
Magnetic connections to the plunging region can exert stresses on the inner
edge of an accretion disk around a black hole. We recompute the relativistic
corrections to the thin-disk dynamics equations when these stresses take the
form of a time-steady torque on the inner edge of the disk. The additional
dissipation associated with these stresses is concentrated relatively close
outside the marginally stable orbit, scaling as r to the -7/2 at large radius.
As a result of these additional stresses: spin-up of the central black hole is
retarded; the maximum spin-equilibrium accretion efficiency is 36%, and occurs
at a/M=0.94; the disk spectrum is extended toward higher frequencies; line
profiles (such as Fe K-alpha) are broadened if the line emissivity scales with
local flux; limb-brightening, especially at the higher frequencies, is
enhanced; and the returning radiation fraction is substantially increased, up
to 58%. This last effect creates possible explanations for both synchronized
continuum fluctuations in AGN, and polarization rises shortward of the Lyman
edge in quasars. We show that no matter what additional stresses occur, when
a/M < 0.36, the second law of black hole dynamics sets an absolute upper bound
on the accretion efficiency.Comment: 11 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Resident Corneal Cells Communicate with Neutrophils Leading to the Production of IP-10 during the Primary Inflammatory Response to HSV-1 Infection
In this study we show that murine and human neutrophils are capable of secreting IP-10 in response to communication from the HSV-1 infected cornea and that they do so in a time frame associated with the recruitment of CD8+ T cells and CXCR3-expressing cells. Cellular markers were used to establish that neutrophil influx corresponded in time to peak IP-10 production, and cellular depletion confirmed neutrophils to be a significant source of IP-10 during HSV-1 corneal infection in mice. A novel ex vivo model for human corneal tissue infection with HSV-1 was used to confirm that cells resident in the cornea are also capable of stimulating neutrophils to secrete IP-10. Our results support the hypothesis that neutrophils play a key role in T-cell recruitment and control of viral replication during HSV-1 corneal infection through the production of the T-cell recruiting chemokine IP-10
Causal Viscosity in Accretion Disc Boundary Layers
The structure of the boundary layer region between the disc and a
comparatively slowly rotating star is studied using a causal prescription for
viscosity. The vertically integrated viscous stress relaxes towards its
equilibrium value on a relaxation timescale , which naturally yields a
finite speed of propagation for viscous information. For a standard alpha
prescription with alpha in the range 0.1-0.01, and ratio of viscous speed to
sound speed in the range 0.02-0.5, details in the boundary layer are strongly
affected by the causality constraint. We study both steady state polytropic
models and time dependent models, taking into account energy dissipation and
transport. Steady state solutions are always subviscous with a variety of
profiles which may exhibit near discontinuities. For alpha =0.01 and
small viscous speeds, the boundary layer adjusted to a near steady state. A
long wavelength oscillation generated by viscous overstability could be seen at
times near the outer boundary. Being confined there, the boundary layer
remained almost stationary. However, for alpha =0.1 and large viscous speeds,
short wavelength disturbances were seen throughout which could significantly
affect the power output in the boundary layer. This could be potentially
important in producing time dependent behaviour in accreting systems such as
CVs and protostars.Comment: 10 LateX pages, requires lamuphys.sty and psfig.sty, 3 figures
included, to appear in the Proceedings of the EARA Workshop on Accretion
Disks (Garching, Oct. 96), Lecture Notes in Physic
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
Gravitational radiation from precessing accretion disks in gamma-ray bursts
We study the precession of accretion disks in the context of gamma-ray burst
inner engines. Our aim is to quantitatively estimate the characteristics of
gravitational waves produced by the precession of the transient accretion disk
in gamma-ray bursts. We evaluate the possible periods of disk precession caused
by the Lense-Thirring effect using an accretion disk model that allows for
neutrino cooling. Assuming jet ejection perpendicular to the disk plane and a
typical intrinsic time-dependence for the burst, we find gamma-ray light curves
that have a temporal microstructure similar to that observed in some reported
events. The parameters obtained for the precession are then used to evaluate
the production of gravitational waves. We find that the precession of accretion
disks of outer radius smaller than cm and accretion rates above 1 solar
mass per second could be detected by Advanced LIGO if they occur at distances
of less than 100 Mpc. We conclude that the precession of a neutrino-cooled
accretion disk in long gamma-ray bursts can be probed by gravitational wave
astronomy. Precession of the disks in short gamma-ray events is undetectable
with the current technology.Comment: 5 pages, 5 figures, accepted for publication in A&
Advection-Dominated Accretion Model of Sagittarius A*: Evidence for a Black Hole at the Galactic Center
Sgr A* at the Galactic Center is a puzzling source. It has a mass
M=(2.5+/-0.4) x 10^6 solar masses which makes it an excellent black hole
candidate. Observations of stellar winds and other gas flows in its vicinity
suggest a mass accretion rate approximately few x 10^{-6} solar masses per
year. However, such an accretion rate would imply a luminosity > 10^{40} erg/s
if the radiative efficiency is the usual 10 percent, whereas observations
indicate a bolometric luminosity <10^{37} erg/s. The spectrum of Sgr A* is
unusual, with emission extending over many decades of wavelength. We present a
model of Sgr A* which is based on a two-temperature optically-thin
advection-dominated accretion flow. The model is consistent with the estimated
mass and accretion rate, and fits the observed fluxes in the cm/mm and X-ray
bands as well as upper limits in the sub-mm and infrared bands; the fit is less
good in the radio below 86 GHz and in gamma-rays above 100 MeV. The very low
luminosity of Sgr A* is explained naturally in the model by means of advection.
Most of the viscously dissipated energy is advected into the central mass by
the accreting gas, and therefore the radiative efficiency is extremely low,
approximately 5 x 10^{-6}. A critical element of the model is the presence of
an event horizon at the center which swallows the advected energy. The success
of the model could thus be viewed as confirmation that Sgr A* is a black hole.Comment: 41 pages (Latex) including 6 Figures and 2 Tables. Final Revised
Version changes to text, tables and figures. ApJ, 492, in pres
Efficiency of Electron-Positron Pair Productionby Neutrino Flux from Accretion Disk of a Kerr Black Hole
Dominant processes of neutrino production and neutrino-induced \ep-pair
production are examined in the model of a disk hyper-accreting onto a Kerr
black hole. The efficiency of plasma production by a neutrino flux from the
disk, obtained for the both cases of presence and absence of a magnetic field,
is found to be no more than several tenths of percent and, therefore, not
enough for the origin of cosmological gamma-ray bursts.Comment: 8 pages, 1 figur
Cannonballs in the context of Gamma Ray Bursts: Formation sites ?
We investigate possible formation sites of the cannonballs (in the gamma ray
bursts context) by calculating their physical parameters, such as density,
magnetic field and temperature close to the origin. Our results favor scenarios
where the cannonballs form as instabilities (knots) within magnetized jets from
hyperaccreting disks. These instabilities would most likely set in beyond the
light cylinder where flow velocity with Lorentz factors as high as 2000 can be
achieved. Our findings challenge the cannonball model of gamma ray bursts if
these indeed form inside core-collapse supernovae (SNe) as suggested in the
literature; unless hyperaccreting disks and the corresponding jets are common
occurrences in core-collapse SNe.Comment: 10 pages, 12 figure
Ballooning Instability in Polar Caps of Accreting Neutron Stars
We assess the stability of Kruskal-Schwarzschild (magnetic Rayleigh-Taylor)
type modes for accreted matter on the surface of a neutron star confined by a
strong (>= 1.E12 G) magnetic field. Employing the energy principle to analyze
the stability of short-wavelength ballooning modes, we find that line-tying to
the neutron star crust stabilizes these modes until the overpressure at the top
of the neutron star crust exceeds the magnetic pressure by a factor ~ 8(a/h),
where a and h are respectively the lateral extent of the accretion region and
the density scale height. The most unstable modes are localized within a
density scale height above the crust. We calculate the amount of mass that can
be accumulated at the polar cap before the onset of instability.Comment: 8 pages, 2 figures, accepted for publication by ApJ, uses AASTEX 5.0
and emulateapj5.sty (included
- …