916 research outputs found
An upper limit on the X-ray luminosity of the black hole - microlens OGLE-1999-BUL-32
We present an upper limit on the 3--20 keV X-ray flux from the black hole -
microlens OGLE-1999-BUL-32, based on RXTE/PCA scans over the Galactic Center
region in 1999-2000. It is shown that the X-ray luminosity of the black hole
did not exceed L(3-20 keV)<3e33(d/1kpc)^2 ergs/s (where d is the distance to
the black hole). Near the maximum of the background star amplification by the
microlens (July 6, 1999), the upper limit on the X-ray flux corresponds to an
X-ray luminosity L(3-20 keV)<7e33(d/1kpc)^2 ergs/s.Comment: 4 pages, 3 figures. Accepted for publication in Astronomy Letter
The Convective Urca Process with Implicit Two-Dimensional Hydrodynamics
Consideration of the role of the convective flux in the thermodymics of the
convective Urca neutrino loss process in degenerate, convective, quasi-static,
carbon-burning cores shows that the convective Urca process slows down the
convective current around the Urca-shell, but, unlike the "thermal" Urca
process, does not reduce the entropy or temperature for a given convective
volume. Here we demonstrate these effects with two-dimensional numerical
hydrodynamical calculations. These two-dimensional implicit hydrodynamics
calculations invoke an artificial speeding up of the nuclear and weak rates.
They should thus be regarded as indicative, but still qualitative. We find
that, compared to a case with no Urca-active nuclei, the case with Urca effects
leads to a higher entropy in the convective core because the energy released by
nuclear burning is confined to a smaller volume by the effective boundary at
the Urca shell. All else being equal, this will tend to accelerate the
progression to dynamical runaway. We discuss the open issues regarding the
impact of the convective Urca process on the evolution to the "smoldering
phase" and then to dynamical runaway.Comment: 22 pages, 11 figures, accepted for publication in the Astrophysical
Journa
Merging White Dwarf/Black Hole Binaries and Gamma-Ray Bursts
The merger of compact binaries, especially black holes and neutron stars, is
frequently invoked to explain gamma-ray bursts (GRB's). In this paper, we
present three dimensional hydrodynamical simulations of the relatively
neglected mergers of white dwarfs and black holes. During the merger, the white
dwarf is tidally disrupted and sheared into an accretion disk. Nuclear
reactions are followed and the energy release is negligible. Peak accretion
rates are ~0.05 Msun/s (less for lower mass white dwarfs) lasting for
approximately a minute. Many of the disk parameters can be explained by a
simple analytic model which we derive and compare to our simulations. This
model can be used to predict accretion rates for white dwarf and black hole (or
neutron star) masses which are not simulated in this paper. Although the
mergers studied here create disks with larger radii, and longer accretion times
than those from the merger of double neutron stars, a larger fraction of the
merging star's mass becomes part of the disk. Thus the merger of a white dwarf
and a black hole could produce a long duration GRB. The event rate of these
mergers may be as high as 1/Myr per galaxy.Comment: 17 pages text + 9 figures, minor corrections to text and tables,
added references, accepted by Ap
The Role of Kinetic Energy Flux in the Convective Urca Process
The previous analysis of the convective Urca neutrino loss process in
degenerate, convective, quasi-static, carbon-burning cores by Barkat and
Wheeler omitted specific consideration of the role of the kinetic energy flux.
The arguments of Barkat and Wheeler that steady-state composition gradients
exist are correct, but chemical equilibrium does not result in net cooling.
Barkat and Wheeler included a "work" term that effectively removed energy from
the total energy budget that could only have come from the kinetic energy,
which must remain positive. Consideration of the kinetic energy in the
thermodynamics of the convective Urca process shows that the convective Urca
neutrinos reduce the rate of increase of entropy that would otherwise be
associated with the input of nuclear energy and slow down the convective
current, but, unlike the "thermal" Urca process do not reduce the entropy or
temperature.Comment: 16 pages, AAS LaTex, in press, Astrophysical Journal, September 20,
Vol 52
A gamma ray burst with small contamination
We present a scenario (SupraNova) for the formation of GRBs occurring when a
supramassive neutron star (SMNS) loses so much angular momentum that
centrifugal support against self--gravity becomes impossible, and the star
implodes to a black hole. This may be the baryon--cleanest environment proposed
so far, because the SN explosion in which the SMNS formed swept the medium
surrounding the remnant, and the quickly spinning remnant loses energy through
magnetic dipole radiation at a rate exceeding its Eddington luminosity by some
four orders of magnitude. The implosion is adiabatic because neutrinos have
short mean free paths, and silent, given the prompt collapse of the polar caps.
However, a mass ~ 0.1 M_solar in the equatorial belt can easily reach
centrifugal equilibrium. The mechanism of energy extraction is via the
conversion of the Poynting flux (due to the large--scale magnetic field locked
into the minitorus) into a magnetized relativistic wind. Occasionally this
model will produce quickly decaying, or non--detectable afterglows.Comment: To appear in The Astrophysical Journal Letters. AASTeX LateX, no
figure
Global General Relativistic Magnetohydrodynamic Simulations of Accretion Tori
This paper presents an initial survey of the properties of accretion flows in
the Kerr metric from three-dimensional, general relativistic
magnetohydrodynamic simulations of accretion tori. We consider three fiducial
models of tori around rotating, both prograde and retrograde, and nonrotating
black holes; these three fiducial models are also contrasted with axisymmetric
simulations and a pseudo-Newtonian simulation with equivalent initial
conditions to delineate the limitations of these approximations.Comment: Submitted to ApJ. 30 pages, 21 figures. Animations and
high-resolution version of figures available at
http://www.astro.virginia.edu/~jd5
IAC-DIDAS-N: A Dynamic Interactive Decision Analysis and Support System for Multicriteria Analysis of Nonlinear Models with Nonlinear Model Generator Supporting Model Analysis
This paper is one of the series of 11 Working Papers presenting the software for interactive decision support and software tools for developing decision support systems. These products constitute the outcome of the contracted study agreement between the System and Decision Sciences Program at IIASA and several Polish scientific institutions. The theoretical part of these results is presented in the IIASA Working Paper WP-88-071 entitled "Theory, Software and Testing Examples in Decision Support Systems". This volume contains the theoretical and methodological backgrounds of the software systems developed within the project.
This paper presents the user documentation for decision analysis and support systems of DIDAS family designed for supporting decision problems when the model of the system under study can be formulated in terms of set of nonlinear equations. The program presented in the paper, called IAC-DIDAS-N is provided with a nonlinear model generator and editor that support definition, edition and symbolic differentiation of nonlinear models for multiobjective decision analysis. A specially introduced standard of defining nonlinear programming models for multiobjective optimization helps to connect the model generator with other parts of the system. Optimization runs involved in interactive, multiobjective decision analysis are performed by a new version of nonlinear programming algorithm specially adapted for multiobjective problems. This algorithm is based on shifted penalty functions and projected conjugate directions techniques.
An attachment to this paper presents user documentation for a pilot version of a nonlinear model generator with facilities for symbolic differentiation and other means of fundamental model analysis
Nearby Microlensing Events - Identification of the Candidates for the SIM
The Space Interferometry Mission (SIM) is the instrument of choice when it
comes to observing astrometric microlensing events where nearby, usually
high-proper-motion stars (``lenses''), pass in front of more distant stars
(``sources''). Each such encounter produces a deflection in the source's
apparent position that when observed by SIM can lead to a precise mass
determination of the nearby lens star. We search for lens-source encounters
during the 2005-2015 period using Hipparcos, ACT and NLTT to select lenses, and
USNO-A2.0 to search for the corresponding sources, and rank these by the SIM
time required for a 1% mass measurement.
For Hipparcos and ACT lenses, the lens distance and lens-source impact
parameter are precisely determined so the events are well characterized. We
present 32 candidates beginning with a 61 Cyg A event in 2012 that requires
only a few minutes of SIM time. Proxima Centauri and Barnard's star each
generate several events. For NLTT lenses, the distance is known only to a
factor of 3, and the impact parameter only to 1''. Together, these produce
uncertainties of a factor ~10 in the amount of SIM time required. We present a
list of 146 NLTT candidates and show how single-epoch CCD photometry of the
candidates could reduce the uncertainty in SIM time to a factor of ~1.5.Comment: ApJ accepted, 31 pages (inc. 5 tables), 5 figures. t SIM refine
The Influences of Outflow on the Dynamics of Inflow
Both numerical simulations and observations indicate that in an
advection-dominated accretion flow most of the accretion material supplied at
the outer boundary will not reach the inner boundary. Rather, they are lost via
outflow. Previously, the influence of outflow on the dynamics of inflow is
taken into account only by adopting a radius-dependent mass accretion rate
with . In this paper, based on a 1.5
dimensional description to the accretion flow, we investigate this problem in
more detail by considering the interchange of mass, radial and azimuthal
momentum, and the energy between the outflow and inflow. The physical
quantities of the outflow is parameterized based on our current understandings
to the properties of outflow mainly from numerical simulations of accretion
flows. Our results indicate that under reasonable assumptions to the properties
of outflow, the main influence of outflow has been properly included by
adopting .Comment: 16 pages, 5 figures. accepted for publication in Ap
Observational Prospects for Afterglows of Short Duration Gamma-ray Bursts
If the efficiency for producing -rays is the same in short duration
(\siml 2 s) Gamma-Ray Bursts (GRBs) as in long duration GRBs, then the
average kinetic energy of short GRBs must be times less than that of
long GRBs. Assuming further that the relativistic shocks in short and long
duration GRBs have similar parameters, we show that the afterglows of short
GRBs will be on average 10--40 times dimmer than those of long GRBs. We find
that the afterglow of a typical short GRB will be below the detection limit
(\siml 10 \microJy) of searches at radio frequencies. The afterglow would be
difficult to observe also in the optical, where we predict R \simg 23 a few
hours after the burst. The radio and optical afterglow would be even fainter if
short GRBs occur in a low-density medium, as expected in NS-NS and NS-BH merger
models. The best prospects for detecting short-GRB afterglows are with early
(\siml 1 day) observations in X-rays.Comment: 5 pages, 2 figures, submitted to ApJ lette
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