2,097 research outputs found
Mechanical Model for Relativistic Blast Waves
Relativistic blast waves can be described by a mechanical model. In this
model, the "blast" -- the compressed gas between the forward and reverse shocks
-- is viewed as one hot body. Equations governing its dynamics are derived from
conservation of mass, energy, and momentum. Simple analytical solutions are
obtained in the two limiting cases of ultra-relativistic and non-relativistic
reverse shock. Equations are derived for the general explosion problem.Comment: 8 pages, accepted to ApJ Letter
Inertia of Heat in Advective Accretion Disks around Kerr Black Holes
In the innermost region of the advective accretion disk orbiting a black hole
of high spin, the inertia of heat stored in the accreting gas is comparable to
that of the gas rest mass itself. Accounting for this effect, we derive
additional terms in the disk structure equations, and show that the heat
inertia plays a significant role in the global energy conservation and dynamics
of accretion in the relativistic advective disks.Comment: 6 pages, Latex, submitted to ApJ
Estimates for Lorentz factors of gamma-ray bursts from early optical afterglow observations
The peak time of optical afterglow may be used as a proxy to constrain the
Lorentz factor Gamma of the gamma-ray burst (GRB) ejecta. We revisit this
method by including bursts with optical observations that started when the
afterglow flux was already decaying; these bursts can provide useful lower
limits on Gamma. Combining all analyzed bursts in our sample, we find that the
previously reported correlation between Gamma and the burst luminosity L_gamma
does not hold. However, the data clearly shows a lower bound Gamma_min which
increases with L_gamma. We suggest an explanation for this feature: explosions
with large jet luminosities and Gamma < Gamma_min suffer strong adiabatic
cooling before their radiation is released at the photosphere; they produce
weak bursts, barely detectable with present instruments. To test this
explanation we examine the effect of adiabatic cooling on the GRB location in
the L_gamma - Gamma plane using a Monte Carlo simulation of the GRB population.
Our results predict detectable on-axis "orphan" afterglows. We also derive
upper limits on the density of the ambient medium that decelerates the
explosion ejecta. We find that the density in many cases is smaller than
expected for stellar winds from normal Wolf-Rayet progenitors. The burst
progenitors may be peculiar massive stars with weaker winds or there might
exist a mechanism that reduces the stellar wind a few years before the
explosion.Comment: 24 pages, 7 figures, 2 tables, accepted for publication in Ap
Orbital roulette: a new method of gravity estimation from observed motions
The traditional way of estimating the gravitational field from observed
motions of test objects is based on the virial relation between their kinetic
and potential energy. We find a more efficient method. It is based on the
natural presumption that the objects are observed at a random moment of time
and therefore have random orbital time phases. The proposed estimator, which we
call "orbital roulette", checks the randomness of the phases. The method has
the following advantages: (1) It estimates accurately Keplerian (point-mass)
potentials as well as non-Keplerian potentials where the unknown gravitating
mass is distributed in space. (2) It is a complete statistical estimator: it
checks a trial potential and accepts it or rules it out with a certain
significance level; the best-fit measurement is thus supplemented with error
bars at any confidence level. (3) It needs no a priori assumptions about the
distribution of orbital parameters of the test bodies. We test our estimator
with Monte-Carlo-generated motions and demonstrate its efficiency. Useful
applications include the Galactic Center, dark-matter halo of the Galaxy, and
clusters of stars or galaxies.Comment: 30 pages, accepted to Ap
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