47 research outputs found
Stirring the Embers: High Sensitivity VLBI Observations of GRB030329
We present high sensitivity Very Long Baseline Interferometry (VLBI)
observations 806 days after the gamma-ray burst of 2003 March 29 (GRB030329).
The angular diameter of the radio afterglow is measured to be 0.347 +- 0.09
mas, corresponding to 0.99 +- 0.26 pc at the redshift of GRB030329 (z =
0.1685). The evolution of the image size favors a uniform external density over
an R^-2 wind-like density profile (at distances of R >~10^18 cm from the
source), although the latter cannot be ruled out yet. The current apparent
expansion velocity of the image size is only mildly relativistic, suggesting a
non-relativistic transition time of t_NR ~ 1 yr. A rebrightening, or at least a
significant flattening in the flux decay, is expected within the next several
years as the counter-jet becomes visible (this has not yet been observed). An
upper limit of <1.9c is set on the proper motion of the flux centroid.Comment: 16 pages, 5 figures, accepted for publication in Astrophysical
Journa
The relation between accretion rate and jet power in X-ray luminous elliptical galaxies
Using Chandra X-ray observations of 9 nearby, X-ray luminous ellipticals with
good optical velocity dispersion measurements, we show that a tight correlation
exists between the Bondi accretion rates calculated from the X-ray data and
estimated black hole masses, and the power emerging from these systems in
relativistic jets. The jet powers, inferred from the energies and timescales
required to inflate the cavities observed in the surrounding X-ray emitting
gas, can be related to the accretion rates by a power law model. A significant
fraction (2.2^{+1.0}_{-0.7} per cent, for P_jet=10^{43} erg/s) of the energy
associated with the rest mass of material entering the accretion radius
eventually emerges in the jets. The data also hint that this fraction may rise
slightly with increasing jet power. Our results have significant implications
for studies of accretion, jet formation and galaxy formation. The tight
correlation between P_Bondi and P_jet suggests that the Bondi formulae provide
a reasonable description of the accretion process, despite the likely presence
of magnetic pressure and angular momentum in the accreting gas, and that the
accretion flows are approximately stable over timescales of a few million
years. Our results show that the black hole `engines' at the hearts of large
elliptical galaxies and groups can feed back sufficient energy to stem cooling
and star formation, leading naturally to the observed exponential cut off at
the bright end of the galaxy luminosity function.Comment: Accepted for publication in MNRAS. 10 pages, 4 figures. Includes an
enhanced statistical analysis and some additional data. Conclusions unchange
Post-Newtonian SPH calculations of binary neutron star coalescence. I. Method and first results
We present the first results from our Post-Newtonian (PN) Smoothed Particle
Hydrodynamics (SPH) code, which has been used to study the coalescence of
binary neutron star (NS) systems. The Lagrangian particle-based code
incorporates consistently all lowest-order (1PN) relativistic effects, as well
as gravitational radiation reaction, the lowest-order dissipative term in
general relativity. We test our code on sequences of single NS models of
varying compactness, and we discuss ways to make PN simulations more relevant
to realistic NS models. We also present a PN SPH relaxation procedure for
constructing equilibrium models of synchronized binaries, and we use these
equilibrium models as initial conditions for our dynamical calculations of
binary coalescence. Though unphysical, since tidal synchronization is not
expected in NS binaries, these initial conditions allow us to compare our PN
work with previous Newtonian results.
We compare calculations with and without 1PN effects, for NS with stiff
equations of state, modeled as polytropes with . We find that 1PN
effects can play a major role in the coalescence, accelerating the final
inspiral and causing a significant misalignment in the binary just prior to
final merging. In addition, the character of the gravitational wave signal is
altered dramatically, showing strong modulation of the exponentially decaying
waveform near the end of the merger. We also discuss briefly the implications
of our results for models of gamma-ray bursts at cosmological distances.Comment: RevTeX, 37 pages, 17 figures, to appear in Phys. Rev. D, minor
corrections onl
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The Giant Flare From SGR 1806-20 And Its Radio Afterglow
The multi-wavelength observations of the 2004 December 27 Giant Flare (GF) from SGR 1806-20 and its long-lived radio afterglow are briefly reviewed. The GF appears to have been produced by a dramatic reconfiguration of the magnetic field near the surface of the neutron star, possibly accompanied by fractures in the crust. The explosive release of over 10{sup 46} erg (isotropic equivalent) powered a one-sided mildly relativistic outflow. The outflow produced a new expanding radio nebula, that is still visible over a year after the GF. Also considered are the constraints on the total energy in the GF, the energy and mass in the outflow, and on the external density, as well as possible implications for short {gamma}-ray bursts and potential signatures in high energy neutrinos, photons, or cosmic rays. Some possible future observations of this and other GFs are briefly discussed