289 research outputs found
Black hole neutron star coalescence as a source of gamma-ray bursts
We present the results of hydrodynamic (SPH) simulations showing the
coalescence of a black hole with a neutron star to be a promising theoretical
source of short duration gamma-ray bursts. The favorable features of the
process include rapid onset, millisecond variability, a duration much longer
than the dynamical timescale, and a range of outcomes sufficient to allow
variety in the properties of individual gamma-ray bursts. Interestingly, the
process of coalescence differs rather markedly from past predictions.Comment: 6 pages, 4 Postscript figures, uses aipproc.sty . To appear in 4th
Gamma Ray Bursts Symposium, C.A. Meegan and P. Cushman, ed
Keplerian frequencies and innermost stable circular orbits of rapidly rotating strange stars
It has been suggested that the frequency in the co-rotating innermost stable
circular orbit (ISCO) about a compact stellar remnant can be determined through
X-ray observations of low-mass X-ray binaries, and that its value can be used
to constrain the equation of state of ultradense matter.
Upon constructing numerical models of rapidly rotating strange (quark) stars
in general relativity, we find that for stars rotating at the equatorial
mass-shedding limit, the ISCO is indeed above the stellar surface, for a wide
range of central energy densities at a height equal to 11% of the
circumferential stellar radius, which scales inversely with the square root of
the energy density, of self-bound quark matter at zero presure. In contrast to
static stars, the ISCO frequencies of rapidly rotating strange stars can be as
low as 0.9 kHz for a 1.3 solar mass strange star. Hence, the presence of
strange stars in low-mass X-ray binaries cannot be excluded on the basis of the
currently observed frequencies of kHz QPOs, such as the cut-off frequency of
1066 Hz in 4U 1820-30.Comment: 5 pg., 4 fig
Holonomy invariance, orbital resonances, and kilohertz QPOs
Quantized orbital structures are typical for many aspects of classical
gravity (Newton's as well as Einstein's). The astronomical phenomenon of
orbital resonances is a well-known example. Recently, Rothman, Ellis and
Murugan (2001) discussed quantized orbital structures in the novel context of a
holonomy invariance of parallel transport in Schwarzschild geometry. We present
here yet another example of quantization of orbits, reflecting both orbital
resonances and holonomy invariance. This strong-gravity effect may already have
been directly observed as the puzzling kilohertz quasi-periodic oscillations
(QPOs) in the X-ray emission from a few accreting galactic black holes and
several neutron stars
Origin of pulsed emission from the young supernova remnant SN 1987A
To overcome difficulties in understanding the origin of the submillisecond optical pulses from SN 1987A a model similar to that of Kundt and Krotscheck for pulsed synchrotron emission from the Crab was applied. The interaction of the expected ultrarelativistic e(sup + or -) pulsar wind with the pulsar dipole electromagnetic wave reflected from the walls of a pulsar cavity within the SN 1987A nubula can generate pulsed optical emission with efficiency at most eta(sub max) is approximately equal to 10(exp -3). The maximum luminosity of the source is reproduced and other observational constraints can be satisfied for an average wind energy flow is approximately equal to 10(exp 38) erg/(s steradian) and for electron Lorentz factor gamma is approximately equal to 10(exp 5). This model applied to the Crab yields pulsations of much lower luminosity and frequency
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