250 research outputs found
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
On the high coherence of kilo-Hz Quasi-Periodic Oscillations
We have carried out a systematic study of the properties of the kilo-Hertz
quasi-periodic oscillations (QPO) observed in the X-ray emission of the neutron
star low-mass X-ray binary 4U1608-52, using archival data obtained with the
Rossi X-ray Timing Explorer. We have investigated the quality factor, Q, of the
oscillations (defined as the ratio of the frequency of the QPO peak to its full
width at half maximum). In order to minimise the effect of long-term frequency
drifts, power spectra were computed over the shortest times permitted by the
data statistics. We show that the high Q of ~200 reported by Berger et al.
(1996) for the lower frequency kilo-Hz QPO in one of their observations is by
no means exceptional, as we observe a mean Q value in excess of 150 in 14 out
of the 21 observations analysed and Q can remain above 200 for thousands of
seconds. The frequency of the QPO varies over the wide range 560--890 Hz and we
find a systematic trend for the coherence time of the QPO, estimated as tau=Q
/(pi nu), to increase with the frequency, up to a maximum level at ~ 800 Hz,
beyond which it appears to decrease, at frequencies where the QPO weakens.
There is a more complex relationship between tau and the QPO root mean squared
amplitude (RMS), in which positive and negative correlations can be found. A
higher-frequency QPO, revealed by correcting for the frequency drift of the
560-890 Hz one, has a much lower Q (~10) which does not follow the same
pattern. We discuss these results in the framework of competing QPO models and
show that those involving clumps orbiting within or above the accretion disk
are ruled out.Comment: Accepted for publication in MNRAS, 8 pages, 6 figures, 3 Table
Innermost stable circular orbits around relativistic rotating stars
We investigate the innermost stable circular orbit (ISCO) of a test particle
moving on the equatorial plane around rotating relativistic stars such as
neutron stars. First, we derive approximate analytic formulas for the angular
velocity and circumferential radius at the ISCO making use of an approximate
relativistic solution which is characterized by arbitrary mass, spin, mass
quadrupole, current octapole and mass -pole moments. Then, we show that
the analytic formulas are accurate enough by comparing them with numerical
results, which are obtained by analyzing the vacuum exterior around numerically
computed geometries for rotating stars of polytropic equation of state. We
demonstrate that contribution of mass quadrupole moment for determining the
angular velocity and, in particular, the circumferential radius at the ISCO
around a rapidly rotating star is as important as that of spin.Comment: 12 pages, 2 figures, accepted for publication in Phys. Rev.
A model for upper kHz QPO coherence of accreting neutron star
{We investigate the coherence of the twin kilohertz quasi-periodic
oscillations (kHz QPOs) in the low-mass X-ray binary (LMXB) theoretically. The
profile of upper kHz QPO, interpreted as Keplerian frequency, is ascribed to
the radial extent of the kHz QPO emission region, associated with the
transitional layer at the magnetosphere-disk boundary, which corresponds to the
coherence of upper kHz QPO. The theoretical model for Q-factor of upper kHz QPO
is applied to the observational data of five Atoll and five Z sources, and the
consistence is implied.Comment: accepted by A&
Population synthesis of neutron stars, strange (quark) stars and black holes
We compute and present the distribution in mass of single and binary neutron
stars, strange stars, and black holes. The calculations were performed using a
stellar population synthesis code. We follow all phases of single and binary
evolution, starting from a ZAMS binary and ending in the creation of one
compact object (neutron star, black hole, strange star) and a white dwarf, or
two compact objects (single or binary). We assume that neutron stars are formed
in the collapse of iron/nickel cores in the mass range M0 < M < M1, quark stars
in the range M1 M2 and find that
the population of quark stars can easily be as large as the population of black
holes, even if there is only a small mass window for their formation.Comment: 4 pages, 4 figures, to appear in the proceedings of "The 4th Integral
Workshop
Astrophysical Probes of the Constancy of the Velocity of Light
We discuss possible tests of the constancy of the velocity of light using
distant astrophysical sources such as gamma-ray bursters (GRBs), Active
Galactic Nuclei (AGNs) and pulsars. This speculative quest may be motivated by
some models of quantum fluctuations in the space-time background, and we
discuss explicitly how an energy-dependent variation in photon velocity \delta
c/ c \sim - E / M arises in one particular quantum-gravitational model. We then
discuss how data on GRBs may be used to set limits on variations in the
velocity of light, which we illustrate using BATSE and OSSE observations of the
GRBs that have recently been identified optically and for which precise
redshifts are available. We show how a regression analysis can be performed to
look for an energy-dependent effect that should correlate with redshift. The
present data yield a limit M \gsim 10^{15} GeV for the quantum gravity scale.
We discuss the prospects for improving this analysis using future data, and how
one might hope to distinguish any positive signal from astrophysical effects
associated with the sources.Comment: 37 pages LaTeX, 9 eps figures included, uses aasms4.st
Testing slim-disk models on the thermal spectra of LMC X-3
Slim-disk models describe accretion flows at high luminosities, while
reducing to the standard thin disk form in the low luminosity limit. We have
developed a new spectral model, slimbb, within the framework of XSPEC, which
describes fully relativistic slim-disk accretion and includes photon
ray-tracing that starts from the disk photosphere, rather than the equatorial
plane. We demonstrate the features of this model by applying it to RXTE spectra
of the persistent black-hole X-ray binary LMC X-3. LMC X-3 has the virtues of
exhibiting large intensity variations while maintaining itself in soft spectral
states which are well described using accretion-disk models, making it an ideal
candidate to test the aptness of slimbb. Our results demonstrate consistency
between the low-luminosity (thin-disk) and high luminosity (slim-disk) regimes.
We also show that X-ray continuum-fitting in the high accretion rate regime can
powerfully test black-hole accretion disk models.Comment: 6 pages, 5 figures, submitted to A&
Distribution of compact object mergers around galaxies
Compact object mergers are one of the currently favored models for the origin
of GRBs. The discovery of optical afterglows and identification of the nearest,
presumably host, galaxies allows the analysis of the distribution of burst
sites with respect to these galaxies. Using a model of stellar binary evolution
we synthesize a population of compact binary systems which merge within the
Hubble time. We include the kicks in the supernovae explosions and calculate
orbits of these binaries in galactic gravitational potentials. We present the
resulting distribution of merger sites and discuss the results in the framework
of the observed GRB afterglows.Comment: 8 pages, 5 figures, submitted to MNRA
Large scale magnetic fields and their dissipation in GRB fireballs
We consider possible geometries of magnetic fields in GRB outflows, and their
evolution with distance from the source. For magnetically driven outflows, with
an assumed ratio of magnetic to kinetic energy density of order unity, the
field strengths are sufficient for efficient production of gamma-rays by
synchrotron emission in the standard internal shock scenario, without the need
for local generation of small scale fields. In these conditions, the MHD
approximation is valid to large distances (>10^19cm). In outflows driven by
nonaxisymmetric magnetic fields, changes of direction of the field cause
dissipation of magnetic energy by reconnection. This dissipation takes place
outside the photosphere of the outflow, and can convert a significant fraction
of the magnetic energy flux into radiation.Comment: 12 pages including 3 figures, submitted to A&
TeV Astrophysics Constraints on Planck Scale Lorentz Violation
We analyze observational constraints from TeV astrophysics on Lorentz
violating nonlinear dispersion for photons and electrons without assuming any a
priori equality between the photon and electron parameters. The constraints
arise from thresholds for vacuum Cerenkov radiation, photon decay and
photo-production of electron-positron pairs. We show that the parameter plane
for cubic momentum terms in the dispersion relations is constrained to an order
unity region in Planck units. We find that the threshold configuration can
occur with an asymmetric distribution of momentum for pair creation, and with a
hard photon for vacuum Cerenkov radiation.Comment: 4 pages, RevTeX4, 1 figure. Some references and a footnote added,
improved discussion on the photon annihilation and GZK cutoff. Minor changes
of wording. Main results unchanged. Version to appear as a Rapid
Communication in PR
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