13,590 research outputs found
Motion of charged test particles in Reissner--Nordstr\"om spacetime
We investigate the circular motion of charged test particles in the
gravitational field of a charged mass described by the Reissner-Nordstr\"om
(RN) spacetime. We study in detail all the spatial regions where circular
motion is allowed around either black holes or naked singularities. The effects
of repulsive gravity are discussed by finding all the circles at which a
particle can have vanishing angular momentum. We show that the geometric
structure of stable accretion disks, made of only test particles moving along
circular orbits around the central body, allows us to clearly distinguish
between black holes and naked singularities.Comment: 58 pages, 41 figures. To be published in Phys. Rev. D. This work
follows the paper "Circular motion of neutral test particles in
Reissner-Nordstr\'om spacetime" published in Phys.\ Rev.\ D \ Vol.83, No.2
with DOI: 10.1103/PhysRevD.83.024021 (arXiv:1012.5411v1 [astro-ph.HE]
Measuring the neutron star equation of state using X-ray timing
One of the primary science goals of the next generation of hard X-ray timing
instruments is to determine the equation of state of the matter at supranuclear
densities inside neutron stars, by measuring the radius of neutron stars with
different masses to accuracies of a few percent. Three main techniques can be
used to achieve this goal. The first involves waveform modelling. The flux we
observe from a hotspot on the neutron star surface offset from the rotational
pole will be modulated by the star's rotation, giving rise to a pulsation.
Information about mass and radius is encoded into the pulse profile via
relativistic effects, and tight constraints on mass and radius can be obtained.
The second technique involves characterising the spin distribution of accreting
neutron stars. The most rapidly rotating stars provide a very clean constraint,
since the mass-shedding limit is a function of mass and radius. However the
overall spin distribution also provides a guide to the torque mechanisms in
operation and the moment of inertia, both of which can depend sensitively on
dense matter physics. The third technique is to search for quasi-periodic
oscillations in X-ray flux associated with global seismic vibrations of
magnetars (the most highly magnetized neutron stars), triggered by magnetic
explosions. The vibrational frequencies depend on stellar parameters including
the dense matter equation of state. We illustrate how these complementary X-ray
timing techniques can be used to constrain the dense matter equation of state,
and discuss the results that might be expected from a 10m instrument. We
also discuss how the results from such a facility would compare to other
astronomical investigations of neutron star properties. [Modified for arXiv]Comment: To appear in Reviews of Modern Physics as a Colloquium, 23 pages, 9
figure
Constraining the neutron star equation of state using Pulse Profile Modeling
One very promising technique for measuring the dense matter Equation of State
exploits hotspots that form on the neutron star surface due to the pulsar
mechanism, accretion streams, or during thermonuclear explosions in the neutron
star ocean. This article explains how Pulse Profile Modeling of hotspots is
being used by the Neutron Star Interior Composition Explorer (NICER), an X-ray
telescope installed on the International Space Station in 2017 - and why the
technique is a mission driver for the next, larger-area generation of
telescopes including the enhanced X-ray Timing and Polarimetry (eXTP) mission
and the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays
(STROBE-X).Comment: To appear in the AIP Conference Proceedings of the Xiamen-CUSTIPEN
Workshop on the EOS of Dense Neutron-Rich Matter in the Era of Gravitational
Wave Astronomy (January 3 - 7, 2019, Xiamen, China
Tomographic Constraints on High-Energy Neutrinos of Hadronuclear Origin
Mounting evidence suggests that the TeV-PeV neutrino flux detected by the
IceCube telescope has mainly an extragalactic origin. If such neutrinos are
primarily produced by a single class of astrophysical sources via hadronuclear
() interactions, a similar flux of gamma-ray photons is expected. For the
first time, we employ tomographic constraints to pinpoint the origin of the
IceCube neutrino events by analyzing recent measurements of the cross
correlation between the distribution of GeV gamma rays, detected by the Fermi
satellite, and several galaxy catalogs in different redshift ranges. We find
that the corresponding bounds on the neutrino luminosity density are up to one
order of magnitude tighter than those obtained by using only the spectrum of
the gamma-ray background, especially for sources with mild redshift evolution.
In particular, our method excludes any hadronuclear source with a spectrum
softer than as a main component of the neutrino background, if its
evolution is slower than . Starburst galaxies, if able to accelerate
and confine cosmic rays efficiently, satisfy both spectral and tomographic
constraints.Comment: 6 pages, 3 figures; accepted for publication in Physical Review
Letter
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