12 research outputs found
Gravitational perturbations of a Kerr black hole in gravity
Modified theories of gravity are often built such that they contain general
relativity as a limiting case. This inclusion property implies that the Kerr
metric is common to many families of theories. For example, all analytic
theories with vanishing constant term admit the Kerr solution. In any given
theory, however, the response of the gravitational field to astrophysical
disturbances is tied to the structure of the field equations. As such, even if
black holes are Kerr, the underlying theory can, in principle, be probed
through gravitational distortions. In this paper, we study linear perturbations
of a Kerr black hole in gravity using the Newman-Penrose formalism. We
show that, as in general relativity, the equations governing the perturbed
metric, which depend on the quadratic term of the function , completely
decouple.Comment: 8 pages. Accepted for publication in Phys. Rev.
Evolutionary implications of a magnetar interpretation for GLEAM-X J162759.5-523504.3
The radio pulsar GLEAM-X J162759.5-523504.3 has an extremely long spin period
(P = 1091.17\, \mbox{s}), and yet seemingly continues to spin down rapidly
(\dot{P} < 1.2 \times 10^{-9}\, \mbox{ss}^{-1}). The magnetic field strength
that is implied, if the source is a neutron star undergoing magnetic dipole
braking, could exceed 10^{16}\,\mbox{G}. This object may therefore be the
most magnetised neutron star observed to date. In this paper, a critical
analysis of a magnetar interpretation for the source is provided. (i) A minimum
polar magnetic field strength of B \sim 5 \times 10^{15}\,\mbox{G} appears to
be necessary for the star to activate as a radio pulsar, based on conventional
`death valley' assumptions. (ii) Back-extrapolation from magnetic braking and
Hall-plastic-Ohm decay suggests that a large angular momentum reservoir was
available at birth to support intense field amplification. (iii) The
observational absence of X-rays constrains the star's field strength and age,
as the competition between heating from field decay and Urca cooling implies a
surface luminosity as a function of time. If the object is an isolated, young
(\sim 10\, \mbox{kyr}) magnetar with a present-day field strength of B
\gtrsim 10^{16}\,\mbox{G}, the upper limit (\approx 10^{30}\, \mbox{erg
s}^{-1}) set on its thermal luminosity suggests it is cooling via a direct
Urca mechanism.Comment: 12 pages, 6 figures. Accepted for publication in MNRA
Measuring spin in coalescing binaries of neutron stars showing double precursors
Gamma-ray bursts resulting from binary neutron-star mergers are sometimes
preceded by precursor flares. These harbingers may be ignited by quasi-normal
modes, excited by orbital resonances, shattering the stellar crust of one of
the inspiralling stars up to seconds before coalescence. In the
rare case that a system displays two precursors, successive overtones of either
interface- or -modes may be responsible for the overstrainings. Since the
free-mode frequencies of these overtones have an almost constant ratio, and the
inertial-frame frequencies for rotating stars are shifted relative to static
ones, the spin frequency of the flaring component can be constrained as a
function of the equation of state, the binary mass ratio, the mode quantum
numbers, and the spin-orbit misalignment angle. As a demonstration of the
method, we find that the precursors of GRB090510 hint at a spin frequency range
of for the shattering star if we
allow for an arbitrary misalignment angle, assuming -modes account
for the events.Comment: 11 pages, 6 figures, 2 tables, with an appendix containing 1 figur
General-relativistic treatment of tidal -mode resonances in coalescing binaries of neutron stars. II. As triggers for precursor flares of short gamma-ray bursts
In some short gamma-ray bursts, precursor flares occurring seconds
prior to the main episode have been observed. These flares may then be
associated with the last few cycles of the inspiral when the orbital frequency
is a few hundred Hz. During these final cycles, tidal forces can resonantly
excite quasi-normal modes in the inspiralling stars, leading to a rapid
increase in their amplitude. It has been shown that these modes can exert
sufficiently strong strains onto the neutron star crust to instigate yieldings.
Due to the typical frequencies of -modes being , their
resonances with the orbital frequency match the precursor timings and warrant
further investigation. Adopting realistic equations of state and solving the
general-relativistic pulsation equations, we study -mode resonances in
coalescing quasi-circular binaries, where we consider various stellar rotation
rates, degrees of stratification, and magnetic field structures. We show that
for some combination of stellar parameters, the resonantly excited - and
-modes may lead to crustal failure and trigger precursor flares.Comment: 14 pages, 8 figures, 2 tables, submitted to MNRA
Magnetic equilibria of relativistic axisymmetric stars: The impact of flow constants
Symmetries and conservation laws associated with the ideal Einstein-Euler
system, for stationary and axisymmetric stars, can be utilized to define a set
of flow constants. These quantities are conserved along flow lines in the sense
that their gradients are orthogonal to the four-velocity. They are also
conserved along surfaces of constant magnetic flux, making them powerful tools
to identify general features of neutron star equilibria. One important
corollary of their existence is that mixed poloidal-toroidal fields are
inconsistent with the absence of meridional flows except in some singular
sense, a surprising but powerful result first proven by Bekenstein and Oron. In
this work, we revisit the flow constant formalism to rederive this result
together with several new ones concerning both nonlinear and perturbative
magnetic equilibria. Our investigation is supplemented by some numerical
solutions for multipolar magnetic fields on top of a Tolman-VII background,
where strict power-counting of the flow constants is used to ensure a
self-consistent treatment.Comment: 20 pages, 6 figures. Minor changes to match published versio
Constraining equation of state groups from -mode asteroseismology
Buoyancy-restored modes inside neutron stars depend sensitively on both the
microphysical (e.g., composition and entropy gradients) and macrophysical
(e.g., stellar mass and radius) properties of the star. Asteroseismology
efforts for -modes are therefore particularly promising avenues for
recovering information concerning the nuclear equation of state. In this work
it is shown that the overall low-temperature -space consists of multiple
groups corresponding to different classes of equation of state (e.g., hadronic
vs. hybrid). This is in contrast to the case of pressure-driven modes, for
example, which tend to follow a universal relation regardless of microphysical
considerations. Using a wide library of currently-viable equations of state,
perturbations of static, stratified stars are calculated in general relativity
to demonstrate in particular how -space groupings can be classified
according to the mean mass density, temperature, central speed of sound, and
tidal deformability. Considering present and future observations regarding
gravitational waves, accretion outbursts, quasi-periodic oscillations, and
precursor flashes from gamma-ray bursts, it is shown how one might determine
which group the -modes belong to.Comment: 14 pages, 10 figures, 1 table; accepted by MNRA