562 research outputs found
Thermonuclear burst oscillations
Burst oscillations, a phenomenon observed in a significant fraction of Type I
(thermonuclear) X-ray bursts, involve the development of highly asymmetric
brightness patches in the burning surface layers of accreting neutron stars.
Intrinsically interesting as nuclear phenomena, they are also important as
probes of dense matter physics and the strong gravity, high magnetic field
environment of the neutron star surface. Burst oscillation frequency is also
used to measure stellar spin, and doubles the sample of rapidly rotating (above
10 Hz) accreting neutron stars with known spins. Although the mechanism remains
mysterious, burst oscillation models must take into account thermonuclear flame
spread, nuclear processes, rapid rotation, and the dynamical role of the
magnetic field. This review provides a comprehensive summary of the
observational properties of burst oscillations, an assessment of the status of
the theoretical models that are being developed to explain them, and an
overview of how they can be used to constrain neutron star properties such as
spin, mass and radius.Comment: Preprint of article submitted to Annual Reviews of Astronomy and
Astrophysics (2012). 35 page
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
The energy dependence of burst oscillations from the accreting millisecond pulsar XTE J1814-338
The nature of the asymmetry that gives rise to Type I X-ray burst
oscillations on accreting neutron stars remains a matter of debate. Of
particular interest is whether the burst oscillation mechanism differs between
the bursting millisecond pulsars and the non-pulsing systems. One means to
diagnose this is to study the energy dependence of the burst oscillations: here
we present an analysis of oscillations from 28 bursts observed during the 2003
outburst of the accreting millisecond pulsar XTE J1814-338. We find that the
fractional amplitude of the burst oscillations falls with energy, in contrast
to the behaviour found by Muno et al. (2003) in the burst oscillations from a
set of non-pulsing systems. The drop with energy mirrors that seen in the
accretion-powered pulsations; in this respect XTE J1814-338 behaves like the
other accreting millisecond pulsars. The burst oscillations show no evidence
for either hard or soft lags, in contrast to the persistent pulsations, which
show soft lags of up to 50 s. The fall in amplitude with energy is
inconsistent with current surface mode and simple hot spot models of burst
oscillations. We discuss improvements to the models and uncertainties in the
physics that might resolve these issues.Comment: Some amendments to the discussion section, accepted for publication
in MNRA
Intermittency and Lifetime of the 625 Hz QPO in the 2004 Hyperflare from the Magnetar SGR 1806-20 as evidence for magnetic coupling between the crust and the core
Quasi-periodic oscillations (QPOs) detected in the 2004 giant flare from SGR
1806-20 are often interpreted as global magneto-elastic oscillations of the
neutron star. There is, however, a large discrepancy between theoretical
models, which predict that the highest frequency oscillations should die out
rapidly, and the observations, which suggested that the highest-frequency
signals persisted for ~100s in X-ray data from two different spacecraft. This
discrepancy is particularly important for the high-frequency QPO at ~625 Hz.
However, previous analyses did not systematically test whether the signal could
also be there in much shorter data segments, more consistent with the
theoretical predictions. Here, we test for the presence of the high-frequency
QPO at 625 Hz in data from both the Rossi X-ray Timing Explorer (RXTE) and the
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) systematically both in
individual rotational cycles of the neutron star, as well as averaged over
multiple successive rotational cycles at the same phase. We find that the QPO
in the RXTE data is consistent with being only present in a single cycle, for a
short duration of ~0.5s, whereas the RHESSI data are as consistent with a
short-lived signal that appears and disappears as with a long-lived QPO. Taken
together, this data provides evidence for strong magnetic interaction between
the crust and the core.Comment: Accepted for publication in ApJ. The data and simulations are
available at
http://figshare.com/articles/SGR_1806_20_Giant_Flare_Data_and_Simulations/1126082
, the code can be downloaded from
https://github.com/dhuppenkothen/giantflare-paper , some documentation is
under
http://nbviewer.ipython.org/github/dhuppenkothen/giantflare-paper/blob/master/documents/giantflare-analysis.ipyn
On the dependence of X-ray burst rate on accretion and spin rate
Nuclear burning and its dependence on the mass accretion rate are fundamental
ingredients for describing the complicated observational phenomenology of
neutron stars in binary systems. Motivated by high quality burst rate data
emerging from large statistical studies, we report general calculations
relating bursting rate to mass accretion rate and neutron star rotation
frequency. In this first work we neglect general relativistic effects and
accretion topology, though we discuss where their inclusion should play a role.
The relations we derive are suitable for different burning regimes and provide
a direct link between parameters predicted by theory and what is to be expected
in observations. We illustrate this for analytical relations of different
unstable burning regimes that operate on the surface of an accreting neutron
star. We also use the observed behaviour of burst rate to suggest new
constraints on burning parameters. We are able to provide an explanation for
the long standing problem of the observed decrease of burst rate with
increasing mass accretion that follows naturally from these calculations: when
accretion rate crosses a certain threshold, ignition moves away from its
initially preferential site and this can cause a net reduction of the burst
rate due to the effects of local conditions that set local differences in both
burst rate and stabilization criteria. We show under which conditions this can
happen even if locally the burst rate keeps increasing with accretion.Comment: Accepted for publication on Ap
Constraints on Neutron Star Crusts From Oscillations in Giant Flares
We show that the fundamental seismic shear mode, observed as a quasi-periodic
oscillation in giant flares emitted by highly-magnetized neutron stars, is
particularly sensitive to the nuclear physics of the crust. The identification
of an oscillation at ~ 30 Hz as the fundamental crustal shear mode requires a
nuclear symmetry energy that depends very weakly on density near saturation. If
the nuclear symmetry energy varies more strongly with density, then lower
frequency oscillations, previously identified as torsional Alfven modes of the
fluid core, could instead be associated with the crust. If this is the case,
then future observations of giant flares should detect oscillations at around
18 Hz. An accurate measurement of the neutron skin thickness of lead will also
constrain the frequencies predicted by the model.Comment: 5 pages, 3 figures; Version to appear in Phys. Rev. Let
The violent neutron star
Neutron stars enable us to study both the highest densities and the highest
magnetic fields in the known Universe. In this article I review what can be
learned about such fundamental physics using magnetar bursts. Both the
instability mechanisms that trigger the bursts, and the subsequent dynamical
and radiative response of the star, can be used to explore stellar and
magnetospheric structure and composition.Comment: 6 pages, invited review, to appear in the proceedings of the ERPM
conference, Zielona Gora, April 201
The impulsive phase of magnetar giant flares: assessing linear tearing as the trigger mechanism
Giant -ray flares comprise the most extreme radiation events observed
from magnetars. Developing on (sub)millisecond timescales and generating vast
amounts of energy within a fraction of a second, the initial phase of these
extraordinary bursts present a significant challenge for candidate trigger
mechanisms. Here we assess and critically analyse the linear growth of the
relativistic tearing instability in a globally twisted magnetosphere as the
trigger mechanism for giant -ray flares. Our main constraints are given
by the observed emission timescales, the energy output of the giant flare
spike, and inferred dipolar magnetic field strengths. We find that the minimum
growth time of the linear mode is comparable to the -folding rise time, i.e.
ms. With this result we constrain basic geometric parameters of
the current sheet. We also discuss the validity of the presumption that the
-folding emission timescale may be equated with the growth time of an MHD
instability.Comment: 15 pages, 4 figures, MNRAS in pres
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