30 research outputs found
The Great Wall: Urca Cooling Layers in the Accreted NS Crust
Accreting neutron stars host a number of astronomical observables which can
be used to infer the properties of the underlying dense matter. These
observables are sensitive to the heating and cooling processes taking place in
the accreted neutron star (NS) crust. Within the past few years it has become
apparent that electron-capture/beta-decay (urca) cycles can operate within the
NS crust at high temperatures. Layers of nuclei undergoing urca cycling can
create a thermal barrier, or Great Wall, between heating occurring deep in the
crust and the regions above the urca layers. This paper briefly reviews the
urca process and the implications for observables from accreting neutron stars.Comment: Invited talk at Capture Gammay-Ray Spectroscopy 16 (Shanghai), to
appear in EPJ Web of Conference
Pulse Profile Modeling of Thermonuclear Burst Oscillations I: The Effect of Neglecting Variability
We study the effects of the time-variable properties of thermonuclear X-ray
bursts on modeling their millisecond-period burst oscillations. We apply the
pulse profile modeling technique that is being used in the analysis of
rotation-powered millisecond pulsars by the Neutron Star Interior Composition
Explorer (NICER) to infer masses, radii, and geometric parameters of neutron
stars. By simulating and analyzing a large set of models, we show that
overlooking burst time-scale variability in temperatures and sizes of the hot
emitting regions can result in substantial bias in the inferred mass and
radius. To adequately infer neutron star properties, it is essential to develop
a model for the time variable properties or invest a substantial amount of
computational time in segmenting the data into non-varying pieces. We discuss
prospects for constraints from proposed future X-ray telescopes.Comment: Accepted for publication in MNRA
Pulse Profile Modelling of Thermonuclear Burst Oscillations II: Handling variability
Pulse profile modelling is a relativistic ray-tracing technique that can be
used to infer masses, radii and geometric parameters of neutron stars. In a
previous study, we looked at the performance of this technique when applied to
thermonuclear burst oscillations from accreting neutron stars. That study
showed that ignoring the variability associated with burst oscillation sources
resulted in significant biases in the inferred mass and radius, particularly
for the high count rates that are nominally required to obtain meaningful
constraints. In this follow-on study, we show that the bias can be mitigated by
slicing the bursts into shorter segments where variability can be neglected,
and jointly fitting the segments. Using this approach, the systematic
uncertainties on the mass and radius are brought within the range of the
statistical uncertainty. With about 10 source counts, this yields
uncertainties of approximately 10% for both the mass and radius. However, this
modelling strategy requires substantial computational resources. We also
confirm that the posterior distributions of the mass and radius obtained from
multiple bursts of the same source can be merged to produce outcomes comparable
to that of a single burst with an equivalent total number of counts.Comment: submitted to MNRAS. The Zenodo link will go public after peer review.
Comments are welcom
Catching Element Formation In The Act
Gamma-ray astronomy explores the most energetic photons in nature to address
some of the most pressing puzzles in contemporary astrophysics. It encompasses
a wide range of objects and phenomena: stars, supernovae, novae, neutron stars,
stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays
and relativistic-particle acceleration, and the evolution of galaxies. MeV
gamma-rays provide a unique probe of nuclear processes in astronomy, directly
measuring radioactive decay, nuclear de-excitation, and positron annihilation.
The substantial information carried by gamma-ray photons allows us to see
deeper into these objects, the bulk of the power is often emitted at gamma-ray
energies, and radioactivity provides a natural physical clock that adds unique
information. New science will be driven by time-domain population studies at
gamma-ray energies. This science is enabled by next-generation gamma-ray
instruments with one to two orders of magnitude better sensitivity, larger sky
coverage, and faster cadence than all previous gamma-ray instruments. This
transformative capability permits: (a) the accurate identification of the
gamma-ray emitting objects and correlations with observations taken at other
wavelengths and with other messengers; (b) construction of new gamma-ray maps
of the Milky Way and other nearby galaxies where extended regions are
distinguished from point sources; and (c) considerable serendipitous science of
scarce events -- nearby neutron star mergers, for example. Advances in
technology push the performance of new gamma-ray instruments to address a wide
set of astrophysical questions.Comment: 14 pages including 3 figure