2,169 research outputs found
Extracting Neutron Star Properties from X-ray Burst Oscillations
Many thermonuclear X-ray bursts exhibit brightness oscillations. The
brightness oscillations are thought to be due to the combined effects of
non-uniform nuclear burning and rotation of the neutron star. The waveforms of
the oscillations contain information about the size and number of burning
regions. They also contain substantial information about the mass and radius of
the star, and hence about strong gravity and the equation of state of matter at
supranuclear densities. We have written general relativistic ray-tracing codes
that compute the waveforms and spectra of rotating hot spots as a function of
photon energy. Using these codes, we survey the effect on the oscillation
waveform and amplitude of parameters such as the compactness of the star, the
spot size, the surface rotation velocity, and whether there are one or two
spots. We also fit phase lag versus photon energy curves to data from the
millisecond X-ray pulsar, SAX J1808--3658.Comment: To appear in Proc. of the 10th Annual October Astrophysics Conference
in Maryland: Cosmic Explosions, 4 page
The emerging specialty of cardiothoracic surgical critical care: The leadership role of cardiothoracic surgeons on the multidisciplinary team
The Fifth Monarchists: Forgotten Radicals of the English Revolution
The Fifth Monarchists were a radical group of Puritans during the period of the English Civil War who sought to seize power in England in order to prepare for what they believed was Christ\u27s inevitable return in the near future to reign in England. Previous research concerning them is scarce, and what scholarship there is does little to explain the importance of the events surrounding them. This study seeks to explain the historical significance of this group through exploring the goals of the group and the means by which they set out to accomplish them. An assortment of primary sources from the period were used, including pamphlets, letters, diaries, and speeches. The study demonstrates that there was a significant relationship between this group and the governing officials for a time that gave them unique opportunities to begin pursuing their goals, only for them to ultimately fail. Thus, this study demonstrates that the Fifth Monarchists’ failure to achieve their goals was significant through showing that they had reasonable chances of making a lasting impact as well as through showing what changes that could have entailed
Stellar Dynamics at the Galactic Center with an Extremely Large Telescope
We discuss experiments achievable via monitoring of stellar dynamics near the
massive black hole at the Galactic center with a next generation, extremely
large telescope (ELT). Given the likely observational capabilities of an ELT
and current knowledge of the stellar environment at the Galactic center, we
synthesize plausible samples of stellar orbits around the black hole. We use
the Markov Chain Monte Carlo method to evaluate the constraints that orbital
monitoring places on the matter content near the black hole. Results are
expressed as functions of the number N of stars with detectable orbital motions
and the astrometric precision dtheta and spectroscopic precision dv at which
stellar proper motions and radial velocities are monitored. For N = 100, dtheta
= 0.5 mas, and dv = 10 km/s -- a conservative estimate of the capabilities of a
30 meter telescope -- the extended matter distribution enclosed by the orbits
will produce measurable deviations from Keplerian motion if >1000 Msun is
enclosed within 0.01 pc. The black hole mass and distance to the Galactic
center will be measured to better than ~0.1%. Lowest-order relativistic
effects, such as the prograde precession, will be detectable if dtheta < 0.5
mas. Higher-order effects, including frame dragging due to black hole spin,
requires dtheta < 0.05 mas, or the favorable discovery of a compact, highly
eccentric orbit. Finally, we calculate the rate at which monitored stars
undergo detectable nearby encounters with background stars. Such encounters
probe the mass function of stellar remnants that accumulate near the black
hole. We find that ~30 encounters will be detected over a 10 yr baseline for
dtheta = 0.5 mas.Comment: 14 pages, 5 figures; discussion no longer aperture-specific (TMT ->
ELT), matches ApJ versio
Carbon Detonation and Shock-Triggered Helium Burning in Neutron Star Superbursts
The strong degeneracy of the 12C ignition layer on an accreting neutron star
results in a hydrodynamic thermonuclear runaway, in which the nuclear heating
time becomes shorter than the local dynamical time. We model the resulting
combustion wave during these superbursts as an upward propagating detonation.
We solve the reactive fluid flow and show that the detonation propagates
through the deepest layers of fuel and drives a shock wave that steepens as it
travels upward into lower density material. The shock is sufficiently strong
upon reaching the freshly accreted H/He layer that it triggers unstable 4He
burning if the superburst occurs during the latter half of the regular Type I
bursting cycle; this is likely the origin of the bright Type I precursor bursts
observed at the onset of superbursts. The cooling of the outermost shock-heated
layers produces a bright, ~0.1s, flash that precedes the Type I burst by a few
seconds; this may be the origin of the spike seen at the burst onset in 4U
1820-30 and 4U 1636-54, the only two bursts observed with RXTE at high time
resolution. The dominant products of the 12C detonation are 28Si, 32S, and
36Ar. Gupta et al. showed that a crust composed of such intermediate mass
elements has a larger heat flux than one composed of iron-peak elements and
helps bring the superburst ignition depth into better agreement with values
inferred from observations.Comment: 11 pages, 11 figures, accepted to ApJ; discussion about onset of
detonation discussed in new detail, including a new figur
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