891 research outputs found
A positive Bondi--type mass in asymptotically de Sitter spacetimes
The general structure of the conformal boundary of
asymptotically de Sitter spacetimes is investigated. First we show that
Penrose's quasi-local mass, associated with a cut of the conformal
boundary, can be zero even in the presence of outgoing gravitational radiation.
On the other hand, following a Witten--type spinorial proof, we show that an
analogous expression based on the Nester--Witten form is finite only if the
Witten spinor field solves the 2-surface twistor equation on , and it
yields a positive functional on the 2-surface twistor space on ,
provided the matter fields satisfy the dominant energy condition. Moreover,
this functional is vanishing if and only if the domain of dependence of the
spacelike hypersurface which intersects in the cut
is locally isometric to the de Sitter spacetime. For non-contorted cuts this
functional yields an invariant analogous to the Bondi mass.Comment: 51 pages; typos corrected, one reference added; final version,
appearing in Class. Quantum Gra
Analysis of variability in the burst oscillations of the accreting millisecond pulsar XTE J1814-338
The accreting millisecond pulsar XTE J1814-338 exhibits oscillations at the
known spin frequency during Type I X-ray bursts. The properties of the burst
oscillations reflect the nature of the thermal asymmetry on the stellar
surface. We present an analysis of the variability of the burst oscillations of
this source, focusing on three characteristics: fractional amplitude, harmonic
content and frequency. Fractional amplitude and harmonic content constrain the
size, shape and position of the emitting region, whilst variations in frequency
indicate motion of the emitting region on the neutron star surface. We examine
both long-term variability over the course of the outburst, and short-term
variability during the bursts. For most of the bursts, fractional amplitude is
consistent with that of the accretion pulsations, implying a low degree of fuel
spread. There is however a population of bursts whose fractional amplitudes are
substantially lower, implying a higher degree of fuel spread, possibly forced
by the explosive burning front of a precursor burst. For the first harmonic,
substantial differences between the burst and accretion pulsations suggest that
hotspot geometry is not the only mechanism giving rise to harmonic content in
the latter. Fractional amplitude variability during the bursts is low; we
cannot rule out the hypothesis that the fractional amplitude remains constant
for bursts that do not exhibit photospheric radius expansion (PRE). There are
no significant variations in frequency in any of the bursts except for the one
burst that exhibits PRE. This burst exhibits a highly significant but small
(Hz) drop in frequency in the burst rise. The timescale of the
frequency shift is slower than simple burning layer expansion models predict,
suggesting that other mechanisms may be at work.Comment: 20 pages, 20 figures, accepted for publication in ApJ. Uses
emulateapj.cl
A comment on Liu and Yau's positive quasi-local mass
Liu and Yau (Phys.Rev.Lett. 90, 231102, 2003) propose a definition of quasi-local mass for any space-like, topological 2-sphere with positive Gauss curvature (and subject to a second, convexity, condition). They are able to show it is positive using a result of Shi and Tam (J.Diff.Geom. 62, 79, 2002). However, as we show here, their definition can give a strictly positive mass for a sphere in flat space
A review of total energy-momenta in GR with a positive cosmological constant
A review is given of the various approaches to and expressions for total
energy-momentum and mass in the presence of a positive cosmological constant in
Einstein's field equations, together with a discussion of the key conceptual
questions, main ideas and techniques behind them.Comment: 26 pages, nine figures and two references adde
The Gypsy Trail
https://digitalcommons.library.umaine.edu/mmb-vp/5726/thumbnail.jp
Grade Incomplete: Examining the Securities and Exchange Commission\u27s Attempt to Implement Credit Rating and Certain Corporate Governance Reforms of Dodd-Frank
Following the financial crisis of 2007-2009, Congress passed the Dodd-Frank Act with stated goals, among others, of creating a sound economic foundation and protecting consumers. The Dodd-Frank Act creates several new agencies and restructures the financial regulatory system, yet controversies remain on the promulgation of new rules and the overall effectiveness in accomplishing the stated goals of the Act.
This Article briefly discusses the status of rulemaking by newly created agencies and the restructured financial regulatory system mandated by the Dodd- Frank Act three years after its passage. Next, we focus on certain aspects of the SEC and its charge from Dodd-Frank to implement new agencies and regulations. Specifically, we examine the SEC efforts to establish the Office of Credit Ratings and its regulations and the SEC’s efforts related to additional executive compensation disclosure regulations required by Dodd-Frank
Post-Glitch RXTE-PCA Observations of the Vela Pulsar
We report the results of analysis of observations of the Vela Pulsar by PCA
on RXTE. Our data consists of two parts. The first part contains observations
at 1, 4, and 9 days after the glitch in 1996 and has 27000 sec. total exposure
time. The second part of observations were performed three months after this
glitch and have a total exposure time of 93000 sec. We found pulsations in both
sets. The observed spectrum is a power-law with no apparent change in flux or
count rate. The theoretical expectations of increase in flux due to internal
heating after a glitch are smaller than the uncertainty of the observations.Comment: 6 pages, 5 figures in 9 ps/eps files. Accepted for publication in A&A
Main Journa
X-ray Bursts from the Accreting Millisecond Pulsar XTE J1814-338
Since the discovery of the accreting millisecond pulsar XTE J1814-338 a total
of 27 thermonuclear bursts have been observed from the source with the
Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer
(RXTE). Spectroscopy of the bursts, as well as the presence of continuous burst
oscillations, suggests that all but one of the bursts are sub-Eddington. The
remaining burst has the largest peak bolometric flux of 2.64 x E^-8
erg/sec/cm^2, as well as a gap in the burst oscillations, similar to that seen
in Eddington limited bursts from other sources. Assuming this burst was
Eddington limited we obtain a source distance of about 8 kpc. All the bursts
show coherent oscillations at the 314.4 Hz spin frequency. The burst
oscillations are strongly frequency and phase locked to the persistent
pulsations. Only two bursts show evidence for frequency drift in the first few
seconds following burst onset. In both cases the initial drift corresponds to a
spin down of a few tenths of a Hz. The large oscillation amplitude during the
bursts confirms that the burst flux is modulated at the spin frequency. We
detect, for the first time, a significant first harmonic component in burst
oscillations. The ratio of countrate in the first harmonic to that in the
fundamental can be > 0.25 and is, on average, less than that of the persistent
pulsations. If the pulsations result from a single bright region on the
surface, the harmonic strength suggests the burst emission is beamed, perhaps
due to a stronger magnetic field than in non-pulsing LMXBs. Alternatively, the
harmonic content could result from a geometry with two bright regions.Comment: AASTeX, 15 pages, 4 figures. Accepted for publication in the
Astrophysical Journal Letter
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