5,255 research outputs found
On the consistency of neutron-star radius measurements from thermonuclear bursts
The radius of neutron stars can in principle be measured via the
normalisation of a blackbody fitted to the X-ray spectrum during thermonuclear
(type-I) X-ray bursts, although few previous studies have addressed the
reliability of such measurements. Here we examine the apparent radius in a
homogeneous sample of long, mixed H/He bursts from the low-mass X-ray binaries
GS 1826-24 and KS 1731-26. The measured blackbody normalisation (proportional
to the emitting area) in these bursts is constant over a period of up to 60s in
the burst tail, even though the flux (blackbody temperature) decreased by a
factor of 60-75% (30-40%). The typical rms variation in the mean normalisation
from burst to burst was 3-5%, although a variation of 17% was found between
bursts observed from GS 1826-24 in two epochs. A comparison of the
time-resolved spectroscopic measurements during bursts from the two epochs
shows that the normalisation evolves consistently through the burst rise and
peak, but subsequently increases further in the earlier epoch bursts. The
elevated normalisation values may arise from a change in the anisotropy of the
burst emission, or alternatively variations in the spectral correction factor,
f_c, of order 10%. Since burst samples observed from systems other than GS
1826-24 are more heterogeneous, we expect that systematic uncertainties of at
least 10% are likely to apply generally to measurements of neutron-star radii,
unless the effects described here can be corrected for.Comment: 9 pages, 6 figures; accepted by Ap
Topological censorship for Kaluza-Klein space-times
The standard topological censorship theorems require asymptotic hypotheses
which are too restrictive for several situations of interest. In this paper we
prove a version of topological censorship under significantly weaker
conditions, compatible e.g. with solutions with Kaluza-Klein asymptotic
behavior. In particular we prove simple connectedness of the quotient of the
domain of outer communications by the group of symmetries for models which are
asymptotically flat, or asymptotically anti-de Sitter, in a Kaluza-Klein sense.
This allows one, e.g., to define the twist potentials needed for the reduction
of the field equations in uniqueness theorems. Finally, the methods used to
prove the above are used to show that weakly trapped compact surfaces cannot be
seen from Scri.Comment: minor correction
Outer trapped surfaces are dense near MOTSs
We show that any vacuum initial data set containing a marginally outer
trapped surface S and satisfying a "no KIDs" condition can be perturbed near S
so that S becomes strictly outer trapped in the new vacuum initial data set.
This, together with the results in [9], gives a precise sense in which generic
initial data containing marginally outer trapped surfaces lead to geodesically
incomplete spacetimes.Comment: 14 pages, minor rewording
Uniqueness of static black-holes without analyticity
We show that the hypothesis of analyticity in the uniqueness theory of
vacuum, or electrovacuum, static black holes is not needed. More generally, we
show that prehorizons covering a closed set cannot occur in well-behaved
domains of outer communications.Comment: 6 pages, 1 figur
The Cosmic Censor Forbids Naked Topology
For any asymptotically flat spacetime with a suitable causal structure
obeying (a weak form of) Penrose's cosmic censorship conjecture and satisfying
conditions guaranteeing focusing of complete null geodesics, we prove that
active topological censorship holds. We do not assume global hyperbolicity, and
therefore make no use of Cauchy surfaces and their topology. Instead, we
replace this with two underlying assumptions concerning the causal structure:
that no compact set can signal to arbitrarily small neighbourhoods of spatial
infinity (``-avoidance''), and that no future incomplete null geodesic is
visible from future null infinity. We show that these and the focusing
condition together imply that the domain of outer communications is simply
connected. Furthermore, we prove lemmas which have as a consequence that if a
future incomplete null geodesic were visible from infinity, then given our
-avoidance assumption, it would also be visible from points of spacetime
that can communicate with infinity, and so would signify a true naked
singularity.Comment: To appear in CQG, this improved version contains minor revisions to
incorporate referee's suggestions. Two revised references. Plain TeX, 12
page
Radius-expansion burst spectra from 4U 1728-34: an ultracompact binary?
Recent theoretical and observational studies have shown that ashes from
thermonuclear burning may be ejected during radius-expansion bursts, giving
rise to photoionisation edges in the X-ray spectra. We report a search for such
features in Chandra spectra observed from the low-mass X-ray binary 4U 1728-34.
We analysed the spectra from four radius-expansion bursts detected in 2006
July, and two in 2002 March, but found no evidence for discrete features. We
estimate upper limits for the equivalent widths of edges of a few hundred eV,
which for the moderate temperatures observed during the bursts, are comparable
with the predictions. During the 2006 July observation 4U 1728-34 exhibited
weak, unusually frequent bursts (separated by <2 hr in some cases), with
profiles and alpha-values characteristic of hydrogen-poor fuel. Recurrence
times as short as those measured are insufficient to exhaust the accreted
hydrogen at solar composition, suggesting that the source accretes hydrogen
deficient fuel, for example from an evolved donor. The detection for the first
time of a 10.77 min periodic signal in the persistent intensity, perhaps
arising from orbital modulation, supports this explanation, and suggests that
this system is an ultracompact binary similar to 4U 1820-30.Comment: 9 pages, 6 figures, accepted by Ap
Mathematical general relativity: a sampler
We provide an introduction to selected recent advances in the mathematical
understanding of Einstein's theory of gravitation.Comment: Some updates. A shortened version, to appear in the Bulletin of the
AMS, is available online at
http://www.ams.org/journals/bull/0000-000-00/S0273-0979-2010-01304-
Regularity of Horizons and The Area Theorem
We prove that the area of sections of future event horizons in space-times
satisfying the null energy condition is non-decreasing towards the future under
any one of the following circumstances: 1) the horizon is future geodesically
complete; 2) the horizon is a black hole event horizon in a globally hyperbolic
space-time and there exists a conformal completion with a ``H-regular'' Scri
plus; 3) the horizon is a black hole event horizon in a space-time which has a
globally hyperbolic conformal completion. (Some related results under less
restrictive hypotheses are also established.) This extends a theorem of
Hawking, in which piecewise smoothness of the event horizon seems to have been
assumed. No assumptions about the cosmological constant or its sign are made.
We prove smoothness or analyticity of the relevant part of the event horizon
when equality in the area inequality is attained - this has applications to the
theory of stationary black holes, as well as to the structure of compact Cauchy
horizons. In the course of the proof we establish several new results
concerning the differentiability properties of horizons.Comment: final version to appear in Annales Henri Poincare, various changes as
suggested by a referee, in particular a section on Krolak's area theorems
adde
On the mass aspect function and positive energy theorems for asymptotically hyperbolic manifolds
We prove positivity of energy for a class of asymptotically locally
hyperbolic manifolds in dimensions . The result is established by
first proving deformation-of-mass-aspect theorems in dimensions . Our
positivity results extend to the case when more stringent conditions
are imposed.Comment: trivial typos correcte
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