26,763 research outputs found
XMM-Newton observations of two transient millisecond X-ray pulsars in quiescence
We report on XMM-Newton observations of two X-ray transient millisecond
pulsars (XRTMSPs). We detected XTE J0929-314 with an unabsorbed luminosity of
\~7x10^{31} erg/s. (0.5-10 keV) at a fiducial distance of 10 kpc. The quiescent
spectrum is consistent with a simple power law spectrum. The upper limit on the
flux from a cooling neutron star atmosphere is about 20% of the total flux. XTE
J1807-294 instead was not detected. We can put an upper limit on the source
quiescent 0.5-10 keV unabsorbed luminosity <4x10^{31} erg/s at 8 kpc. These
observations strenghten the idea that XRTMSPs have quiescent luminosities
significantly lower than classical neutron star transients.Comment: 4 pages including 1 figures. Accepted for publication in A&A Letter
Special Varieties and classification Theory
A new class of compact K\"ahler manifolds, called special, is defined, which
are the ones having no surjective meromorphic map to an orbifold of general
type. The special manifolds are in many respect higher-dimensional
generalisations of rational and elliptic curves. For example, we show that
being rationally connected or having vanishing Kodaira dimension implies being
special. Moreover, for any compact K\"ahler we define a fibration , which we call its core, such that the general fibres of are
special, and every special subvariety of containing a general point of
is contained in the corresponding fibre of . We then conjecture and prove
in low dimensions and some cases that: 1) Special manifolds have an almost
abelian fundamental group. 2) Special manifolds are exactly the ones having a
vanishing Kobayashi pseudometric. 3) The core is a fibration of general type,
which means that so is its base ,when equipped with its orbifold
structure coming from the multiple fibres of . 4) The Kobayashi
pseudometric of is obtained as the pull-back of the orbifold Kobayashi
pseudo-metric on , which is a metric outside some proper algebraic
subset. 5) If is projective,defined over some finitely generated (over
) subfield of the complex number field, the set of -rational
points of is mapped by the core into a proper algebraic subset of .
These two last conjectures are the natural generalisations to arbitrary of
Lang's conjectures formulated when is of general type.Comment: 72 pages, latex fil
Birational stability of the cotangent bundle
We define a birational version of the stability of cotangent sheaves for
complex projective manifolds, and more generally for smooth orbifolds. We then
show, using standard conjectures in birational classification, that these
cotangent sheaves are birationally stable, unless the orbifold is uniruled
Special orbifolds and birational classification: a survey
We shall show how to decompose, by functorial and canonical fibrations,
arbitrary -dimensional complex projective {Although the geometric results
apply to compact K\" ahler manifolds without change, we consider here for
simplicity this special case only.} varieties into varieties (or rather `
geometric orbifolds\rq of one of the three \pure geometries determined by
the `sign' (negative, zero, or positive) of the canonical bundle. These
decompositions being birationally invariant, birational versions of these \pure
geometries, based on the \canonical (or ` Kodaira\rq dimension will be
considered, rather.
A crucial feature of these decompositions is indeed that, in order to deal
with multiple fibres of fibrations, they need to take place in the larger
category of `geometric orbifolds' . These are `virtual ramified
covers' of varieties, which `virtually eliminate' multiple fibres of
fibrations. Although formally the same as the `pairs' of the LMMP (see
\cite{kmm}, \cite{KM}, \cite{BCHM} and the references there), they are here
fully geometric objects equipped with the usual geometric invariants of
varieties, such as sheaves of (symmetric) differential forms, fundamental
group, Kobayashi pseudometric, integral points, morphisms and rational maps.
It is intended to expose (with some addtional topics or developments), as
briefly and simply as possible, and essentially skipping the proofs, the main
content of arXiv:math/0110051 and arXiv:0705.0737 respectively published in
ANN. Inst. Fourier (2004), and to appear in J.Inst. Math. Jussieu.Comment: Expanded version of a talk given in May 2009 at the Schiermonnikoog
Conference on birational geometry (Holland, organised by C. Faber, E.
Looijenga and G. Van der Gee
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