1,850 research outputs found
Observational evidence for the origin of X-ray sources in globular clusters
Low-mass X-ray binaries, recycled pulsars, cataclysmic variables and
magnetically active binaries are observed as X-ray sources in globular
clusters. We discuss the classification of these systems, and find that some
presumed active binaries are brighter than expected. We discuss a new
statistical method to determine from observations how the formation of X-ray
sources depends on the number of stellar encounters and/or on the cluster mass.
We show that cluster mass is not a proxy for the encounter number, and that
optical identifications are essential in proving the presence of primordial
binaries among the low-luminosity X-ray sources.Comment: 10 pages, 7 figures, to appear in IAUS 246, Dynamical evolution of
dense stellar systems, ed. E. Vesperin
X-ray sources in globular clusters
Observations with BeppoSAX, RXTE and Chandra suggest that many of the bright
X-ray sources in globular clusters have ultrashort binary periods. This is
remarkable as such systems are not easily formed. With accurate optical
astrometry of HST images, the large numbers of low-luminosity X-ray sources
discovered with Chandra can be classified as quiescent low-mass X-ray binaries,
pulsars, cataclysmic variables, and magnetically active binaries. The number of
cataclysmic variables is found to scale with the number of close stellar
encounters.Comment: Invited review in: "Interacting binaries", July 4-10 Cefalu, eds.
Antonelli et al., to be published with AIP. 10 pages,6 figure
A new look at distances and velocities of neutron stars
We take a fresh look at the determination of distances and velocities of
neutron stars. The conversion of a parallax measurement into a distance, or
distance probability distribution, has led to a debate quite similar to the one
involving Cepheids, centering on the question whether priors can be used when
discussing a single system. With the example of PSRJ0218+4232 we show that a
prior is necessary to determine the probability distribution for the distance.
The distance of this pulsar implies a gamma-ray luminosity larger than 10% of
its spindown luminosity. For velocities the debate is whether a single
Maxwellian describes the distribution for young pulsars. By limiting our
discussion to accurate (VLBI) measurements we argue that a description with two
Maxwellians, with distribution parameters sigma1=77 and sigma2=320 km/s, is
significantly better. Corrections for galactic rotation, to derive velocities
with respect to the local standards of rest, are insignificant.Comment: Has appeared in Journal of Astrophysics and Astronomy special issue
on 'Physics of Neutron Stars and Related Objects', celebrating the 75th
birth-year of G. Srinivasan. Ten pages, nine figure
A census with ROSAT of low-luminosity X-ray sources in globular clusters
I analyze 101 observations from the ROSAT archive to search for X-ray sources
in or near 55 globular clusters. New sources are found in the cores of NGC362
(a double source), NGC6121 (marginally significant), NGC6139, and NGC6266; and
outside the cores of NGC6205, NGC6352 and NGC6388. More accurate positions are
determined for the X-ray sources in some ten clusters. The improved position
for the source in NGC6341 excludes the suggested ultraviolet counterpart. It is
shown that one of the two sources reported near the core NGC6626 is spurious,
as is the detection of a pulsar period in the PSPC data of this cluster; the
central source is resolved in three sources. One source reported previously in
NGC6304 is demoted to an upper limit. For 20 cluster cores better upper limits
to the X-ray luminosity are obtained.
From a statistical analysis I argue that several sources outside the cluster
cores may well belong to the clusters. All spectral energy distributions
observed so far are relatively soft, with bremsstrahlung temperatures =~0.9keV;
there is evidence however that bremsstrahlung spectra do not correctly describe
the spectra. The X-ray luminosity per unit mass for the cluster as a whole does
not depend on the concentration; the luminosity per unit mass for the core may
increase with the cluster concentration.Comment: 24 pages, 8 figures, 5 tables, accepted for publication in Astronomy
and Astrophysic
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