1,721,123 research outputs found
On Systems of Selfgravitating Bosons and Fermions Undergoing Condensation.Newtonian Approach
No full text
X-Ray Emission from Neutron Stars in Dark Clusters
No full textTwo classes of neutron stars may exist in the galactic
halo: neutron stars originating in the disk with high velocities
and neutron stars originating in globular clusters (via Type II supernovae
and/or accretion induced collapse). Moreover, the halo
dark matter is likely in the form of dark clusters composed of
MACHOs and cold molecular clouds. Here we propose a model
in which neutron stars crossing dark clusters in the galactic halo accrete material and emit in the X-ray band. The detection of these X-ray sources whose luminosity can span several orders of magnitude from 1033 to 1037 erg s−1 could be already done by ROSAT
Dark matter in bright X-ray ellipticals
No full textInvestigates the total mass distribution of bright X-ray ellipticals (with diffuse X-ray luminosity higher than 1041erg s-1) studying the gravitational equilibrium of three component models constituted by stars, hot diffuse gas and dark matter. Assuming a constant mass-to-light ratio for the stars described with anisotropic distribution function in phase space, an emission in X-rays from the hot, diffuse gas, and a dark component in the form of massive fermions, the authors derive constraints on the dark matter distribution by using optical and X-ray data. Comparison of model results with observations does not allow a tight determination of the amount of dark matter which results in 0.4 - 9 times the amount of the stellar mass. The authors investigate the elliptical NGC 4472 for which spectroscopic data of relatively good quality are available
Collapsed white dwarfs as gamma-ray bursts sources
No full textIt has been suggested by Usov (1992) that accreting white dwarfs, collapsing
to neutron stars may be the sources of the gamma-ray bursts observed at cosmological
distances, provided they rotate very fast and have enormously high magnetic fields. In this
model the burst's duration r is given by the ratio of pulsar kinetic energy and magneticdipole
luminosity, so that in order to account for the shortest (r ,,~ 0.1 s) bursts, the pulsars
must rotate very fast (with period P ,-~ 0.5 ms) and have magnetic fields of 1016 - 101T
G. Though the high pulsar frequency was anticipated (Qadir and Rafique, 1986) and has
been shown to be plausible (Abramowicz, 1990), the extremely high magnetic fields seem
anomalous as observed neutron stars have fields below ~ 1013 G.
The problem with Usov's proposal is reduced by incorporating the relativistic corrections
for fast rotating magnetic dipoles (Belinsky et al., 1994) or magnetic stars (De Paolis
and Qadir, 1994). These corrections substantially enhance the radiation efficiency due
to the existence of a magnetic synchrotron effect so that the magnetic field required for
the explanation of the shortest gamma-ray bursts is strongly reduced. As such the model
becomes much more plausible
Dark matter in X-ray-emitting elliptical galaxies
No full textTo investigate the dynamical mass distribution in elliptical galaxies, we study the gravitational equilibrium of spherically symmetric three-component models constituted by stars, hot diffuse gas, and dark matter. Assuming a constant mass-to-light ratio for the stars, described with an isotropic distribution function in phase space (King), an emission in the X-ray band from the hot gas, and a dark component in the form of massive fermions, we derive constraints on the dark matter distribution by using optical and X-ray brightness profiles. We find that, while optical profiles are not useful to discriminate between models with and without dark matter, fits to X-ray observations of high-luminosity ellipticals require the presence of a third dark component, mainly distributed in the outer radial regions, beyond the half-luminosity radius. The total amount of dark matter is, however, poorly determined because X-ray brightness profiles can be fitted by models with values of dark mass Md in the range 0.4-9 times the stellar mass M* inside R25. In any case, the lower limit for Md/M* can be weakened for low-luminosity ellipticals, for which X-ray observations can be fitted by models with a negligible amount of dark matter. A better determination of the ration Md/M* is obtained if stellar kinematical (velocity dispersion) data are also considered. Limits on the mass of fermions composing the dark matter are also discussed, and they are found to be in agreement with previous particle mass determinations based on the analysis of rotation curves in spiral galaxies. The possibility of fitting all the available observational data by using two-component (stars and hot gas) models with the stellar component described by an anisotropic distribution function in the phase space is also explored. For high-luminosity ellipticals also these models cannot fit X-ray observations because the corresponding gas temperatures are systematically higher than the observed values
Pulsars tracing the black hole in globular clusters
No full textResults of precise measurements of the periods of pulsars discovered in the central regions of globular clusters are shown to be approaching the capabilities of testing the existence of a central black hole. For example, in the case of M 15 the available data on two pulsars PSR 2129+1210A and PSR 2129+1209D seem to exclude the existence of a black hole, the presence of which was instead supported by recent Hubble Space Telescope data on surface brightness profile (Yanny et al. 1994AJ....107.1745Y). The fluctuations of the gravitational field caused by the stars of the system are enough to explain the acceleration observed for both pulsars. In the case of the Galactic center, additional data are needed for similar definite conclusions
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