1,824 research outputs found
The Formation of High-Mass Black Holes in Low Mass X-ray Binaries
In this note we suggest that high-mass black holes; i.e., black holes of
several solar masses, can be formed in binaries with low-mass main-sequence
companions, provided that the hydrogen envelope of the massive star is removed
in common envelope evolution which begins only after the massive star has
finished He core burning. That is, the massive star is in the supergiant stage,
which lasts only years, so effects of mass loss by He winds are
small. Since the removal of the hydrogen envelope of the massive star occurs so
late, it evolves essentially as a single star, rather than one in a binary.
Thus, we can use evolutionary calculations of Woosley & Weaver (1995) of single
stars. We find that the black holes in transient sources can be formed from
stars with ZAMS masses in the interval 20-35\msun. The black hole mass is
only slightly smaller than the He core mass, typically \sim 7\msun.Comment: 19 pages, substantial changes, accepted in New Astronom
Evolution and Merging of Binaries with Compact Objects
In the light of recent observations in which short gamma-ray bursts are
interpreted as arising from black-hole(BH), neutron-star(NS) or NS-NS mergings
we would like to review our research on the evolution of compact binaries,
especially those containing NS's. These were carried out with predictions for
LIGO in mind, but are directly applicable to short gamma-ray bursts in the
interpretation above.
Most important in our review is that we show that the standard scenario for
evolving NS-NS binaries always ends up with a low-mass BH (LMBH), NS binary.
Bethe and Brown (1998) showed that this fate could be avoided if the two giants
in the progenitor binary burned He at the same time, and that in this way the
binary could avoid the common envelope evolution of the NS with red giant
companion which sends the first born NS into a BH in the standard scenario. The
burning of He at the same time requires, for the more massive giants such as
the progenitors of the Hulse-Taylor binary NS that the two giants be within 4%
of each other in ZAMS mass. Applying this criterion to all binaries results in
a factor 5 of LMBH-NS binaries as compared with NS-NS binaries.
Our scenario of NS-NS binaries as having been preceded by a double He-star
binary is collecting observational support in terms of the nearly equal NS
masses within a given close binary.Comment: 32 pages, 1 figure, substantial changes from v
The Determination of Nuclear Level Densities from Experimental Information -
A novel Information Theory based method for determining the density of states
from prior information is presented. The energy dependence of the density of
states is determined from the observed number of states per energy interval and
model calculations suggest that the method is sufficiently reliable to
calculate the thermal properties of nuclei over a reasonable temperature range.Comment: 7 pages + 6 eps figures, REVTEX 3.
A Theory of Gamma-Ray Bursts
We present a specific scenario for the link between GRB and hypernovae, based
on Blandford-Znajek extraction of black-hole spin energy. Such a mechanism
requires a high angular momentum in the progenitor object. The observed
association of gamma-ray bursts with type Ibc supernovae leads us to consider
massive helium stars that form black holes at the end of their lives as
progenitors. We combine the numerical work of MacFadyen & Woosley with analytic
calculations, to show that about 1E53 erg each are available to drive the fast
GRB ejecta and the supernova. The GRB ejecta are driven by the power output
through the open field lines, whereas the supernova is powered by closed filed
lines and jet shocks. We also present a much simplified approximate derivation
of these energetics.
Helium stars that leave massive black-hole remnants in special ways, namely
via soft X-ray transients or very massive WNL stars. Since binaries naturally
have high angular momentum, we propose a link between black-hole transients and
gamma-ray bursts. Recent observations of one such transient, GRO J1655-40/Nova
Scorpii 1994, explicitly support this connection: its high space velocity
indicates that substantial mass was ejected in the formation of the black hole,
and the overabundance of alpha-nuclei, especially sulphur, indicates that the
explosion energy was extreme, as in SN 1998bw/GRB 980425. (abstract shortened)Comment: 32 pages, 8 figures, accepted for publication in New Astronom
Back-to-back emission of the electrons in double photoionization of helium
We calculate the double differential distributions and distributions in
recoil momenta for the high energy non-relativistic double photoionization of
helium. We show that the results of recent experiments is the pioneering
experimental manifestation of the quasifree mechanism for the double
photoionization, predicted long ago in our papers. This mechanism provides a
surplus in distribution over the recoil momenta at small values of the latter,
corresponding to nearly "back-to-back" emission of the electrons. Also in
agreement with previous analysis the surplus is due to the quadrupole terms of
the photon-electron interaction. We present the characteristic angular
distribution for the "back-to-back" electron emission. The confirmation of the
quasifree mechanism opens a new area of exiting experiments, which are expected
to increase our understanding of the electron dynamics and of the bound states
structure. The results of this Letter along with the recent experiments open a
new field for studies of two-electron ionization not only by photons but by
other projectiles, e.g. by fast electrons or heavy ions.Comment: 10 pages, 2 figure
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