38,615 research outputs found
Modelling incomplete fusion dynamics of weakly-bound nuclei at near-barrier energies
The classical dynamical model for reactions induced by weakly-bound nuclei at
near-barrier energies is developed further. It allows a quantitative study of
the role and importance of incomplete fusion dynamics in asymptotic
observables, such as the population of high-spin states in reaction products as
well as the angular distribution of direct alpha-production. Model calculations
indicate that incomplete fusion is an effective mechanism for populating
high-spin states, and its contribution to the direct alpha production yield
diminishes with decreasing energy towards the Coulomb barrier. It also becomes
notably separated in angles from the contribution of no-capture breakup events.
This should facilitate the experimental disentanglement of these competing
reaction processes.Comment: 12 pages, 7 figures (for better resolution figures please contact the
author), Accepted in Journal of Physics
A propeller scenario for the gamma-ray emission of low-mass X-ray binaries: The case of XSS J12270-4859
XSS J12270-4859 is the only low mass X-ray binary (LMXB) with a proposed
persistent gamma-ray counterpart in the Fermi-LAT domain, 2FGL 1227.7-4853.
Here, we present the results of the analysis of recent INTEGRAL observations,
aimed at assessing the long-term variability of the hard X-ray emission, and
thus the stability of the accretion state. We confirm that the source behaves
as a persistent hard X-ray emitter between 2003 and 2012. We propose that XSS
J12270-4859 hosts a neutron star in a propeller state, a state we investigate
in detail, developing a theoretical model to reproduce the associated X-ray and
gamma-ray properties. This model can be understood as being of a more general
nature, representing a viable alternative by which LMXBs can appear as
gamma-ray sources. In particular, this may apply to the case of millisecond
pulsars performing a transition from a state powered by the rotation of their
magnetic field, to a state powered by matter in-fall, such as that recently
observed from the transitional pulsar PSR J1023+0038. While the surface
magnetic field of a typical NS in a LMXB is lower by more than four orders of
magnitude than the much more intense fields of neutron stars accompanying
high-mass binaries, the radius at which the matter in-flow is truncated in a
NS-LMXB system is much lower. The magnetic field at the magnetospheric
interface is then orders of magnitude larger at this interface, and as
consequence, so is the power to accelerate electrons. We demonstrate that the
cooling of the accelerated electron population takes place mainly through
synchrotron interaction with the magnetic field permeating the interface, and
through inverse Compton losses due to the interaction between the electrons and
the synchrotron photons they emit. We found that self-synchrotron Compton
processes can explain the high energy phenomenology of XSS J12270-4859.Comment: 12 pages, 3 figures, accepted for publication in MNRAS. References
update
configuration of the system
We study the configuration of the system by
considering as a coupled channel. We solve the Faddeev equations
for these systems and find confirmation of the existence of a new
resonance around 1920 MeV with predicted in a single-channel
potential model and also found in a Faddeev calculation as an
state, with the generated in the ,
interaction.Comment: Published versio
Spin frequency distributions of binary millisecond pulsars
Rotation-powered millisecond radio pulsars have been spun up to their present
spin period by a - yr long X-ray-bright phase of accretion of
matter and angular momentum in a low-to-intermediate mass binary system.
Recently, the discovery of transitional pulsars that alternate cyclically
between accretion and rotation-powered states on time scales of a few years or
shorter, has demonstrated this evolutionary scenario. Here, we present a
thorough statistical analysis of the spin distributions of the various classes
of millisecond pulsars to assess the evolution of their spin period between the
different stages. Accreting sources that showed oscillations exclusively during
thermonuclear type I X-ray bursts (nuclear-powered millisecond pulsars) are
found to be significantly faster than rotation-powered sources, while accreting
sources that possess a magnetosphere and show coherent pulsations (accreting
millisecond pulsars) are not. On the other hand, if accreting millisecond
pulsars and eclipsing rotation-powered millisecond pulsars form a common class
of transitional pulsars, these are shown to have a spin distribution
intermediate between the faster nuclear-powered millisecond pulsars and the
slower non-eclipsing rotation-powered millisecond pulsars. We interpret these
findings in terms of a spin-down due to the decreasing mass-accretion rate
during the latest stages of the accretion phase, and in terms of the different
orbital evolutionary channels mapped by the various classes of pulsars. We
summarize possible instrumental selection effects, showing that even if an
unbiased sample of pulsars is still lacking, their influence on the results of
the presented analysis is reduced by recent improvements in instrumentation and
searching techniques.Comment: Accepted for publication in A&A (6 pages, 4 figures
- …