240 research outputs found
The role of mass, equation of state and superfluid reservoir in large pulsar glitches
Observations of pulsar glitches may provide insights on the internal physics
of neutron stars and recent studies show how it is in principle possible to
constrain pulsar masses with timing observations. The reliability of these
estimates depend on the current uncertainties about the structure of neutron
stars and on our ability to model the dynamics of the superfluid neutrons in
the internal layers. We assume a simplified model for the rotational dynamics
of a neutron star and estimate an upper bound to the mass of 25 pulsars from
their largest glitch and average activity: the aim is to understand to which
extent the mass constraints are sensitive to the choice of the unknown
structural properties of neutron stars, like the extension of the superfluid
region and the equation of state. Reasonable values, within the range measured
for neutron star masses, are obtained only if the superfluid domain extends for
at least a small region inside the outer core, which is compatible with
calculations of the neutron S-wave pairing gap. Moreover, the mass constraints
stabilise when the superfluid domain extends to densities over nuclear
saturation, irrespective of the equation of state tested.Comment: 11 pages, 6 figure
On some categorical-algebraic conditions in S-protomodular categories
In the context of protomodular categories, several additional conditions have
been considered in order to obtain a closer group-like behavior. Among them are
locally algebraic cartesian closedness and algebraic coherence. The recent
notion of S-protomodular category, whose main examples are the category of
monoids and, more generally, categories of monoids with operations and
Jo\'{o}nsson-Tarski varieties, raises a similar question: how to get a
description of S-protomodular categories with a strong monoid-like behavior. In
this paper we consider relative versions of the conditions mentioned above, in
order to exhibit the parallelism with the "absolute" protomodular context and
to obtain a hierarchy among S-protomodular categories
How to centralize and normalize quandle extensions
We show that quandle coverings in the sense of Eisermann form a (regular
epi)-reflective subcategory of the category of surjective quandle
homomorphisms, both by using arguments coming from categorical Galois theory
and by constructing concretely a centralization congruence. Moreover, we show
that a similar result holds for normal quandle extensions.Comment: 17 page
On the representability of actions for topological algebras
The actions of a group B on a group X correspond bijectively to the group homomorphisms B ⶠAut(X), proving that the functor âactions on Xâ is representable by the group of automorphisms of X. Making the detour through pseudotopological spaces, we generalize this result to the topological case, for quasi-locally compact groups and some other algebraic structures. We investigate next the case of arbitrary topological algebras for a semi-abelian theory and prove that the representability of topological actions reduces to the preservation of coproducts by the functor Act(â,X)
Core and crust contributions in overshooting glitches: the Vela pulsar 2016 glitch
During the spin-up phase of a large pulsar glitch - a sudden decrease of the rotational period of a neutron star - the angular velocity of the star may overshoot, namely reach values greater than that observed for the new post-glitch equilibrium. These transient phenomena are expected on the basis of theoretical models for pulsar internal dynamics, and their observation has the potential to provide an important diagnostic for glitch modelling. In this article, we present a simple criterion to assess the presence of an overshoot, based on the minimal analytical model that is able to reproduce an overshooting spin-up. We employed it to fit the data of the 2016 glitch of the Vela pulsar, obtaining estimates of the fractional moments of inertia of the internal superfluid components involved in the glitch, of the rise and decay timescales of the overshoot, and of the mutual friction parameters between the superfluid components and the normal one. We studied the cases with and without strong entrainment in the crust: in the former, we found an indication of a large inner core strongly coupled to the observable component, and of a reservoir of angular momentum extending into the core to densities below nuclear saturation; while in the latter, a large reservoir extending above nuclear saturation and a standard normal component without inner core were found
Insights into the physics of neutron star interiors from pulsar glitches
The presence of superfluid phases in the interior of a neutron star affects
its dynamics, as neutrons can flow relative to the non-superfluid (normal)
components of the star with little or no viscosity. A probe of superfluidity
comes from pulsar glitches, sudden jumps in the observed rotational period of
radio pulsars. Most models of glitches build on the idea that a superfluid
component of the star is decoupled from the spin-down of the normal component,
and its sudden recoupling leads to a glitch. This transition in the strength of
the hydrodynamic coupling is explained in terms of quantum vortices (long-lived
vortices that are naturally present in the neutron superfluid at the
microscopic scale). After introducing some basic ideas, we derive (as a
pedagogical exercise) the formal scheme shared by many glitch studies. Then, we
apply these notions to present some recent advances and discuss how
observations can help us to indirectly probe the internal physics of neutron
stars.Comment: 30 pages, 7 figures. Chapter 7 of the volume "Astrophysics in the XXI
Century with Compact Stars", Eds. C.A.Z. Vasconcellos and F. Weber, World
Scientific (2022), submitted in August 202
A criterion for reflectiveness of normal extensions
We give a new sufficient condition for the normal extensions in an admissible Galois structure to be reflective. We then show that this condition is indeed fulfilled when X is the (protomodular) reflective subcategory of S-special objects of a Barr-exact S-protomodular category C, where S is the class of split epimorphic trivial extensions in C. Next to some concrete examples where the criterion may be applied, we also study the adjunction between a Barr-exact unital category and its abelian core, which we prove to be admissible
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