arXiv subjects

Pawel Haensel

Publications and source records attributed to Pawel Haensel.

14 recordsLinked to original sources

arXiv2024

Layers of electron captures in the crust of accreting neutron stars

The accumulation of accreted matter onto the neutron star surface triggers exothermic reactions in the crust. The heat released as a result influences the luminosity exhibited by the X-ray transient. The most common approach to the kinetics of exothermic reactions in the crust of accreting neutron stars is to consider an infinite reaction rate. Here, we investigate accretion-related heat release in the accreted outer crust of a neutron star by including a time-dependent accretion cycle and experimentally based reaction rates in the kinetics of electron captures above the reaction threshold. A simple model was used to compute the zero temperature equation of state of a crust in which two nuclei can coexist. We solved the abundance of parent nuclei as a function of the depth in the star and the time variable using astrophysically motivated features of the accreting system. We calculated the heat release and neutrino loss associated to reactions in the outer crust. We report the existence of layers in the outer crust, which contain both parent and grand-daughter nuclei of electron captures. The reactions can occur deeper in the shell than the reaction threshold, thus releasing more heat per accreted baryon for a given accretion rate. The electron capture layers continue to exist even when the accretion has stopped. The heat sources are time- and pressure-dependent in accreting crusts of neutron stars. The total heat released is a function of astrophysical (active and quiescent time) and microscopic (reaction rate) parameters Therefore, we conclude these parameters should be considered individually and carefully for a range of different sources.

astro-ph.HE↗

arXiv2022

Internal Heating in Magnetars: Role of Electron Captures

The role of electron captures by nuclei in the shallow heating of magnetars is further investigated using both nuclear measurements and the theoretical atomic mass table HFB-27. Starting from the composition of the outer crust in full equilibrium, we have calculated the onset of electron captures and the heat released due to the slow decay of the magnetic field. Numerical results are found to be similar to those previously obtained with the HFB-24 atomic mass model and are consistent with neutron-star cooling data.

astro-ph.HE↗

arXiv2022

Partially accreted crusts of neutron stars

Neutron stars in low-mass binary systems are subject to accretion. The common assumption in studying the properties of the neutron star crust is the fully accreted crust approximation. However, observations of some X-ray transient sources indicate that the original crust has not been completely replaced by accreted material, but is partly composed of the compressed original crust. A two-part (or hybrid) crust made of the original crust that is compressed and of the accreted material crashing onto it was reconstructed as a function of the accretion stage. The differences in the composition and energy sources for the fully accreted and hybrid crusts influence the cooling and transport properties. A simple semi-empirical formula of a compressible liquid drop was used. We compared the nuclear reactions triggered by compression in the original crust and in the accreted matter part of the hybrid crust. We discuss another crust compression astrophysical phenomenon related to spinning neutron stars. The compression of the originally catalyzed outer crust triggers exothermic reactions (electron captures and pycnonuclear fusions) that deposit heat in the crust. The heat sources are cataloged as a function of the compression until the fully accreted crust approximation is reached. The pressure at which neutron drip occurs is a nonmonotonic function of the depth, leading to a temporary neutron drip anomaly. The additional potential source of energy for partially accreted crusts is the occurrence of a density inversion phenomenon between some compressed layers. The original crust of a neutron star cannot be neglected when the original crust is not fully replaced by the accreted matter. The amount of heat associated with the compression of the original crust is on the same order of magnitude as that from the sources acting in the accreted part of the hybrid crust.

astro-ph.HE↗

arXiv2021

Influence of the crust on the neutron star macrophysical quantities and universal relations

Measurements of neutron-star macrophysical properties thanks to multimessenger observations offer the possibility to constrain the properties of nuclear matter. Indeed cold and dense matter as found inside neutron stars, in particular in their core, is not accessible to terrestrial laboratories.We investigate the consequences of using equations of state that employ models for the core and the crust that are not calculated consistently on the neutron-star macrophysical properties, on some of the so-called universal relations and on the constraints obtained from gravitational-wave observations. We use various treatments found in the literature to connect together nonconsistent core and crust equations of state. We then compute the mass, the radius, the tidal deformability, and the moment of inertia for each model. Finally, we assess the discrepancies in the neutron-star macrophysical properties obtained when consistent models for the whole star and nonconsistent ones are employed. The use of crust models nonconsistent with the core introduces an error on the macrophysical parameters which can be as large as the estimated accuracy of current and next generation telescopes. The precision of some of the universal relations reported in the literature is found to be overestimated. We confirm that the equation of the crust has limited influence on the macrophysical properties. The discrepancy between results obtained for a fully consistent equation of state and a nonconsistent one can be reduced if one connects the core and the crust models at baryon densities around 0.08-0.1 fm$^{-3}$. The equation of the crust cannot be probed with current multimessenger observations and near-future ones.

astro-ph.HE↗

arXiv2021

Heating in Magnetar Crusts from Electron Captures

The persistent thermal luminosity of magnetars and their outbursts suggest the existence of some internal heat sources located in their outer crust. The compression of matter accompanying the decay of the magnetic field may trigger exothermic electron captures and, possibly, pycnonuclear fusions of light elements that may have been accreted onto the surface from the fallback of supernova debris, from a disk or from the interstellar medium. This scenario bears some resemblance to deep crustal heating in accreting neutron stars, although the matter composition and the thermodynamic conditions are very different. The maximum possible amount of heat that can be released by each reaction and their locations are determined analytically taking into account the Landau--Rabi quantization of electron motion. Numerical results are also presented using experimental, as well as theoretical nuclear data. Whereas the heat deposited is mainly determined by atomic masses, the locations of the sources are found to be very sensitive to the magnetic field strength, thus providing a new way of probing the internal magnetic field of magnetars. Most sources are found to be concentrated at densities $10^{10}-10^{11}$ g cm$^{-3}$ with heat power $W^\infty\sim 10^{35}-10^{36}$ erg/s, as found empirically by comparing cooling simulations with observed thermal luminosity. The change of magnetic field required to trigger the reactions is shown to be consistent with the age of known magnetars. This suggests that electron captures and pycnonuclear fusion reactions may be a viable heating mechanism in magnetars. The present results provide consistent microscopic inputs for neutron star cooling simulations, based on the same model as that underlying the Brussels-Montreal unified equations of state.

astro-ph.HE↗

arXiv2020

Experimental constraints on shallow heating in accreting neutron-star crusts

The observed thermal relaxation of transiently accreting neutron stars during quiescence periods in low-mass X-ray binaries suggests the existence of unknown heat sources in the shallow layers of neutron-star crust. Making use of existing experimental nuclear data, we estimate the maximum possible amount of heat that can be deposited in the outer crust of an accreting neutron star due to electron captures and pycnonuclear fusion reactions triggered by the burial of X-ray burst ashes.

astro-ph.HE↗

arXiv2014

EMMI Rapid Reaction Task Force Meeting on 'Quark Matter in Compact Star'

The recent measurement of two solar mass pulsars has initiated an intense discussion on its impact on our understanding of the high-density matter in the cores of neutron stars. A task force meeting was held from October 7-10, 2013 at the Frankfurt Institute for Advanced Studies to address the presence of quark matter in these massive stars. During this meeting, the recent oservational astrophysical data and heavy-ion data was reviewed. The possibility of pure quark stars, hybrid stars and the nature of the QCD phase transition were discussed and their observational signals delineated.

astro-ph.HE↗

arXiv2012

Lev Landau and the conception of neutron stars

We review the history of neutron star physics in the 1930s that is related to L. Landau. According to recollections of Rosenfeld (1974, Proc. 16th Solvay Conference on Physics, p. 174), Landau improvised the concept of neutron stars in a discussion with Bohr and Rosenfeld just after the news of the discovery of the neutron reached Copenhagen in February 1932. We present arguments that the discussion took place in March 1931, before the discovery of the neutron, and that they in fact discussed the paper written by Landau in Zurich in February 1931 but not published until February 1932 (Phys. Z. Sowjetunion, 1, 285). In his paper Landau mentioned the possible existence of dense stars which look like one giant nucleus; this can be regarded as an early theoretical prediction or anticipation of neutron stars, prior to the discovery of the neutron. The coincidence of the dates of the neutron's discovery and the paper's publication has led to an erroneous association of the paper with the discovery of the neutron. In passing, we outline the contribution of Landau to the theory of white dwarfs and to the hypothesis of stars with neutron cores.

physics.hist-ph↗

arXiv2009

Dynamic migration of rotating neutron stars due to a phase transition instability

Using numerical simulations based on solving the general relativistic hydrodynamic equations, we study the dynamics of a phase transition in the dense core of isolated rotating neutron stars, triggered by the back bending instability reached via angular momentum loss. In particular, we investigate the dynamics of a migration from an unstable configuration into a stable one, which leads to a mini-collapse of the neutron star and excites sizeable pulsations in its bulk until it acquires a new stable equilibrium state. We consider equations of state with softening at high densities, a simple analytic one with a mixed hadron-quark phase in an intermediate pressure interval and pure quark matter at very high densities, and a microphysical one that has a first-order phase transition, originating from kaon condensation. Although the marginally stable initial models are rigidly rotating, we observe that during the collapse (albeit little) differential rotation is created. We analyze the emission of gravitational radiation, which in some models is amplified by mode resonance effects, and assess its prospective detectability by interferometric detectors. We expect that the most favorable conditions for dynamic migration exist in very young magnetars. We find that the damping of the post-migration pulsations strongly depends on the character of the equation of state softening. The damping of pulsations in the models with the microphysical equation of state is caused by dissipation associated with matter flowing through the density jump at the edge of the dense core. If at work, this mechanism dominates over all other types of dissipation, like bulk viscosity in the exotic-phase core, gravitational radiation damping, or numerical viscosity.

astro-ph.SR↗

arXiv2007

The final phase of inspiral of neutron stars: realistic equations of state

Coalescing compact star binaries are expected to be among the strongest sources of gravitational radiation to be seen by laser interferometers. We present calculations of the final phase of inspiral of equal mass irrotational neutron star binaries and strange quark star binaries. Six types of equations of state at zero temperature are used - three realistic nuclear equations of state of various softness and three different MIT bag models of strange quark matter. We study the precoalescing stage within the Isenberg-Wilson-Mathews approximation of general relativity using a multidomain spectral method. The gravitational-radiation driven evolution of the binary system is approximated by a sequence of quasi-equilibrium configurations at fixed baryon number and decreasing separation. We find that the innermost stable circular orbit (ISCO) is given by an orbital instability for binary strange quark stars and by the mass-shedding limit for neutron star binaries. The gravitational wave frequency at the ISCO, which marks the end of the inspiral phase, is found to be around 1100-1460 Hz for two 1.35 solar masses irrotational strange stars described by the MIT bag model and between 800 Hz and 1230 Hz for neutron stars.

gr-qc↗

arXiv2006

Entrainment parameters in cold superfluid neutron star core

Hydrodynamical simulations of neutron star cores, based on a two fluid description in terms of a neutron-proton superfluid mixture, require the knowledge of the Andreev-Bashkin entrainment matrix which relates the momentum of one constituent to the currents of both constituents. This matrix is derived for arbitrary nuclear asymmetry at zero temperature and in the limits of small relative currents in the framework of the energy density functional theory. The Skyrme energy density functional is considered as a particular case. General analytic formulae for the entrainment parameters and various corresponding effective masses are obtained. These formulae are applied to the liquid core of a neutron star, composed of an homogeneous plasma of nucleons, electrons and possibly muons in beta equilibrium.

nucl-th↗

arXiv2006

Entrainment coefficient and effective mass for conduction neutrons in neutron star crust: II Macroscopic treatment

Phenomena such as pulsar frequency glitches are believed to be attributable to differential rotation of a current of ``free'' superfluid neutrons at densities above the ``drip'' threshold in the ionic crust of a neutron star. Such relative flow is shown to be locally describable by adaption of a canonical two fluid treatment that emphasizes the role of the momentum covectors constructed by differentiation of action with respect to the currents, with allowance for stratification whereby the ionic number current may be conserved even when the ionic charge number Z is altered by beta processes. It is demonstrated that the gauge freedom to make different choices of the chemical basis determining which neutrons are counted as ``free'' does not affect their ``superfluid'' momentum covector, which must locally have the form of a gradient (though it does affect the ``normal'' momentum covector characterising the protons and those neutrons that are considered to be ``confined'' in the nuclei). It is shown how the effect of ``entrainment'' (whereby the momentum directions deviate from those of the currents) is controlled by the (gauge independent) mobility coefficient K, estimated in recent microscopical quantum mechanical investigations, which suggest that the corresponding (gauge dependent) ``effective mass'' m* of the free neutrons can become very large in some layers. The relation between this treatment of the crust layers and related work (using different definitions of ``effective mass'') intended for the deeper core layers is discussed.

astro-ph↗

arXiv2005

Effect of BCS pairing on entrainment in neutron superfluid current in neutron star crust

The relative current density $n^i$ of conduction neutrons in a neutron star crust beyond the neutron drip threshold can be expected to be related to the corresponding particle momentum covector $p_i$ by a linear relation of the form $n^i=K^{ij}p_j$ in terms of a physically well defined mobility tensor $K^{ij}$. This result is describable as an ``entrainment'' whose effect - wherever the crust lattice is isotropic - will simply be to change the ordinary neutron mass m to an effective mass $m_\star$ such that in terms of the relevant number density n of unconfined neutrons we shall have $K^{ij}=n/m_\starγ^{ij}$. In a preceding analysis based on an independent particle treatment using Bloch boundary conditions to obtain the distribution of energy $E_k$ and associated group velocity $v_k^i=\partial E_k/\partial\hbar k_i$ as a function of wavenumber $k_i$, it was shown that the mobility tensor would be given by $K^{ij}\propto\int d^3 k v_k^i v_k^jδ\{E_k-μ\}$, where $μ$ is the Fermi energy. Using the approach due to Bogoliubov, it is shown here that the effect of BCS pairing with an energy gap Δ_F and corresponding quasiparticle energy function \EE_k=\sqrt{(E_k-μ)^2+Δ_F^2}$ will just be to replace the Dirac distributional integrand by a smoother distribution : $K^{ij}\propto \int d^3 k v_k^i v_k^j Δ_F^2 /\EE^3_k$. It is also shown how the pairing condensation gives rise to superfluidity in the technical sense of providing (meta) stability against resistive perturbations for a current that is not too strong (its momentum $p_i$ must be small enough to give $2|p_i v_k^i|<\EE_k^2/|E_k-μ|$ for all modes). It is concluded that the prediction of a very large effective mass enhancement in the middle layers of the crust will not be significantly affected by the pairing mechanism.

astro-ph↗

arXiv2005

Entrainment coefficient and effective mass for conduction neutrons in neutron star crust: simple microscopic models

In the inner crust of a neutron star, at densities above the ``drip'' threshold, unbound ``conduction'' neutrons can move freely past through the ionic lattice formed by the nuclei. The relative current density $n^i= n \bar v^i$ of such conduction neutrons will be related to the corresponding mean particle momentum $p_i$ by a proportionality relation of the form $n^i= {\cal K}p^i$ in terms of a physically well defined mobility coefficient $\cal K$ whose value in this context has not been calculated before. Using methods from ordinary solid state and nuclear physics, a simple quantum mechanical treatment based on the independent particle approximation, is used here to formulate $\cal K$ as the phase space integral of the relevant group velocity over the neutron Fermi surface. The result can be described as an ``entrainment'' that changes the ordinary neutron mass m to a macroscopic effective mass per neutron that will be given -- subject to adoption of a convention specifying the precise number density n of the neutrons that are considered to be ``free'' -- by $m_\star=n/{\cal K}$. The numerical evaluation of the mobility coefficient is carried out for nuclear configurations of the ``lasagna'' and ``spaghetti'' type that may be relevant at the base of the crust. Extrapolation to the middle layers of the inner crust leads to the unexpected prediction that $m_\star$ will become very large compared with m.

nucl-th↗