R-mode Instability Research Articles

R-mode instability of neutron stars: LOCV approach

R-mode instability of neutron stars: LOCV approach

Signatures of hadron-quark phase transition through the r-mode instability in twin stars

Signatures of hadron-quark phase transition through the r-mode instability in twin stars

Formation of long-period radio pulsars

Abstract This study investigates the influence of different braking mechanisms on the formation of three long-period radio pulsars (PSRs J0250 + 5854, J2251-3711, and J0901-4046): plasma-filled magnetosphere in combination with magnetic field decay, fall-back disc, and r-mode instability. These braking mechanisms can also affect the thermal evolution of pulsars. By comparing the model-predicted values with observational data such as spin periods, period derivatives, and upper limits of the bolometric luminosity of these pulsars, we find that these three braking mechanisms can reasonably explain the spin period and the period derivative within a certain range of parameters for these sources. The model-predicted bolometric luminosity associated with magnetic field dissipation exceeds the upper limit for PSR J0901-4046 but falls below the upper limits for PSR J0250 + 5854 and PSR J2251-3711. The model-predicted bolometric luminosity within the fall-back disc model exceeds the observed results, whereas the luminosity within the r-mode instability falls below the observed upper limit for these three pulsars. However, a conflict arises when we consider the pulsar radio activity and the accretion phases within the fall-back disc model simultaneously. By combining data from radio and X-ray observations, along with precise measurements of surface thermal emissions, we can enhance our understanding of the braking mechanisms involved in the formation of long-period radio pulsars or constrain key model parameters. Finding more long-period pulsars in the future and conducting multi-band observations will enhance our understanding of the formation and nature of long-period radio pulsars.

R-modes as a new probe of dark matter in neutron stars

In this work, we perform the first systematic investigation of effects of the presence of dark matter on r-mode oscillations in neutron stars (NSs). Using a self-interacting dark matter (DM) model based on the neutron decay anomaly and a hadronic model obtained from the posterior distribution of a recent Bayesian analysis, we impose constraints on the DM self-interaction strength using recent multimessenger astrophysical observations. We also put new constraints on the DM fraction for this model of DM. The constrained DM interaction strength is then used to estimate DM self-interaction cross section and shear viscosity resulting from DM, which is found to be several orders of magnitude smaller than shear viscosity due to hadronic matter. Assuming chemical equilibrium among DM fermions and neutrons, we estimate the bulk viscosity resulting from the dark decay of neutrons considering different scenarios for the temperature dependence of the reaction rate and investigate the effect on the r-mode instability window. We conclude that DM shear and bulk viscosity may significantly modify the r-mode instability window compared with the minimal hadronic viscosities, depending on the temperature dependence of the process. We also found that for the window to be compatible with the X-ray and pulsar observational data, the rate for the dark decay process must be fast.

Hyperon bulk viscosity and r-modes of neutron stars

ABSTRACT We propose and apply a new parametrization of the modified chiral effective model to study rotating neutron stars with hyperon cores in the framework of the relativistic mean-field theory. The inclusion of mesonic cross couplings in the model has improved the density content of the symmetry energy slope parameters, which are in agreement with the findings from recent terrestrial experiments. The bulk viscosity of the hyperonic medium is analyzed to investigate its role in the suppression of gravitationally driven r-modes. The hyperonic bulk viscosity coefficient caused by non-leptonic weak interactions and the corresponding damping time-scales are calculated and the r-mode instability windows are obtained. The present model predicts a significant reduction of the unstable region due to a more effective damping of oscillations. We find that from ∼108 K to ∼109 K, hyperonic bulk viscosity completely suppresses the r-modes leading to a stable region between the instability windows. Our analysis indicates that the instability can reduce the angular velocity of the star up to ∼0.3 ΩK, where ΩK is the Kepler frequency of the star.

Observability of HOFNARs at SRG/eROSITA

Neutron stars can appear as sources of different nature. In this paper we address the observability of a hypothetical class of neutron stars—HOt and Fast Non-Accreting Rotators, HOFNARs. These objects are heated due to the r-mode instability. With surface temperatures ∼106 K they are expected to be thermal soft X-ray emitters. We perform a population synthesis modeling of HOFNARs to predict the number of potentially detectable sources in the eROSITA all-sky survey. For surface temperatures ∼106 K we obtain ∼500 sources above the detection limit 0.01 cts s−1 and ∼100 easier identifiable sources with >0.1 cts s−1. Temperatures ≳1.2 × 106 K start to be in contradiction with non-detection of HOFNARs by ROSAT. Only for T ≲ 5 × 105 K numbers predicted for eROSITA turn out to be so low that identification does not look possible. We conclude that eROSITA has good chances to discover HOFNARs, if they exist. Non-detection will put very stringent limits on the properties of this type of neutron stars.

Hyperons in Finite and Infinite Nuclear Systems

In this work, we shortly review the role and properties of hyperons in finite and infinite nuclear systems such as hypernuclei and neutron stars. Particularly, we describe different production mechanisms of hypernuclei, discuss some aspects of their γ-ray spectroscopy and their weak decay modes, and give a few strokes on their theoretical description. We reexamine also the role played by hyperons on the properties of neutron and proto-neutron stars with a special emphasis on the well-known “hyperon puzzle”, of which we discuss some of the solutions that have been proposed to tackle this problem. Finally, we review the role of hyperons on the cooling properties of newly born neutron stars and on the so-called r-mode instability.

Transport Properties of Superfluid Phonons in Neutron Stars

We review the effective field theory associated with the superfluid phonons that we use for the study of transport properties in the core of superfluid neutrons stars in their low temperature regime. We then discuss the shear and bulk viscosities together with the thermal conductivity coming from the collisions of superfluid phonons in neutron stars. With regard to shear, bulk, and thermal transport coefficients, the phonon collisional processes are obtained in terms of the equation of state and the superfluid gap. We compare the shear coefficient due to the interaction among superfluid phonons with other dominant processes in neutron stars, such as electron collisions. We also analyze the possible consequences for the r-mode instability in neutron stars. As for the bulk viscosities, we determine that phonon collisions contribute decisively to the bulk viscosities inside neutron stars. For the thermal conductivity resulting from phonon collisions, we find that it is temperature independent well below the transition temperature. We also obtain that the thermal conductivity due to superfluid phonons dominates over the one resulting from electron-muon interactions once phonons are in the hydrodynamic regime. As the phonons couple to the Z electroweak gauge boson, we estimate the associated neutrino emissivity. We also briefly comment on how the superfluid phonon interactions are modified in the presence of a gravitational field or in a moving background.

R-mode Stability of GW190814's Secondary Component as a Supermassive and Superfast Pulsar

Abstract The nature of GW190814's secondary component m 2 of mass 2.50–2.67 M ⊙ in the mass gap between the currently known maximum mass of neutron stars and the minimum mass of black holes is currently under hot debate. Among the many possibilities proposed in the literature, m 2 was suggested to be a superfast pulsar, while its r-mode stability against runaway gravitational radiation through the Chandrasekhar–Friedman–Schutz mechanism is still unknown. Previously, Fortin et al. constructed a sample of 33 unified equations of state using the same nuclear interactions from the crust to the core consistently; from that sample we use those equations that fulfill all currently known astrophysical and nuclear physics constraints to compare the minimum frequency required for m 2 to rotationally sustain a mass greater than 2.50 M ⊙ with the critical frequency above which the r-mode instability occurs. We use two extreme damping models assuming that the crust is either perfectly rigid or elastic. Using the stability of 19 observed low-mass X-ray binaries as an indication that the rigid crust damping of the r-mode dominates within the models studied, we find that m 2 is r-mode-stable while rotating with a frequency higher than 870.2 Hz (0.744 times its Kepler frequency of 1169.6 Hz) as long as its temperature is lower than about 3.9 × 107 K, further supporting the proposal that GW190814's secondary component is a supermassive and superfast pulsar.

Influence of direct Urca on the r-mode spin down features of newborn neutron star pulsars

We perform a study on the time duration, final frequency, and exit temperature of the r-mode instability in newborn neutron stars of normal fluid composition under the assumption that the r-mode is the dominating spin-down mechanism. It is found that these spin-down features will vary widely depending on the star mass when direct Urca becomes operative. The occurrence of direct Urca is decided from the stiffness of the symmetry energy of the underlying equation of state, which is still uncertain. The change in the spin-down features in stars of different masses has been studied by computing the magnitude of the bulk viscosity due to the extent of direct Urca coverage in the core. The range for the variation of the symmetry energy stiffness, characterized by the slope parameter L, is decided from the tidal deformability data of the GW170817 event and the maximum mass constraint. The study is done by applying a family of equation of states obtained with the finite range simple effective interaction, constrained at low and intermediate densities by nuclear data and the flow analysis in heavy-ion collisions and at high densities by causality.

Role of nucleon-nucleon correlation in transport coefficients and gravitational-wave-driven r-mode instability of neutron stars

The thermal conductivity and shear viscosity of dense nuclear matter, along with the corresponding shear viscosity timescale of canonical neutron stars (NSs), are investigated, where the effect of Fermi surface depletion (i.e., the Z-factor effect) induced by the nucleon-nucleon correlation is taken into account. The factors which are responsible for the transport coefficients, including the equation of state for building the stellar structure, nucleon effective masses, in-medium cross sections, and the Z-factor at Fermi surfaces, are all calculated in the framework of the Brueckner theory. The Fermi surface depletion is found to enhance the transport coefficients by several times at high densities, which is more favorable to damping the gravitational-wave-driven r-mode instability of NSs. Yet, the onset of the Z-factor-quenched neutron triplet superfluidity provides the opposite effects, which can be much more significant than the above mentioned Z-factor effect itself. Therefore, different from the previous understanding, the nucleon shear viscosity is still smaller than the lepton one in the superfluid NS matter at low temperatures. Accordingly, the shear viscosity cannot stabilize canonical NSs against r-mode oscillations even at quite low core temperatures 106 K.

Bulk viscosity for interacting strange quark matter and r-mode instability windows for strange stars * *Supported by National Natural Science Foundation of China (12005005, 11947098, 11705163, 11875052, No. 11575190) and key research projects of universities in Henan province (20A140003)

We investigate the bulk viscosity of strange quark matter in the framework of the equivparticle model, where analytical formulae are obtained for certain temperature ranges, which can be readily applied to those with various quark mass scalings. In the case of adopting a quark mass scaling with both linear confinement and perturbative interactions, the obtained bulk viscosity increases by orders of magnitude compared with those in bag model scenarios. Such an enhancement is mainly due to the large quark equivalent masses adopted in the equivparticle model, which are essentially attributed to the strong interquark interactions and are related to the dynamical chiral symmetry breaking. Due to the high bulk viscosity, the predicted damping time of oscillations for a canonical 1.4 strange star is less than one millisecond, which is shorter than previous findings. Consequently, the obtained -mode instability window for the canonical strange stars well accommodates the observational frequencies and temperatures for pulsars in low-mass X-ray binaries (LMXBs).

R-mode instability of neutron stars in Low-mass X-ray binaries: effects of Fermi surface depletion and superfluidity of dense matter

Abstract The nucleon-nucleon correlation between nucleons leads to the Fermi surface depletion measured by a Z-factor in momentum distribution of dense nuclear matter. The roles of the Fermi surface depletion effect (Z-factor effect) and its quenched neutron triplet superfluidity of nuclear matter in viscosity and hence in the gravitational-wave-driven r-mode instability of neutron stars (NSs) are investigated. The bulk viscosity is reduced by both the two effects, especially the superfluid effect at low temperatures which is also able to reduce the inferred core temperature of NSs. Intriguingly, due to the neutron superfluidity, the core temperature of the NSs in known low-mass X-ray binaries (LMXBs) are found to be clearly divided into two groups: high and low temperatures which correspond to NSs with short and long recurrence times for nuclear-powered bursts respectively. Yet, a large number of NSs in these LMXBs are still located in the r-mode instability region. If the density-dependent symmetry energy is stiff enough, the occurence of direct Urca process reduces the inferred core temperature by about one order of magnitude. Accordingly, the contradiction between the predictions and observations is alleviated to some extent, but some NSs are still located inside the unstable region.

Gravitational waves from r-mode instability of massive young sub- and super-Chandrasekhar white dwarfs

In the present work we investigate the r-mode instability windows, spindown and spindown rates of sub- and super-Chandrasekhar magnetized white dwarfs in presence of Landau quantization of the electron gas and magnetic braking. The gravitational wave strain amplitudes due to r-mode instability is also calculated. The dominant damping mechanism is taken to be the shear viscosity arising due to scattering of the degenerate electrons from the ion liquid. We find that the critical frequencies of Landau quantized magnetized white dwarfs are the lowest, those of non-Landau quantized ones are higher and those of non-magnetized ones are the highest at the same temperature. This implies that magnetic braking and Landau quantization both enhance r-mode instability. We have also seen that there is rapid spindown of magnetized white dwarfs due to additional magnetic braking term but there is no considerable effect of Landau quantization on the spindown and spindown rates for magnetic field strengths relevant for white dwarf interiors. We find that the r-mode gravitational wave strain amplitude for a rapidly rotating super-Chandrasekhar white dwarf at 1 kpc is $\sim 10^{-27}$, making isolated massive rapidly rotating hot magnetized white dwarfs prime candidates for search of gravitational waves in the future.

Vortex pinning in the superfluid core of neutron stars and the rise of pulsar glitches

ABSTRACT Timing of the Crab and Vela pulsars has recently revealed very peculiar evolutions of their spin frequency during the early stage of a glitch. We show that these differences can be interpreted from the interactions between neutron superfluid vortices and proton fluxoids in the core of these neutron stars. In particular, pinning of individual vortices to fluxoids is found to have a dramatic impact on the mutual friction between the neutron superfluid and the rest of the star. The number of fluxoids attached to vortices turns out to be a key parameter governing the global dynamics of the star. These results may have implications for the interpretation of other astrophysical phenomena such as pulsar-free precession or the r-mode instability.

Bulk viscosity in neutron stars with hyperon cores

It is well-known that r-mode oscillations of rotating neutron stars may be unstable with respect to the gravitational wave emission. It is highly unlikely to observe a neutron star with the parameters within the instability window, a domain where this instability is not suppressed. But if one adopts the `minimal' (nucleonic) composition of the stellar interior, a lot of observed stars appear to be within the r-mode instability window. One of the possible solutions to this problem is to account for hyperons in the neutron star core. The presence of hyperons allows for a set of powerful (lepton-free) non-equilibrium weak processes, which increase the bulk viscosity, and thus suppress the r-mode instability. Existing calculations of the instability windows for hyperon NSs generally use reaction rates calculated for the $\Sigma^-\Lambda$ hyperonic composition via the contact $W$ boson exchange interaction. In contrast, here we employ hyperonic equations of state where the $\Lambda$ and $\Xi^-$ are the first hyperons to appear (the $\Sigma^-$'s, if they are present, appear at much larger densities), and consider the meson exchange channel, which is more effective for the lepton-free weak processes. We calculate the bulk viscosity for the non-paired $npe\mu\Lambda\Xi^-$ matter using the meson exchange weak interaction. A number of viscosity-generating non-equilibrium processes is considered (some of them for the first time in the neutron-star context). The calculated reaction rates and bulk viscosity are approximated by simple analytic formulas, easy-to-use in applications. Applying our results to calculation of the instability window, we argue that accounting for hyperons may be a viable solution to the r-mode problem.

R-mode stabilization in neutron stars with hyperon cores

R-mode oscillations of neutron stars are known to be generally unstable with respect to the emission of gravitational waves. This instability can be suppressed by dissipative processes in the neutron star core. The standard minimal nucleonic model of the core cannot stabilize r-modes for many observed neutron stars, at the same time the probability to observe them unstable should be very low. One of the solutions to this problem is to account for possible presence of hyperons in the stellar core. Hyperons strongly enhance bulk viscosity in the core, which effectively suppresses the r-mode instability. We revisit the bulk viscosity calculations in the non-superfluid hyperon matter and briefly discuss the effect of baryon superfluidity. Our results suggest that the hyperon solution to the r-mode problem is more promising than it was thought before.

XMM-Newton Detection and Spectrum of the Second Fastest Spinning Pulsar PSR J0952−0607

Abstract With a spin frequency of 707 Hz, PSR J0952−0607 is the second fastest spinning pulsar known. It was discovered in radio by LOFAR in 2017 at an estimated distance of either 0.97 or 1.74 kpc and has a low-mass companion with a 6.42 hr orbital period. We report the discovery of the X-ray counterpart of PSR J0952−0607 using XMM-Newton. The X-ray spectra can be well-fit by a single power law (PL) model (Γ ≈ 2.5) or by a thermal plus PL model ( and Γ ≈ 1.4). We do not detect evidence of variability, such as that due to orbital modulation from pulsar wind and companion star interaction. Because of its fast spin rate, PSR J0952−0607 is a crucial source for understanding the r-mode instability, which can be an effective mechanism for producing gravitational waves. Using the high end of our measured surface temperature, we infer a neutron star core temperature of ∼107 K, which places PSR J0952−0607 within the window for the r-mode to be unstable unless an effect such as superfluid mutual friction damps the fluid oscillation. The measured luminosity limits the dimensionless r-mode amplitude to be less than ∼1 × 10−9.

R-mode constraints from neutron star equation of state

The gravitational radiation has been proposed a long time before, as an explana- tion for the observed relatively low spin frequencies of young neutron stars and of accreting neutron stars in low-mass X-ray binaries as well. In the present work we studied the effects of the neutron star equation of state on the r-mode instability window of rotating neutron stars.

The r-mode instability windows of strange stars

With the constraint from gravitational wave emission of a binary merger system (GW170817) and two-solar-mass pulsar observations, we investigate the r-mode instability windows of strange stars with unpaired and color-flavor-locked phase strange quark matter. Shear viscosities due to surface rubbing and electron-electron scattering are taken into account in this work. The results show that the effects of the equation of state of unpaired strange quark matter are only dominant at low temperature, but do not have significant effects on strange stars in the color-flavor-locked phase. A color-flavor-locked phase strange star, which is surrounded by an insulating nuclear crust, seems to be consistent with observational data of young pulsars. We find that an additional enhanced dissipation mechanisms might exist in SAX J1808.4–3658. Fast spinning young pulsar PSR J0537–6910 is a primary source for detecting gravitational waves from a rotating strange star, and young pulsars might be strange stars with color-flavor-locked phase strange quark matter.