arXiv subjects

Brynmor Haskell

Publications and source records attributed to Brynmor Haskell.

At least 55 records · Page 3Linked to original sources

arXiv2018

Modelling pulsar glitches: the hydrodynamics of superfluid vortex avalanches in neutron stars

The dynamics of quantised vorticity in neutron star interiors is at the heart of most pulsar glitch models. However, the large number of vortices (up to $\approx 10^{13}$) involved in a glitch and the huge disparity in scales between the femtometer scale of vortex cores and the kilometre scale of the star makes quantum dynamical simulations of the problem computationally intractable. In this paper we take a first step towards developing a mean field prescription to include the dynamics of vortices in large scale hydrodynamical simulations of superfluid neutron stars. We consider a one dimensional setup and show that vortex accumulation and differential rotation in the neutron superfluid lead to propagating waves, or `avalanches', as solutions for the equations of motion for the superfluid velocities. We introduce an additional variable, the fraction of free vortices, and test different prescriptions for its advection with the superfluid flow. We find that the new terms lead to solutions with a linear component in the rise of a glitch, and that, in specific setups, they can give rise to glitch precursors and even to decreases in frequency, or `anti-glitches'.

astro-ph.HE↗

arXiv2018

Constraints on pulsar masses from the maximum observed glitch

Pulsar glitches, sudden jumps in frequency in otherwise steadily spinning down radio pulsars, offer a unique glimpse into the superfluid interior of neutron stars. The exact trigger of these events remains, however, elusive and this has hampered attempts to use glitch observations to constrain fundamental physics. In this paper we propose a new method to measure the mass of glitching pulsars, using observations of the maximum glitch recorded in a star, together with state of the art microphysical models of the pinning interaction between superfluid vortices and ions in the crust. Studying systematically all the presently observed large glitchers, we find an inverse correlation between size of the maximum glitch and the pulsar mass. Our procedure will allow current and future observations of glitching pulsars to constrain not only the physics of glitch models but also the equation of state of dense matter in neutron star interiors.

astro-ph.HE↗

arXiv2017

Probing neutron star interiors with pulsar glitches

Pulsar glitches are thought to be probes of the superfluid interior of neutron stars. These sudden jumps in frequency observed in many pulsars are generally assumed to be the macroscopic manifestation of superfluid vortex motion on a microscopic scale. Resolving and modelling such phenomena on the scale of a neutron star is, however, a challenging problem which still remains open, fifty years after the discovery of pulsars. In this article I will review recent theoretical progress, both on the microscopic level and on the macroscopic level, and discuss which constraints on the models can be provided by observations.

astro-ph.HE↗

arXiv2017

Superfluidity and Superconductivity in Neutron Stars

This review focuses on applications of the ideas of superfluidity and superconductivity in neutron stars in a broader context, ranging from the microphysics of pairing in nucleonic superfluids to macroscopic manifestations of superfluidity in pulsars. The exposition of the basics of pairing, vorticity and mutual friction can serve as an introduction to the subject. We also review some topics of recent interest, including the various types of pinning of vortices, glitches, and oscillations in neutron stars containing superfluid phases of baryonic matter.

astro-ph.HE↗

arXiv2017

Are gravitational waves spinning down PSR J1023+0038?

The pulsar J1203+0038 rotates with a frequency $ν\approx 592$ Hz and has been observed to transition between a radio state, during which it is visible as a millisecond radio pulsar, and and a Low Mass X-ray Binary state, during which accretion powered X-ray pulsations are visible. Timing during the two phases reveals that during the LMXB phase the neutron star is spinning down at a rate of $\dotν\approx -3 \times 10^{-15}$ Hz/s, which is approximately 27\% faster than the rate measured during the radio phase, $\dotν\approx -2.4 \times 10^{-15}$ Hz/s, and at odds with the predictions of accretion models. In this letter we suggest that the increase in spin-down rate is compatible with gravitational wave emission, and in particular to the creation of a `mountain' during the accretion phase. We show that asymmetries in pycno-nuclear reaction rates in the crust can lead to a large enough mass quadrupole to explain the observed spin-down rate, which so far has no other self-consistent explanation, and that radio timing at the onset of the next millisecond radio pulsar phase can test this scenario. Another possibility is that an unstable $r$-mode with amplitude $α\approx 5\times10^{-8}$ may be present in the system.

astro-ph.HE↗

arXiv2016

The effect of superfluid hydrodynamics on pulsar glitch sizes and waiting times

Pulsar glitches, sudden jumps in frequency observed in many radio pulsars, may be the macroscopic manifestation of superfluid vortex avalanches on the microscopic scale. Small scale quantum mechanical simulations of vortex motion in a decelerating container have shown that such events are possible and predict power-law distributions for the size of the events, and exponential distributions for the waiting time. Despite a paucity of data, this prediction is consistent with the size and waiting time distributions of most glitching pulsars. Nevertheless a few object appear to glitch quasi-periodically, and exhibit many large glitches, while a recent study of the Crab pulsar has suggested a cut-off deviations from a power-law distribution for smaller glitches. In this paper we incorporate the results of quantum mechanical simulations in a macroscopic scale superfluid hydrodynamics simulation. We show that the effect of vortex coupling to the neutron and proton fluids in the neutron star naturally leads to deviations from power-law distributions for sizes and from exponential distributions for waiting times. In particular we predict a cut-off in the size distribution for small glitches.

astro-ph.SR↗

arXiv2015

Mesoscopic pinning forces in neutron star crusts

The crust of a neutron star is thought to be comprised of a lattice of nuclei immersed in a sea of free electrons and neutrons. As the neutrons are superfluid their angular momentum is carried by an array of quantized vortices. These vortices can pin to the nuclear lattice and prevent the neutron superfluid from spinning down, allowing it to store angular momentum which can then be released catastrophically, giving rise to a pulsar glitch. A crucial ingredient for this model is the maximum pinning force that the lattice can exert on the vortices, as this allows us to estimate the angular momentum that can be exchanged during a glitch. In this paper we perform, for the first time, a detailed and quantitative calculation of the pinning force \emph{per unit length} acting on a vortex immersed in the crust and resulting from the mesoscopic vortex-lattice interaction. We consider realistic vortex tensions, allow for displacement of the nuclei and average over all possible orientation of the crystal with respect to the vortex. We find that, as expected, the mesoscopic pinning force becomes weaker for longer vortices and is generally much smaller than previous estimates, based on vortices aligned with the crystal. Nevertheless the forces we obtain still have maximum values of order $f_{\rm{pin}}\approx 10^{15}$ dyn/cm, which would still allow for enough angular momentum to be stored in the crust to explain large Vela glitches, if part of the star is decoupled during the event.

astro-ph.SR↗

arXiv2015

Pinned vortex hopping in a neutron star crust

The motion of superfluid vortices in a neutron star crust is at the heart of most theories of pulsar glitches. Pinning of vortices to ions can decouple the superfluid from the crust and create a reservoir of angular momentum. Sudden large scale unpinning can lead to an observable glitch. In this paper we investigate the scattering of a free vortex off a pinning potential and calculate its mean free path, in order to assess whether unpinned vortices can skip multiple pinning sites and come close enough to their neighbours to trigger avalanches, or whether they simply hop from one pinning site to another giving rise to a more gradual creep. We find that there is a significant range of parameter space in which avalanches can be triggered, thus supporting the hypothesis that they may lie at the origin of pulsar glitches. For realistic values of the pinning force and superfluid drag parameters we find that avalanches are more likely in the higher density regions of the crust where pinning is stronger. Physical differences in stellar parameters, such as mass and temperature, may lead to a switch between creep-like motion and avalanches, explaining the different characteristics of glitching pulsars.

astro-ph.SR↗

arXiv2015

R-modes in neutron stars: Theory and observations

In this article I will review the theory behind the gravitational wave driven r-mode instability in rapidly rotating neutron stars and discuss which constraints can be derived from observations of spins and temperatures in Low Mass X-ray Binaries. I will discuss how a standard, `minimal' neutron star model is not consistent with the data, and discuss some of the additional physical mechanisms that could reconcile theory with observations. In particular I will focus on additional forms of damping due to exotic cores and on strong mutual friction due to superfluid vortices cutting through superconducting flux tubes, and examine the repercussions these effects could have on the saturation amplitude of the mode. Finally I will also discuss the possibility that oscillations due to r-modes may have been recently observed in the X-ray light curves of two Low Mass X-ray Binaries.

astro-ph.HE↗

arXiv2015

Observational constraints on neutron star crust-core coupling during glitches

We demonstrate that observations of glitches in the Vela pulsar can be used to investigate the strength of the crust-core coupling in a neutron star, and suggest that recovery from the glitch is dominated by torque exerted by the re-coupling of superfluid components of the core that were decoupled from the crust during the glitch. Assuming that the recoupling is mediated by mutual friction between the superfluid neutrons and the charged components of the core, we use the observed magnitudes and timescales of the shortest timescale components of the recoveries from two recent glitches in the Vela pulsar to infer the fraction of the core that is coupled to the crust during the glitch, and hence spun up by the glitch event. Within the framework of a two-fluid hydrodynamic model of glitches, we analyze whether crustal neutrons alone are sufficient to drive the glitch activity observed in the Vela pulsar. We use two sets of neutron star equations of state (EOSs), both of which span crust and core consistently and cover a range of the slope of the symmetry energy at saturation density $30 < L <120$ MeV. One set produces maximum masses $\approx$2.0$M_{\odot}$, the second $\approx$2.6$M_{\odot}$. We also include the effects of entrainment of crustal neutrons by the superfluid lattice. We find that for medium to stiff EOSs, observations imply $>70\%$ of the moment of inertia of the core is coupled to the crust during the glitch, though for softer EOSs $L\approx 30$MeV as little as $5\%$ could be coupled. No EOS is able to reproduce the observed glitch activity with crust neutrons alone, but extending the region where superfluid vortices are strongly pinned into the core by densities as little as 0.016fm$^{-3}$ above the crust-core transition density restores agreement with the observed glitch activity.

astro-ph.SR↗

arXiv2015

Models of Pulsar Glitches

Radio pulsars provide us with some of the most stable clocks in the universe. Nevertheless several pulsars exhibit sudden spin-up events, known as glitches. More than forty years after their first discovery, the exact origin of these phenomena is still open to debate. It is generally thought that they an observational manifestation of a superfluid component in the stellar interior and provide an insight into the dynamics of matter at extreme densities. In recent years there have been several advances on both the theoretical and observational side, that have provided significant steps forward in our understanding of neutron star interior dynamics and possible glitch mechanisms. In this article we review the main glitch models that have been proposed and discuss our understanding, in the light of current observations.

astro-ph.SR↗

arXiv2015

Detecting gravitational waves from mountains on neutron stars in the Advanced Detector Era

Rapidly rotating Neutron Stars (NSs) in Low Mass X-ray Binaries (LMXBs) are thought to be interesting sources of Gravitational Waves (GWs) for current and next generation ground based detectors, such as Advanced LIGO and the Einstein Telescope. The main reason is that many of the NS in these systems appear to be spinning well below their Keplerian breakup frequency, and it has been suggested that torques associated with GW emission may be setting the observed spin period. This assumption has been used extensively in the literature to assess the strength of the likely gravitational wave signal. There is now, however, a significant amount of theoretical and observation work that suggests that this may not be the case, and that GW emission is unlikely to be setting the spin equilibrium period in many systems. In this paper we take a different starting point and predict the GW signal strength for two physical mechanisms that are likely to be at work in LMXBs: crustal mountains due to thermal asymmetries and magnetically confined mountains. We find that thermal crustal mountains in transient LMXBs are unlikely to lead to detectable GW emission, while persistent systems are good candidates for detection by Advanced LIGO and by the Einstein Telescope. Detection prospects are pessimistic for the magnetic mountain case, unless the NS has a buried magnetic field of $B\approx 10^{12}$ G, well above the typically inferred exterior dipole fields of these objects. Nevertheless, if a system were to be detected by a GW observatory, cyclotron resonant scattering features in the X-ray emission could be used to distinguish between the two different scenarios.

astro-ph.SR↗

arXiv2014

Gravitational waves from rapidly rotating neutron stars

Rapidly rotating neutron stars in Low Mass X-ray Binaries have been proposed as an interesting source of gravitational waves. In this chapter we present estimates of the gravitational wave emission for various scenarios, given the (electromagnetically) observed characteristics of these systems. First of all we focus on the r-mode instability and show that a 'minimal' neutron star model (which does not incorporate exotica in the core, dynamically important magnetic fields or superfluid degrees of freedom), is not consistent with observations. We then present estimates of both thermally induced and magnetically sustained mountains in the crust. In general magnetic mountains are likely to be detectable only if the buried magnetic field of the star is of the order of $B\approx 10^{12}$ G. In the thermal mountain case we find that gravitational wave emission from persistent systems may be detected by ground based interferometers. Finally we re-asses the idea that gravitational wave emission may be balancing the accretion torque in these systems, and show that in most cases the disc/magnetosphere interaction can account for the observed spin periods.

astro-ph.SR↗

arXiv2014

Nuclear Equation of State from Observations of Short Gamma-Ray Burst Remnants

The favoured progenitor model for short $γ$-ray bursts (SGRBs) is the merger of two neutron stars that triggers an explosion with a burst of collimated $γ$-rays. Following the initial prompt emission, some SGRBs exhibit a plateau phase in their $X$-ray light curves that indicates additional energy injection from a central engine, believed to be a rapidly rotating, highly magnetised neutron star. The collapse of this `protomagnetar' to a black hole is likely to be responsible for a steep decay in $X$-ray flux observed at the end of the plateau. In this letter, we show that these observations can be used to effectively constrain the equation of state of dense matter. In particular, we show that the known distribution of masses in binary neutron star systems, together with fits to the $X$-ray light curves, provide constraints that exclude the softest and stiffest plausible equations of state. We further illustrate how a future gravitational wave observation with Advanced LIGO/Virgo can place tight constraints on the equation of state, by adding into the picture a measurement of the chirp mass of the SGRB progenitor.

astro-ph.HE↗

arXiv2014

On the universality of I-Love-Q relations in magnetized neutron stars

Recently, general relations among the quadrupole moment (Q), the moment of inertia (I), and the tidal deformability (Love number) of a neutron star were shown to exist. They are nearly independent of the nuclear matter equation of state and would be of great aid in extracting parameters from observed gravitational-waves and in testing general relativity. These relations, however, do not account for strong magnetic fields. We consider this problem by studying the effect of a strong magnetic field on slowly rotating relativistic neutron stars and show that, for simple magnetic field configurations that are purely poloidal or purely toroidal, the relation between Q and I is again nearly universal. However, different magnetic field geometries lead to different I-Q relations, and, in the case of a more realistic twisted-torus magnetic field configuration, the relation depends significantly on the equation of state, losing its universality. I-Love-Q relations must thus be used with very great care, since universality is lost for stars with long spin periods, i.e. P > 10 s, and strong magnetic fields, i.e. B > 10^12 G.

astro-ph.SR↗

arXiv2013

A new mechanism for saturating unstable r-modes in neutron stars

We consider a new mechanism for damping the oscillations of a mature neutron star. The new dissipation channel arises if superfluid vortices are forced to cut through superconducting fluxtubes. This mechanism is interesting because the oscillation modes need to exceed a critical amplitude in order for it to operate. Once it acts the effect is very strong (and nonlinear) leading to efficient damping. The upshot of this is that modes are unlikely to ever evolve far beyond the critical amplitude. We consider the effect of this new dissipation channel on the r-modes, that may be driven unstable by the emission of gravitational waves. Our estimates show that the fluxtube cutting leads to a saturation threshold for the instability that can be smaller than that of other proposed mechanisms. This suggests that the idea may be of direct astrophysical relevance.

astro-ph.SR↗

arXiv2013

Glitch recoveries in radio-pulsars and magnetars

Pulsar glitches are sudden increases in the spin frequency of an otherwise steadily spinning down neutron star. These events are thought to represent a direct probe of the dynamics of the superfluid interior of the star. However glitches can differ significantly from one another, not only in size and frequency, but also in the post-glitch response of the star. Some appear as simple steps in frequency, while others also display an increase in spin-down rate after the glitch. Others still show several exponentially relaxing components in the post-glitch recovery. We show that if glitches are indeed due to large scale unpinning of superfluid vortices, the different regions in which this occurs and respective timescales on which they recouple can lead to the various observed signatures. Furthermore we show that this framework naturally accounts for the peculiar relaxations of glitches in Anomalous X-ray Pulsars.

astro-ph.SR↗

arXiv2013

The hottest superfluid and superconductor in the Universe: Discovery and nuclear physics implications

We present recent work on using astronomical observations of neutron stars to reveal unique insights into nuclear matter that cannot be obtained from laboratories on Earth. First, we discuss our measurement of the rapid cooling of the youngest neutron star in the Galaxy; this provides the first direct evidence for superfluidity and superconductivity in the supra-nuclear core of neutron stars. We show that observations of thermonuclear X-ray bursts on neutron stars can be used to constrain properties of neutron superfluidity and neutrino emission. We describe the implications of rapid neutron star rotation rates on aspects of nuclear and superfluid physics. Finally, we show that entrainment coupling between the neutron superfluid and the nuclear lattice leads to a less mobile crust superfluid; this result puts into question the conventional picture of pulsar glitches as being solely due to the crust superfluid and suggests that the core superfluid also participates.

hep-ph↗