arXiv subjects

S. K. Lander

Publications and source records attributed to S. K. Lander.

At least 19 recordsLinked to original sources

arXiv2025

Timing and Spectral Evolution of the Magnetar 1E 1841-045 in Outburst

We present the timing and spectral analyses of the NICER, NuSTAR, and IXPE observations of the magnetar 1E 1841-045 covering 82 days following its August 2024 bursting activity as well as radio observations utilizing MeerKAT and Effelsberg. We supplement our study with a historical NuSTAR and all 2024 pre-outburst NICER observations. The outburst is marked by an X-ray flux enhancement of a factor 1.6 compared to the historical level, predominantly driven by a newly-formed non-thermal emitting component with a photon index $\Gamma=1.5$. This flux showed a 20% decay at the end of our monitoring campaign. The radio monitoring did not reveal any pulsed radio emission with an upper-limit of 20 mJy and 50 mJy ms on the mean flux density and single pulse fluence, respectively. We detect a spin-up glitch at outburst onset with a $\Delta\nu=6.1\times10^{-8}$ Hz and a $\Delta\dot{\nu}=-1.4\times10^{-14}$ Hz s$^{-1}$, consistent with the near-universality of this behavior among the continuously-monitored magnetars. Most intriguingly, the 1E 1841-045 2-10 keV pulse profile is markedly different compared to pre-outburst; it shows a new, narrow (0.1 cycles) peak that appears to shift towards merging with the main, persistently-present, pulse. This is the second case of pulse-peak migration observed in magnetars after SGR 1830$-$0645, and the two sources exhibit a similar rate of phase shift. This implies that this phenomenon is not unique and might present itself in the broader population. The newly-formed peak for 1E 1841-045 is non-thermal, with emission extending to $\gtrsim20$ keV, in contrast to the case of SGR 1830$-$0645. Our results are consistent with an untwisting magnetic field bundle with migration towards the magnetic pole, perhaps accompanied by plastic motion of the crust.

astro-ph.HE↗

arXiv2024

The Meissner effect in neutron stars

We present the first model aimed at understanding how the Meissner effect in a young neutron star affects its macroscopic magnetic field. In this model, field expulsion occurs on a dynamical timescale, and is realised through two processes that occur at the onset of superconductivity: fluid motions causing the dragging of field lines, followed by magnetic reconnection. Focussing on magnetic fields weaker than the superconducting critical field, we show that complete Meissner expulsion is but one of four possible generic scenarios for the magnetic-field geometry, and can never expel magnetic flux from the centre of the star. Reconnection causes the release of up to $\sim 5\times 10^{46}\,\mathrm{erg}$ of energy at the onset of superconductivity, and is only possible for certain favourable early-phase dynamics and for pre-condensation fields $10^{12}\,\mathrm{G}\lesssim B\lesssim 5\times 10^{14}\,\mathrm{G}$. Fields weaker or stronger than this are predicted to thread the whole star.

astro-ph.HE↗

arXiv2024

Signatures of magnetic flux expulsion from neutron star cores

Shortly after a neutron star is born, the protons in its core begin to form a superconductor. In terrestrial materials, the hallmark of superconductivity is an associated expulsion of magnetic flux, but whether this expulsion process can be effective in neutron stars remains an open question -- one with major implications for the phenomenology of pulsars and magnetars. Earlier theoretical arguments suggested flux must be trapped within the core, yet models of magnetars rely on it being expelled from the core and confined to the crust, where it can evolve on kyr timescales. We show that if expulsion is not complete, a qualitatively new evolutionary branch for neutron stars arises, which can account for the properties of newly discovered long-period radio transients and fast radio bursts in older environments. One recently proposed model that could create such field topologies has additional implications for gravitational wave emission and predicts a characteristic energy release that, if observed, will corroborate the role of reconnection at the onset of superconductivity and can constrain the superconducting proton gap.

astro-ph.HE↗

arXiv2023

Twisted magnetar magnetospheres

Magnetar magnetospheres are strongly twisted, and are able to power sudden energetic events through the rapid release of stored electromagnetic energy. In this paper, we investigate twisted relativistic force-free axisymmetric magnetospheres of rotating neutron stars. We obtain numerical solutions of such configurations using the method of simultaneous relaxation for the magnetic field inside and outside the light-cylinder. We introduce a toroidal magnetic field in the region of closed field-lines that is associated with a poloidal electric current distribution in that region, and explore various mathematical expressions for that distribution. We find that, by increasing the twist, a larger fraction of magnetic field-lines crosses the light-cylinder and opens up to infinity, thus increasing the size of the polar caps and enhancing the spin-down rate. We also find that, for moderately to strongly twisted magnetospheres, the region of closed field-lines ends at some distance inside the light-cylinder. We discuss the implications of these solutions on the variation of magnetar spin-down rates, moding and nulling of pulsars, the relation between the angular shear and the twist and the overall shape of the magnetosphere.

astro-ph.HE↗

arXiv2023

The game of life on a magnetar crust: from $γ$-ray flares to FRBs

This paper presents a model to unify the diverse range of magnetar activity, through the building and release of elastic stress from the crust. A cellular automaton drives both local and global yielding of the crust, leading to braiding of coronal loops and energy release. The model behaves like a real magnetar in many ways: giant flares and small bursts both occur, as well as periods of quiescence whose typical duration is either $\lesssim 1$ yr or $\sim 10-30$ yr. The burst energy distribution broadly follows an earthquake-like power law over the energy range $10^{40}-10^{45}\,{\rm erg}$. The local nature of coronal loops allows for the possibility of high-energy and fast radio bursts from the same magnetar. Within this paradigm, magnetar observations can be used to constrain the poorly-understood mechanical properties of the neutron-star crust.

astro-ph.HE↗

arXiv2022

Pulse Peak Migration during the Outburst Decay of the Magnetar SGR 1830-0645: Crustal Motion and Magnetospheric Untwisting

Magnetars, isolated neutron stars with magnetic field strengths typically $\gtrsim10^{14}$~G, exhibit distinctive months-long outburst epochs during which strong evolution of soft X-ray pulse profiles, along with nonthermal magnetospheric emission components, is often observed. Using near-daily NICER observations of the magnetar SGR 1830-0645 during the first 37 days of a recent outburst decay, a pulse peak migration in phase is clearly observed, transforming the pulse shape from an initially triple-peaked to a single-peaked profile. Such peak merging has not been seen before for a magnetar. Our high-resolution phase-resolved spectroscopic analysis reveals no significant evolution of temperature despite the complex initial pulse shape. Yet the inferred surface hot spots shrink during the peak migration and outburst decay. We suggest two possible origins for this evolution. For internal heating of the surface, tectonic motion of the crust may be its underlying cause. The inferred speed of this crustal motion is $\lesssim100$~m~day$^{-1}$, constraining the density of the driving region to $ρ\sim10^{10}$~g~cm$^{-3}$, at a depth of $\sim200$~m. Alternatively, the hot spots could be heated by particle bombardment from a twisted magnetosphere possessing flux tubes or ropes, somewhat resembling solar coronal loops, that untwist and dissipate on the 30-40~day timescale. The peak migration may then be due to a combination of field-line footpoint motion (necessarily driven by crustal motion) and evolving surface radiation beaming. These novel dataset paints a vivid picture of the dynamics associated with magnetar outbursts, yet it also highlights the need for a more generic theoretical picture where magnetosphere and crust are considered in tandem.

astro-ph.HE↗

arXiv2021

Generating neutron-star magnetic fields: three dynamo phases

Young neutron stars (NSs) have magnetic fields $B$ in the range $10^{12}-10^{15}$ G, believed to be generated by dynamo action at birth. We argue that such a dynamo is actually too inefficient to explain the strongest of these fields. Dynamo action in the mature star is also unlikely. Instead we propose a promising new precession-driven dynamo and examine its basic properties, as well as arguing for a revised mean-field approach to NS dynamos. The precession-driven dynamo could also play a role in field generation in main-sequence stars.

astro-ph.HE↗

arXiv2021

Magnetic fields in late-stage proto-neutron stars

We explore the thermal and magnetic-field structure of a late-stage proto-neutron star. We find the dominant contribution to the entropy in different regions of the star, from which we build a simplified equation of state for the hot neutron star. With this, we numerically solve the stellar equilibrium equations to find a range of models, including magnetic fields and rotation up to Keplerian velocity. We approximate the equation of state as a barotrope, and discuss the validity of this assumption. For fixed magnetic-field strength, the induced ellipticity increases with temperature; we give quantitative formulae for this. The Keplerian velocity is considerably lower for hotter stars, which may set a de-facto maximum rotation rate for non-recycled NSs well below 1 kHz. Magnetic fields stronger than around $10^{14}$ G have qualitatively similar equilibrium states in both hot and cold neutron stars, with large-scale simple structure and the poloidal field component dominating over the toroidal one; we argue this result may be universal. We show that truncating magnetic-field solutions at low multipoles leads to serious inaccuracies, especially for models with rapid rotation or a strong toroidal-field component.

astro-ph.HE↗

arXiv2020

Magnetar birth: rotation rates and gravitational-wave emission

Understanding the evolution of the angle $χ$ between a magnetar's rotation and magnetic axes sheds light on the star's birth properties. This evolution is coupled with that of the stellar rotation $Ω$, and depends on the competing effects of internal viscous dissipation and external torques. We study this coupled evolution for a model magnetar with a strong internal toroidal field, extending previous work by modelling -- for the first time in this context -- the strong proto-magnetar wind acting shortly after birth. We also account for the effect of buoyancy forces on viscous dissipation at late times. Typically we find that $χ\to 90^\circ$ shortly after birth, then decreases towards $0^\circ$ over hundreds of years. From observational indications that magnetars typically have small $χ$, we infer that these stars are subject to a stronger average exterior torque than radio pulsars, and that they were born spinning faster than $\sim 100-300$ Hz. Our results allow us to make quantitative predictions for the gravitational and electromagnetic signals from a newborn rotating magnetar. We also comment briefly on the possible connection with periodic Fast Radio Burst sources.

astro-ph.HE↗

arXiv2019

M51 ULX-7: super-orbital periodicity and constraints on the neutron star magnetic field

In the current work we explore the applicability of standard theoretical models of accretion to the observed properties of M51 ULX-7. The spin-up rate and observed X-ray luminosity are evidence of a neutron star with a surface magnetic field of $2-7\times10^{13}$ G, rotating near equilibrium. Analysis of the X-ray light-curve of the system (Swift/XRT data) reveals the presence of a $\sim$39 d super-orbital period. We argue that the super-orbital periodicity is due to disc precession, and that material is accreted onto the neutron star at a constant rate throughout it. Moreover, by attributing this modulation to the free precession of the neutron star we estimate a surface magnetic field strength of $3-4\times10^{13}$ G. The agreement of these two independent estimates provide strong constraints on the surface polar magnetic field strength of the neutron star.

astro-ph.HE↗

arXiv2019

Magnetic-field evolution in a plastically-failing neutron-star crust

Under normal conditions in a neutron-star crust, ions are locked in place in the crustal lattice and only electrons are mobile, and magnetic-field evolution is thus directly related to the electron velocity. The evolution, however, builds magnetic stresses that can become sufficiently large for the crust to exceed its elastic limit, and to flow plastically. We consider the nature of this plastic flow and the back-reaction on the crustal magnetic field evolution. We formulate a plane-parallel model for the local failure, showing that surface motions are inevitable once the crust yields, in the absence of extra dissipative mechanisms. We perform numerical evolutions of the crustal magnetic field under the joint effect of Hall drift and Ohmic decay, tracking the build-up of magnetic stresses, and diagnosing crustal failure with the von Mises criterion. Beyond this point we solve for the coupled evolution of the plastic velocity and magnetic field. Our results suggest that to have a coexistence of a magnetar corona with small-scale magnetic features, the viscosity of the plastic flow must be roughly $10^{36}-10^{37}\ \textrm{g cm}^{-1}\textrm{s}^{-1}$. We find significant motion at the surface at a rate of $10-100$ centimetres per year, and that the localised magnetic field is weaker than in evolutions without plastic flow. We discuss astrophysical implications, and how our local simulations could be used to build a global model of field evolution in the neutron-star crust.

astro-ph.HE↗

arXiv2018

Neutron-star spindown and magnetic inclination-angle evolution

A rotating fluid star, endowed with a magnetic field, can undergo a form of precessional motion: a sum of rigid-body free precession and a non-rigid response. On secular timescales this motion is dissipated by bulk and shear viscous processes in the stellar interior and magnetospheric braking in the exterior, changing the inclination angle between the rotation and magnetic axes. Using our recent solutions for the non-rigid precessional dynamics, and viscous dissipation integrals derived in this paper, we make the only self-consistent calculation to date of these dissipation rates. We present the first results for the full coupled evolution of spindown and inclination angle for a model of a late-stage proto-neutron star with a strong toroidal magnetic field, allowing for both electromagnetic torques and internal dissipation when evolving the inclination angle. We explore this coupled evolution for a range of initial inclination angles, rotation rates and magnetic field strengths. For fixed initial inclination angle, our results indicate that the neutron-star population naturally evolves into two classes: near-aligned and near-orthogonal rotators -- with typical pulsars falling into the latter category. Millisecond magnetars can evolve into the near-aligned rotators which mature magnetars appear to be, but only for small initial inclination angle and internal toroidal fields stronger than roughly $5\times 10^{15}$ G. Once any model has evolved to either an aligned or orthogonal state, there appears to be no further evolution away from this state at later times.

astro-ph.HE↗

arXiv2018

Heat conduction in rotating relativistic stars

In the standard form of the relativistic heat equation used in astrophysics, information propagates instantaneously, rather than being limited by the speed of light as demanded by relativity. We show how this equation nonetheless follows from a more general, causal theory of heat propagation in which the entropy plays the role of a fluid. In deriving this result, however, we see that it is necessary to make some assumptions which are not universally valid: the dynamical timescales of the process must be long compared with the explicitly causal physics of the theory, the heat flow must be sufficiently steady, and the spacetime static. Generalising the heat equation (e.g. restoring causality) would thus entail retaining some of the terms we neglected. As a first extension, we derive the heat equation for the spacetime associated with a slowly-rotating star or black hole, showing that it only differs from the static result by an additional advection term due to the rotation, and as a consequence demonstrate that a hotspot on a neutron star will be seen to be modulated at the rotation frequency by a distant observer.

astro-ph.HE↗

arXiv2017

Non-rigid precession of magnetic stars

Stars are, generically, rotating and magnetised objects with a misalignment between their magnetic and rotation axes. Since a magnetic field induces a permanent distortion to its host, it provides effective rigidity even to a fluid star, leading to bulk stellar motion which resembles free precession. This bulk motion is however accompanied by induced interior velocity and magnetic field perturbations, which are oscillatory on the precession timescale. Extending previous work, we show that these quantities are described by a set of second-order perturbation equations featuring cross-terms scaling with the product of the magnetic and centrifugal distortions to the star. For the case of a background toroidal field, we reduce these to a set of differential equations in radial functions, and find a method for their solution. The resulting magnetic-field and velocity perturbations show complex multipolar structure and are strongest towards the centre of the star.

astro-ph.SR↗

arXiv2016

Magnetar field evolution and crustal plasticity

The activity of magnetars is believed to be powered by colossal magnetic energy reservoirs. We sketch an evolutionary picture in which internal field evolution in magnetars generates a twisted corona, form which energy may be released suddenly in a single giant flare, or more gradually through smaller outbursts and persistent emission. Given the ages of magnetars and the energy of their giant flares, we suggest that their evolution is driven by a novel mechanism: magnetic flux transport/decay due to persistent plastic flow in the crust, which would invalidate the common assumption that the crustal lattice is static and evolves only under Hall drift and Ohmic decay. We estimate the field strength required to induce plastic flow as a function of crustal depth, and the viscosity of the plastic phase. The star's superconducting core may also play a role in magnetar field evolution, depending on the star's spindown history and how rotational vortices and magnetic fluxtubes interact.

astro-ph.HE↗

arXiv2015

Magnetized neutron stars with superconducting cores: Effect of entrainment

We construct equilibrium configurations of magnetized, two-fluid neutron stars using an iterative numerical method. Working in Newtonian framework we assume that the neutron star has two regions: the core, which is modelled as a two-component fluid consisting of type-II superconducting protons and superfluid neutrons, and the crust, a region composed of normal matter. Taking a new step towards more complete equilibrium models, we include the effect of entrainment, which implies that a magnetic force acts on neutrons, too. We consider purely poloidal field cases and present improvements to an earlier numerical scheme for solving equilibrium equations, by introducing new convergence criteria. We find that entrainment results in qualitative differences in the structure of field lines along the magnetic axis.

astro-ph.HE↗

arXiv2015

Magnetically-driven crustquakes in neutron stars

Crustquake events may be connected with both rapid spin-up `glitches' within the regular slowdown of neutron stars, and high-energy magnetar flares. We argue that magnetic field decay builds up stresses in a neutron star's crust, as the elastic shear force resists the Lorentz force's desire to rearrange the global magnetic-field equilibrium. We derive a criterion for crust-breaking induced by a changing magnetic-field configuration, and use this to investigate strain patterns in a neutron star's crust for a variety of different magnetic-field models. Universally, we find that the crust is most liable to break if the magnetic field has a strong toroidal component, in which case the epicentre of the crustquake is around the equator. We calculate the energy released in a crustquake as a function of the fracture depth, finding that it is independent of field strength. Crust-breaking is, however, associated with a characteristic local field strength of $2.4\times 10^{14}$ G for a breaking strain of $0.001$, or $2.4\times 10^{15}$ G at a breaking strain of $0.1$. We find that even the most luminous magnetar giant flare could have been powered by crustal energy release alone.

astro-ph.HE↗

arXiv2014

The contrasting magnetic fields of superconducting pulsars and magnetars

We study equilibrium magnetic field configurations in a neutron star (NS) whose core has type-II superconducting protons. Unlike the equations for normal matter, which feature no special field strength, those for superconductors contain the lower critical field, of order $10^{15}$ G. We find that the ratio of this critical field to the smooth-averaged stellar field at the crust-core boundary is the key feature dictating the field geometry. Our results suggest that pulsar and magnetar-strength fields have notably different configurations. Field decay for NSs with $B_\textrm{pole}\sim 10^{14}$ G could thus result in substantial internal rearrangements, pushing the toroidal field component out of the core; this may be related to observed magnetar activity. In addition, we calculate the magnetically-induced ellipticities of our models.

astro-ph.HE↗