How do neutron stars maintain inhomogeneous surfaces and migrating "hot spots"? (e.g. SGR 1830-0645)

Question

News item NASA's NICER Tracks a Magnetar's Hot Spots and Phys.org's Properties of magnetar SGR 1830−0645 inspected with NICER reference the January 14 2022 arXiv Pulse Peak Migration during the Outburst Decay of the Magnetar SGR 1830-0645: Crustal Motion and Magnetospheric Untwisting an object that seems to first be reported in the October 13, 2020 Astronomer's Telegram NuSTAR observation of the newly discovered magnetar SGR 1830-0645.

The abstract for the arXiv preprint is shown below, but I'm wondering if a simple explanation is possible.

We hear about how amazingly smooth a neutron star's surface must be due to the incredibly high gravity; it should be within millimeters or less of a hydrostatic equilibrium shape.

Until now I've also assumed that the composition and nature of the surface would also be uniform, or perhaps have a slight pole-to-equator variation. And of course there are two pairs of poles; rotational and magnetic.

But now I see there are believed to be wandering "hot spots" observed on this magnetar's surface moving on the scale of days or months, which is probably a geological timescale for something made of nuclear matter (cf. Dragon's Egg and Did they ever deal with non-relativistic kinematics on Dragon's Egg?).

So I'd like to ask:

Question: How do neutron stars maintain inhomogeneous surfaces and migrating "hot spots"? (e.g. SGR 1830-0645) What keeps their surface from equilibrating to a uniform or at least a static nature?

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Abstract

Magnetars, isolated neutron stars with magnetic field strengths typically ≳10¹⁴ 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 ≲100 m day⁻¹, constraining the density of the driving region to ρ∼10¹⁰ g cm⁻³, at a depth of ∼200 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.

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From the NASA video below:

NICER tracked these spots as they drifted across the star and changed size. Two of them even merged - a behavior not seen before.

Answer 1

It is almost certainly the strong magnetic fields. These contribute a magnetic pressure that allows charged/ionised material to flow readily along field lines, but not to cross them.

In the paper you refer to (Younes et al. 2022), the object studied is a magnetar and has a very strong surface magnetic field of $\sim 10^{10}$ T. The field will not be homogeneous - even if it were dipolar, there would be a concentration of flux at the magnetic poles (which do not necessarily coincide with the rotation axis).

These fields are strong enough, that the magnetic pressure ($\propto B^2$) will dominate the thermal gas pressure. This means that local heating, associated with the dissipation of the stressed magnetic field, either in the crust or in the magnetosphere, could be associated with temperature inhomogeneities at the surface. The strong magnetic fields are then able to prevent (or at least slow) the diffusion of hot material and maintain the identity of the inhomogeneity.

A similar phenomenon is seen in the Sun's atmosphere whereby strong magnetic fields can maintain temperature differences, and may also be responsible in part for the local heating, in the photosphere - manifested as sunspots and plages.

In the magnetar case, it is hypothesised that stresses build up in the crust and there is some sort of seismic event that shifts the crust and releases the magnetic stresses either within the crust itself or by the magnetosphere causing charged particle acceleration into the crust (which would be quite similar to the case of a solar flare).