Neutron starquakes and the dynamic crust

The most strongly magnetized neutron stars, the magnetars, have spectacular outbursts of gamma-ray flares powered by decay of the magnetic field. The rapidly changing field is strong enough that it should be able to stress and rupture the star’s crust, with potentially interesting seismic consequences. This was confirmed in dramatic fashion a few years ago with the discovery of long-lived seismic vibrations excited by rare giant flares. The starquakes associated with the giant flares are, it seems, so catastrophic that they leave the whole star ringing. These discoveries have opened up the possibility of using asteroseismology to study the extreme conditions of the neutron star interior - the dense matter equation of state in crust and core, and the core magnetic field. This chapter reviews the main observational properties of the oscillations seen in the giant flares, and the state of the art in terms of theoretical models of global seismic vibrations in magnetars. I discuss efforts to extend oscillation searches to smaller magnetar bursts (which are more frequent but less energetic), and the role that crust rupturing might play in triggering magnetar flares and exciting global seismic oscillations.

💡 Research Summary

The chapter provides a comprehensive review of seismic activity in magnetars, focusing on the quasi‑periodic oscillations (QPOs) observed during giant gamma‑ray flares and the theoretical frameworks developed to interpret them. Magnetars possess magnetic fields up to ~10^15 G, and the rapid evolution of these fields can stress the solid crust until it ruptures, producing a starquake that excites global seismic oscillations (GSOs).

Observationally, the two best‑studied giant flares—SGR 1806‑20 (2004) and SGR 1900+14 (1998)—revealed a rich spectrum of QPOs ranging from ~18 Hz to ~1800 Hz. Data from RXTE and RHESSI show that each QPO is strongly dependent on the rotational phase of the star, with amplitudes that can reach several percent rms and durations that vary from a fraction of a second to many tens of seconds. Lower‑frequency QPOs (18–30 Hz) appear early in the flare tail, while higher‑frequency signals (e.g., 625 Hz, 1840 Hz) persist for longer intervals. The Q‑values (frequency divided by width) are highly variable, indicating that simple exponential decay models cannot fully explain the observed line broadening; frequency drift, mode splitting, or coupling to a continuum are likely contributors.

Early theoretical work proposed that the most easily excited modes would be toroidal shear (torsional) oscillations confined to the crust, with a fundamental frequency near 30 Hz. This model successfully matched many of the observed frequencies, especially when higher harmonics were invoked. The detection of a 625 Hz QPO, consistent with the first radial overtone of a crustal shear mode, suggested that simultaneous identification of fundamental and overtone could tightly constrain the dense‑matter equation of state. However, the lowest‑frequency QPOs could not be accommodated within a pure crustal shear framework, prompting the suggestion that they might be torsional Alfvén modes of the fluid core, thereby offering a probe of the internal toroidal magnetic field component.

Subsequent studies recognized that the strong magnetic field couples the crust to the fluid core, invalidating the “free‑slip” boundary condition. Magneto‑elastic models that treat the star as a single magnetically coupled system have become the standard. In these models, a uniform or dipolar field of order 10^15 G significantly shifts the frequencies of crustal overtones, especially the radial overtones, and can split degenerate modes. Moreover, the core supports a continuum of Alfvén waves; endpoints of this continuum can appear as drifting QPOs that temporarily resonate with crustal frequencies, explaining observed frequency drifts and broadened peaks. Studies in both Newtonian gravity and full general relativity confirm that magnetic field geometry (dipole, mixed poloidal‑toroidal) strongly influences mode spectra.

The chapter also discusses recent efforts to detect QPOs in more frequent, lower‑energy bursts. By applying Bayesian significance testing and searching over shorter time windows, additional low‑frequency signals (17–117 Hz) have been reported, though the limited signal‑to‑noise ratio prevents definitive confirmation. These findings hint that the same seismic excitation mechanism may operate across a wide range of burst energies.

Finally, the role of crustal rupture in triggering flares is examined. Magnetic stresses build up in the lattice of charged nuclei; once a critical strain is exceeded, the crust cracks, allowing magnetic field lines to rearrange and release energy as a giant flare. The sudden displacement injects energy into the coupled magneto‑elastic system, launching both crustal shear waves and core Alfvén motions, which manifest as the observed QPOs. This scenario links flare energetics, starquake dynamics, and seismic oscillations into a coherent picture.

In summary, the observed QPOs likely arise from a complex interplay of crustal shear modes, radial overtones, core Alfvén continua, and torsional Alfvén modes, all modulated by the ultra‑strong magnetic field. Precise measurement of these frequencies offers a powerful asteroseismic tool to probe the neutron‑star interior—its composition, superfluid content, and internal magnetic field geometry. Future high‑time‑resolution X‑ray missions, combined with gravitational‑wave observations, promise to refine these models and unlock the physics of matter at supra‑nuclear densities.