The resonant excitation of neutron star (NS) modes by tides is investigated as a source of short gamma-ray burst (SGRB) precursors. We find that the driving of a crust-core interface mode can lead to shattering of the NS crust, liberating $\sim {10}^{46}–{10}^{47}\mathrm{erg}$ of energy seconds before the merger of a NS-NS or NS-black-hole binary. Such properties are consistent with Swift/BAT detections of SGRB precursors, and we use the timing of the observed precursors to place weak constraints on the crust equation of state. We describe how a larger sample of precursor detections could be used alongside coincident gravitational wave detections of the inspiral by Advanced LIGO class detectors to probe the NS structure. These two types of observations nicely complement one another, since the former constrains the equation of state and structure near the crust-core boundary, while the latter is more sensitive to the core equation of state.

Magnetars are neutron stars with X-ray and soft γ-ray outbursts thought to be powered by intense internal magnetic fields. Like conventional neutron stars in the form of radio pulsars, magnetars exhibit 'glitches' during which angular momentum is believed to be transferred between the solid outer crust and the superfluid component of the inner crust. The several hundred observed glitches in radio pulsars and magnetars have involved a sudden spin-up (increase in the angular velocity) of the star, presumably because the interior superfluid was rotating faster than the crust. Here we report X-ray timing observations of the magnetar 1E 2259+586 (ref. 8), which exhibited a clear 'anti-glitch'--a sudden spin-down. We show that this event, like some previous magnetar spin-up glitches, was accompanied by multiple X-ray radiative changes and a significant spin-down rate change. Such behaviour is not predicted by models of neutron star spin-down and, if of internal origin, is suggestive of differential rotation in the magnetar, supporting the need for a rethinking of glitch theory for all neutron stars.

Using numerical calculations, we compare three versions of the Barrett- Crane model of 4-dimensional Riemannian quantum gravity. In the version with face and edge amplitudes as described by De Pietri, Freidel, Krasnov, and Rovelli, we show the partition function diverges very rapidly for many triangulated 4-manifolds. In the version with modified face and edge amplitudes due to Perez and Rovelli, we show the partition function converges so rapidly that the sum is dominated by spin foams where all the spins labelling faces are zero except for small, widely separated islands of higher spin. We also describe a new version which appears to have a convergent partition function without drastic spin-zero dominance. Finally, after a general discussion of how to extract physics from spin foam models, we discuss the implications of convergence or divergence of the partition function for other aspects of a spin foam model.

GRB 230307A is an extremely bright long-duration GRB with an observed gamma-ray fluence of ≳3 × 10⁻³ erg cm⁻² (10–1000 keV), second only to GRB 221009A. Despite its long duration, it is possibly associated with a kilonova, thus resembling the case of GRB 211211A. In analogy with GRB 211211A, we distinguish three phases in the prompt gamma-ray emission of GRB 230307A: an initial short duration, spectrally soft emission; a main long duration, spectrally hard burst; and a temporally extended and spectrally soft tail. We interpret the initial soft pulse as a bright precursor to the main burst and compare its properties with models of precursors from compact binary mergers. We find that to explain the brightness of GRB 230307A, a magnetar-like (≳10¹⁵ G) magnetic field should be retained by the progenitor neutron star. Alternatively, in the postmerger scenario, the luminous precursor could point to the formation of a rapidly rotating massive neutron star.

We study the global stability of non-axisymmetric p modes (also called inertial-acoustic modes) trapped in the innermost regions of accretion discs a