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.

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 ∼ 10 46 − 10 47 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.PACS numbers: 97.60. Jd, 98.70.Rz, 95.85.Sz

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.

We demonstrate that resonant shattering flares can occur during close passages of neutron stars in eccentric or hyperbolic encounters. We provide updated estimates for the rate of close encounters of compact objects in dense stellar environments, which we find are substantially lower than given in previous works. While such occurrences are rare, we show that shattering flares can provide a strong electromagnetic counterpart to the gravitational wave bursts expected from such encounters, allowing triggered searches for these events to occur.

We study the global stability of non‐axisymmetric p modes (also called inertial‐acoustic modes) trapped in the innermost regions of accretion discs around black holes. We show that the lowest‐order (highest‐frequency) p modes, with frequencies ω= (0.5–0.7) mΩISCO[where m= 1, 2, 3, … is the azimuthal wavenumber, ΩISCO is the disc rotation frequency at the Innermost Stable Circular Orbit (ISCO)], can be overstable due to general relativistic effects, according to which the radial epicyclic frequency κ is a non‐monotonic function of radius near the black hole. The mode is trapped inside the corotation resonance radius rc (where the wave pattern rotation speed ω/m equals the disc rotation rate Ω) and carries a negative energy. The mode growth arises primarily from wave absorption at the corotation resonance, and the sign of the wave absorption depends on the gradient of the disc vortensity, ζ=κ2/(2ΩΣ) (where Σ is the surface density). When the mode frequency ω is sufficiently high, such that dζ/r > 0 at rc, positive wave energy is absorbed at the corotation, leading to the growth of mode amplitude. The mode growth is further enhanced by wave transmission beyond the corotation barrier. We also study how the rapid radial inflow at the inner edge of the disc affects the mode trapping and growth. Our analysis of the behaviour of the fluid perturbations in the transonic flow near the ISCO indicates that, while the inflow tends to damp the mode, the damping effect is sufficiently small under some conditions (e.g. when the disc density decreases rapidly with decreasing radius at the sonic point) so that net mode growth can still be achieved. We further clarify the role of the Rossby wave instability and show that it does not operate for black hole accretion discs with smooth‐varying vortensity profiles. Overstable non‐axisymmetric p modes driven by the corotational instability provide a plausible explanation for the high‐frequency (≳100 Hz) quasi‐periodic oscillations observed from a number of black‐hole X‐ray binaries in the very high state. The absence of high‐frequency quasi‐periodic oscillations in the soft (thermal) state may result from mode damping due to the radial infall at the ISCO.