Neutron star deformation due to poloidal-toroidal magnetic fields of arbitrary multipole order: a new analytic approach

A recipe is presented to construct an analytic, self-consistent model of a non-barotropic neutron star with a poloidal-toroidal field of arbitrary multipole order, whose toroidal component is confined in a torus around the neutral curve inside the star, as in numerical simulations of twisted tori. The recipe takes advantage of magnetic-field-aligned coordinates to ensure continuity of the mass density at the surface of the torus. The density perturbation and ellipticity of such a star are calculated in general and for the special case of a mixed dipole-quadrupole field as a worked example. The calculation generalises previous work restricted to dipolar, poloidal-toroidal and multipolar, poloidal-only configurations. The results are applied, as an example, to magnetars whose observations (e.g., spectral features and pulse modulation) indicate that the internal magnetic fields may be at least one order of magnitude stronger than the external fields, as inferred from their spin downs, and are not purely dipolar.Comment: 14 pages, 6 figures, 1 table. Accepted for publication in the Monthly Notices of the Royal Astronomical Societ

Gravitational perturbations of a Kerr black hole in f(R)f(R) gravity

Modified theories of gravity are often built such that they contain general relativity as a limiting case. This inclusion property implies that the Kerr metric is common to many families of theories. For example, all analytic $f(R)$ theories with vanishing constant term admit the Kerr solution. In any given theory, however, the response of the gravitational field to astrophysical disturbances is tied to the structure of the field equations. As such, even if black holes are Kerr, the underlying theory can, in principle, be probed through gravitational distortions. In this paper, we study linear perturbations of a Kerr black hole in $f(R)$ gravity using the Newman-Penrose formalism. We show that, as in general relativity, the equations governing the perturbed metric, which depend on the quadratic term of the function $f$, completely decouple.Comment: 8 pages. Accepted for publication in Phys. Rev.

Evolutionary implications of a magnetar interpretation for GLEAM-X J162759.5-523504.3

The radio pulsar GLEAM-X J162759.5-523504.3 has an extremely long spin period (P = 1091.17\, \mbox{s}), and yet seemingly continues to spin down rapidly (\dot{P} < 1.2 \times 10^{-9}\, \mbox{ss}^{-1}). The magnetic field strength that is implied, if the source is a neutron star undergoing magnetic dipole braking, could exceed 10^{16}\,\mbox{G}. This object may therefore be the most magnetised neutron star observed to date. In this paper, a critical analysis of a magnetar interpretation for the source is provided. (i) A minimum polar magnetic field strength of B \sim 5 \times 10^{15}\,\mbox{G} appears to be necessary for the star to activate as a radio pulsar, based on conventional `death valley' assumptions. (ii) Back-extrapolation from magnetic braking and Hall-plastic-Ohm decay suggests that a large angular momentum reservoir was available at birth to support intense field amplification. (iii) The observational absence of X-rays constrains the star's field strength and age, as the competition between heating from field decay and Urca cooling implies a surface luminosity as a function of time. If the object is an isolated, young (\sim 10\, \mbox{kyr}) magnetar with a present-day field strength of B \gtrsim 10^{16}\,\mbox{G}, the upper limit (\approx 10^{30}\, \mbox{erg s}^{-1}) set on its thermal luminosity suggests it is cooling via a direct Urca mechanism.Comment: 12 pages, 6 figures. Accepted for publication in MNRA