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NICER / ISS Science Nugget
for November 23, 2023



Magnetic fields quirkier and quirkier

Neutron stars are born hot, in fiery explosions. Initially, they cool in a unique manner: by emitting copious quantities of neutrinos. Later (after 104 - 105 years, depending on whether light or heavy elements blanket the surface), energy carried away by photons dominates. This unusual thermal evolution is further complicated by the presence of powerful magnetic fields -- they play an important role in channeling heat flows within the star, and their geometry and strength can also be time-dependent. This combined magneto-thermal evolution ultimately determines the surface temperatures of neutron stars that are observable through their thermal X-ray glow by telescopes such as NICER, with non-uniform temperature distributions contributing to visible X-ray "pulses" as hotter and colder regions are carried through our line of sight by the star's rotation.

Motivated in part by NICER findings that neutron star magnetic fields are not necessarily simple "textbook" dipoles centered on the star, a paper recently published in the peer-reviewed UK journal Monthly Notices of the Royal Astronomical Society by A. Igoshev (Univ. of Leeds, UK) and collaborators describes sophisticated three-dimensional modeling of the magneto-thermal evolution of off-center dipolar (with higher-order contributions) field geometries. Including microphysical phenomena -- such as electric currents in the neutron-star crust, diffusion of magnetic field lines, the Hall effect, and others -- the authors explore a number of initial conditions and trace them through to 200,000 years of evolution to derive observable X-ray pulse "lightcurves" and energy spectra due to surface-temperature anisotropies. Among other conclusions, they determine that the initial magnetic field strength is a more significant driver of later evolution than geometric configuration, but the presence or absence of a toroidal field component in the crust is also important. These results may be modified by additional complexities, such as external heating due to particle flows in the star's magnetosphere, to be incorporated into future modeling, all of which will eventually be confronted and put to the test with NICER data.


External field lines (blue arrows) for a vertical magnetic dipole shifted away from the star's center in the polar direction by half the neutron star's radius (R_NS); the grey circle represents the star, to scale. (Image credit: Igoshev et al. 2023) Surface temperature maps for neutron stars with different initial conditions (columns) and over time (rows), in millions of degrees Kelvin (color scale). B0-0 and C0-0 represent 10^13 and 10^14 Gauss, respectively, magnetic fields centered on the star; A3-P represents a 10^13 G field offset in the polar direction by 1/3rd the radius of the star. From top to bottom, the rows indicate temperature distributions at 10, 50, and 200 thousand years.

Left: External field lines (blue arrows) for a vertical magnetic dipole shifted away from the star's center in the polar direction by half the neutron star's radius (R_NS); the grey circle represents the star, to scale. (Image credit: Igoshev et al. 2023)

Right: Surface temperature maps for neutron stars with different initial conditions (columns) and over time (rows), in millions of degrees Kelvin (color scale). B0-0 and C0-0 represent 10^13 and 10^14 Gauss, respectively, magnetic fields centered on the star; A3-P represents a 10^13 G field offset in the polar direction by 1/3rd the radius of the star. From top to bottom, the rows indicate temperature distributions at 10, 50, and 200 thousand years.

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