The Hidden Magnetic Field of The Young Neutron Star in Kesteven 79

TL;DR

This paper introduces a new method to model X-ray emissions from neutron stars with hidden strong sub-surface magnetic fields, explaining observed pulsations despite weak surface dipole fields.

Contribution

Develops the TTFT method to simulate neutron star surface emissions considering complex magnetic and temperature distributions, revealing hidden strong crustal magnetic fields.

Findings

A crustal toroidal magnetic field of ~10^{14} G can produce observed pulse fractions.

Hidden magnetic fields are likely common in young neutron stars.

The TTFT method can be applied to study other magnetic neutron stars.

Abstract

Recent observations of the central compact object in the Kesteven 79 supernova remnant show that this neutron star (NS) has a weak dipole magnetic field (a few x 10^{10} G) but an anomalously large (~ 64%) pulse fraction in its surface X-ray emission. We explore the idea that a substantial sub-surface magnetic field exists in the NS crust, which produces diffuse hot spots on the stellar surface due to anisotropic heat conduction, and gives rise to the observed X-ray pulsation. We develop a general-purpose method, termed "Temperature Template with Full Transport" (TTFT), that computes the synthetic pulse profile of surface X-ray emission from NSs with arbitrary magnetic field and surface temperature distributions, taking into account magnetic atmosphere opacities, beam pattern, vacuum polarization, and gravitational light bending. We show that a crustal toroidal magnetic field of order a few x 10^{14} G or higher, varying smoothly across the crust, can produce sufficiently distinct surface hot spots to generate the observed pulse fraction in the Kes 79 NS. This result suggests that substantial sub-surface magnetic fields, much stronger than the "visible" dipole fields, may be buried in the crusts of some young NSs, and such hidden magnetic fields can play an important role in their observational manifestations. The general TTFT tool we have developed can also be used for studying radiation from other magnetic NSs.