3D Magnetothermal Simulations of Tangled Crustal Magnetic Field in Central Compact Objects

Abstract

Central compact objects (CCOs) are young neutron stars emitting thermal X-rays with bolometric luminosities L X in the range of 10 32 -10 34 erg s −1 . Gourgouliatos, Hollerbach, and Igoshev recently suggested that peculiar emission properties of CCOs can be explained by tangled magnetic field configurations formed in a stochastic dynamo during the proto-neutron star stage. In this case the magnetic field consists of multiple small-scale components with negligible contribution of global dipolar field. We study numerically three-dimensional magnetothermal evolution of tangled crustal magnetic fields in neutron stars. We find that all configurations produce complicated surface thermal patterns that consist of multiple small hot regions located at significant separations from each other. The configurations with initial magnetic energy of (2.5-10) × 10 47 erg have temperatures of hot regions that reach ≈ 0.2 keV, to be compared with the bulk temperature of ≈ 0.1 keV in our simulations with no cooling. A factor of two in temperature is also seen in observations of CCOs. The hot spots produce periodic modulations in light curve with typical amplitudes of 9%-11%. Therefore, the tangled magnetic field configuration can explain thermal emission properties of some CCOs.

Magnetic Field Evolution in Neutron Star Crusts: Beyond the Hall Effect

“…Fifth, apart from the obvious connection to strongly magnetised NSs, the Hall effect has been related to Central Compact Objects [106][107][108][109][110][111][112], in the scenario of the re-emergence of the buried magnetic field. In this context, the magnetic field has been buried following the formation of the NS, and while it provides heat so that the star shines in X-rays, its dipole strength is relatively weak [49,[113][114][115][116]. Finally, the role of the Hall effect has been central in the study of the entire NS population.…”

“…Even in the case of relatively simple magnetic field configuration such as the poloidal dipolar magnetic field, certain regions become thermally isolated from the NS core. In Figure 3 we show the temperature and magnetic field configuration inside the NS crust computed using the PARODY code [41,42,81,116,[152][153][154]. In Figure 4 we show the corresponding surface temperature distribution.…”

Evolution of Neutron Star Magnetic Fields

“…Since Hall evolution leads to the development of a turbulent cascade, in some cases, energy can also be transferred from small to large scales [59,60,[67][68][69]. If a dynamo at the proto-NS stage fails to generate a strong dipolar or quadrupolar magnetic field, but generates fields with dominant l = 10 − 50 (turbulent dynamo), Hall evolution leads to energy redistribution and at least some increase in the strength of the large-scale components.…”

“…These regular magnetic fields form hot and cold regions with sizes ranging from a few km to NS-radius size. T s (10 6 K) In the case where small-scale multipoles dominate the magnetic field, the hot regions becomes much smaller (comparable to the linear size of the multipoles), but a large number of these hot spots are seen simultaneously [69]; see Figure 8. When integrated over the visible hemisphere, taking into account the light-bending in general relativity, these thermal maps become the basis to construct lightcurves, which can be further tested against timing observations.…”

“…Figure 8. Surface temperature distribution at age 6 kyr for model 4[69] for a NS with dominant small-scale poloidal field.…”