Magnetar heating

TL;DR

This paper evaluates four potential mechanisms for magnetar surface heating, finding that core heating and crustal dissipation are insufficient for sustained luminosity, while magnetospheric bombardment and external heating are more plausible explanations.

Contribution

The study systematically assesses and compares four candidate heating mechanisms for magnetars, highlighting their viability and limitations based on observational and theoretical constraints.

Findings

Core heating can sustain observed luminosity if core magnetic fields are >10^{16} G.

Mechanical crustal heating is limited and unlikely to sustain high luminosity.

Magnetospheric bombardment can produce hot spots consistent with observations.

Abstract

We examine four candidate mechanisms that could explain the high surface temperatures of magnetars. (1) Heat flux from the liquid core heated by ambipolar diffusion. It could sustain the observed surface luminosity $L_s\approx 10^{35}$ erg/s if core heating offsets neutrino cooling at a temperature $T_{core}>6\times 10^8$ K. This scenario is viable if the core magnetic field exceeds $10^{16}$ G and the heat-blanketing envelope of the magnetar has a light element composition. We find however that the lifetime of such a hot core should be shorter than the typical observed lifetime of magnetars. (2) Mechanical dissipation in the solid crust. This heating can be quasi-steady, powered by gradual (or frequent) crustal yielding to magnetic stresses. We show that it obeys a strong upper limit. As long as the crustal stresses are fostered by the field evolution in the core or Hall drift in the crust, mechanical heating is insufficient to sustain persistent $L_s\approx 10^{35}$ erg/s. The surface luminosity is increased in an alternative scenario of mechanical deformations triggered by external magnetospheric flares. (3) Ohmic dissipation in the crust, in volume or current sheets. This mechanism is inefficient because of the high conductivity of the crust. Only extreme magnetic configurations with crustal fields $B>10^{16}$ G varying on a 100 meter scale could provide $L_s\approx 10^{35}$ erg/s. (4) Bombardment of the stellar surface by particles accelerated in the magnetosphere. This mechanism produces hot spots on magnetars. Observations of transient magnetars show evidence for external heating.