# Optical excess of dim isolated neutron stars

## TL;DR

This paper proposes a fallback disc model to explain the optical excess in dim isolated neutron stars, attributing it mainly to magnetic stresses heating the inner disc, with consistent ratios of optical luminosity to energy loss rate across sources.

## Contribution

It introduces a model linking optical excess to magnetic heating in fallback discs, providing a unified explanation for XDINs' optical spectra and their relation to rotational energy loss.

## Key findings

- Optical luminosity ratios are similar across XDINs, supporting a common magnetic heating mechanism.
- Magnetic stresses dominate optical emission over X-ray irradiation in these systems.
- The model explains the optical excess without requiring additional energy sources.

## Abstract

The optical excess in the spectra of dim isolated neutron stars (XDINs) is a significant fraction of their rotational energy loss-rate. This is strikingly different from the situation in isolated radio pulsars. We investigate this problem in the framework of the fallback disc model. The optical spectra can be powered by magnetic stresses on the innermost disc matter, as the energy dissipated is emitted as blackbody radiation mainly from the inner rim of the disc. In the fallback disc model, XDINs are the sources evolving in the propeller phase with similar torque mechanisms. In this this model, the ratio of the total magnetic work that heats up the inner disc matter is expected to be similar for different XDINs. Optical luminosities that are calculated consistently with the the optical spectra and the theoretical constraints on the inner disc radii give very similar ratios of the optical luminosity to the rotational energy loss rate for all these sources. These ratios indicate that a significant fraction of the magnetic torque heats up the disc matter while the remaining fraction expels disc matter from the system. For XDINs, the contribution of heating by X-ray irradiation to the optical luminosity is negligible in comparison with the magnetic heating. The correlation we expect between the optical luminosities and the rotational energy loss-rates of XDINs can be a property of the systems with low X-ray luminosities, in particular those in the propeller phase.