Accretion onto WD 2226$-$210, the central star of the Helix Nebula

TL;DR

This paper analyzes X-ray observations of WD 2226-210, revealing stable hard X-ray emission over a decade, and estimates an accretion rate suggesting complex origins possibly involving a dusty disk or sub-stellar companion.

Contribution

It provides a detailed multi-epoch X-ray analysis of WD 2226-210, estimating accretion rates and discussing potential sources of accreted material, which is a novel investigation of its enigmatic emission.

Findings

Hard X-ray luminosity remained constant over a decade.

Estimated accretion rate of approximately 10^{-10} solar masses per year.

Accretion source remains uncertain, possibly a disk or sub-stellar companion.

Abstract

The central star of the Helix Nebula, WD 2226$-$210 presents enigmatic hard X-ray emission and mid-IR excess. The latter has been attributed to a dusty disk or a cloud-like structure around WD 2226$-$210 formed from material of Kuiper Belt-like or comet-like objects in highly eccentric orbits. We present here a detailed analysis of multi-epoch Chandra and XMM-Newton X-ray observations of WD 2226$-$210, comparing these to previous Einstein and ROSAT data. The luminosity of the hard X-ray component of WD 2226$-$210 has remained basically constant in the decade from 1992 to 2002, with very subtle evidence for variability in timescales of hours. Under the assumption that the X-ray emission from WD 2226$-$210 is due to accretion of material, an accretion rate of $\dot{M}\approx10^{-10}$ M$_\odot$ yr$^{-1}$ is estimated. The origin of the material accreted by WD 2226$-$210 is uncertain, and can be attributed to the disk-like structure around it or to a sub-stellar donor companion. The accretion rate proposed for the continuous replenishment by bombardment of the mid-IR-emitting structure around WD 2226$-$210 cannot match that required by the X-ray emission.