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

The central star of the Helix Nebula, WD 2226<inline-formula><tex-math id="TM0002" notation="LaTeX">$-$</tex-math></inline-formula>210 presents enigmatic hard X-ray emission and mid-IR excess. The latter has been attributed to a dusty disc or a cloud-like structure around WD 2226<inline-formula><tex-math id="TM0003" notation="LaTeX">$-$</tex-math></inline-formula>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<inline-formula><tex-math id="TM0004" notation="LaTeX">$-$</tex-math></inline-formula>210, comparing these to previous Einstein and ROSAT data. The luminosity of the hard X-ray component of WD 2226<inline-formula><tex-math id="TM0005" notation="LaTeX">$-$</tex-math></inline-formula>210 has remained basically constant in the decade from 1992 to 2002, with very subtle evidence for variability in time-scales of hours. Under the assumption that the X-ray emission from WD 2226<inline-formula><tex-math id="TM0006" notation="LaTeX">$-$</tex-math></inline-formula>210 is due to accretion of material, an accretion rate of <inline-formula><tex-math id="TM0007" notation="LaTeX">$\dot{M}\approx 10^{-10}$</tex-math></inline-formula> M<inline-formula><tex-math id="TM0008" notation="LaTeX">$_\odot$</tex-math></inline-formula> yr<inline-formula><tex-math id="TM0009" notation="LaTeX">$^{-1}$</tex-math></inline-formula> is estimated. The origin of the material accreted by WD 2226<inline-formula><tex-math id="TM0010" notation="LaTeX">$-$</tex-math></inline-formula>210 is uncertain, and can be attributed to the disc-like structure around it or to a substellar donor companion. The accretion rate proposed for the continuous replenishment by bombardment of the mid-IR-emitting structure around WD 2226<inline-formula><tex-math id="TM0011" notation="LaTeX">$-$</tex-math></inline-formula>210 cannot match that required by the X-ray emission.