Chandra observations of the planetary nebula IC 4593
  The ACIS-S camera on board the Chandra X-ray Observatory has been used to
discover a hot bubble in the planetary nebula (PN) IC4593, the most distant PN
detected by Chandra so far. The data are used to study the distribution of the
X-ray-emitting gas in IC 4593 and to estimate its physical properties. The hot
bubble has a radius of ~2$^{\prime\prime}$ and is found to be confined inside
the optically-bright innermost cavity of IC 4593. The X-ray emission is mostly
consistent with that of an optically-thin plasma with temperature
$kT\approx0.15$ keV (or $T_\mathrm{X}\approx1.7\times10^{6}$ K), electron
density $n_\mathrm{e}\approx15$ cm$^{-3}$, and intrinsic X-ray luminosity in
the 0.3-1.5 keV energy range $L_\mathrm{X}=3.4\times10^{30}$ erg s$^{-1}$. A
careful analysis of the distribution of hard ($E>$0.8 keV) photons in IC 4593
suggests the presence of X-ray emission from a point source likely associated
with its central star (CSPN). If this were the case, its estimated X-ray
luminosity would be $L_\mathrm{X,CSPN}=7\times10^{29}$ erg s$^{-1}$, fulfilling
the log$(L_\mathrm{X,CSPN}/L_\mathrm{bol})\approx-7$ relation for self-shocking
winds in hot stars. The X-ray detection of the CSPN helps explain the presence
of high-ionisation species detected in the UV spectra as predicted by stellar
atmosphere models.
