THE ROSAT X-RAY-SPECTRUM OF THE HELIX NEBULA

We have for the first time discovered the existence of two temperature components in the X-ray emission from planetary nebulae. The observed ROSAT PSPC spectrum of the Helix Nebula exhibits a strong peak at abo ut 0.12 keV and a secondary peak at about 0.8 keV. The best model fit results in a temperature of T1 = 1.4 x 10(5) K for the low-energy comp onent, and a temperature T2 = 8.7 x 10(6) K for the high-energy compon ent, at hydrogen column density N(H) = 1.41 x 10(20) cm-2. The low-ene rgy component can be attributed to the photosphere of the central star . The high-energy component may be caused by a stellar corona. An alte rnative explanation is that this hot component is due to plasma in a h ot bubble predicted by the interacting wind model. A lower limit of th e electron density in the hot plasma is about 10 cm-3.