A sharp X-ray absorption feature in the BL Lacertae object PKS 2155-304

Abstract

An absorption feature appears at 600 eV and extends to higher energies by 50 - 100 eV in a spectrum that covers the range from 0.38 to 1.6 keV obtained with the objective grating spectrometer on the Einstein Observatory. The spectrum can be fitted by a single absorption edge or trough plus two power laws with slopes that are consistent with simultaneous measurements of the 2 - 10 keV flux. The absence of a low-energy cutoff is consistent with a neutral column density of N<SUB>H I</SUB> ≡ 4×10<SUP>20</SUP> cm<SUP>-2</SUP>. Absorption by ionized oxygen is by far the most likely explanation of the sharp feature, and possible detailed interpretations are discussed.