The unique high-energy spectrum of the QSO PKS 0558-504

The 15th mag QSO 0558-504, was observed at X-ray, UV, optical, and radio frequencies during the period of 1984 to 1987. This QSO exhibits very strong optical Fe II emission, and the observations are part of a program to investigate a possible correlation between Fe II emission and the spectral shape of AGN at high energies. The EXOSAT observations (0.2 to 7.0 keV) of PKS 0558-504 revealed an unusually steep X-ray spectrum, which is fit by a single power law with an energy index of 1.31. However, observations with IUE show a rather flat UV spectrum, with a UV energy index of 0.5. The set of optical observations of the QSO shows no evidence of significant variability. The composite, multifrequency spectrum is interpreted as having a large X-ray UV excess, which is similar to PG 1211+143 but extends to higher energy (at least 5 keV). These observations of PKS 0558-504 provide requirements for physical models seeking to explain the X-ray UV bump in active galactic nuclei.