ASCA OBSERVATIONS OF HIGH-REDSHIFT QUASARS

ASCA observations of 4 high redshift radio-loud quasars with 1.44<z<3. 21 are presented. The spectral analysis for three of them (PKS 0332-40 3, PKS 0537-286, PKS 2149-306) reveals that their X-ray continuum emis sion is well represented by a simple power-law model plus absorption w ith photon indices of Gamma = 1.92(-0.20)(+0.30) (PKS 0332-403), Gamma = 1.63(-0.12)(+0.14), (PKS 0537-286) and Gamma = 1.57 +/- 0.05 (PKS 2 149-306). The fourth and most distant object, PKS 1614+051 at z=3.21, was detected, but a detailed spectral analysis is impossible due to th e small number of photons. We find evidence for excess absorption abov e the Galactic N-H-value in the ASCA data of PKS 2149-306, which is no t confirmed by the ROSAT All-Sky Survey PSPC spectrum of this source. This could probably be due to variable absorption. The ROSAT spectrum of PKS 0537-286, deduced from a 10 ksec pointed PSPC observation, is c onsistent with the ASCA results. Thermal bremsstrahlung models also gi ve acceptable fits to the ASCA data with best fit (rest frame) tempera tures of 10.4, 33.5 and 45.8 keV for PKS 0332-403, PKS 0537-286 and PK S 2149-306, respectively. More complicated models for the X-ray contin uum are not required, in particular, tight upper limits on the strengt h of the Fe-K emission line are given. The broad band spectral energy distributions from the radio to the gamma-rays are presented and discu ssed.