THE ROSAT PSPC SPECTRUM OF BRIGHT LOW-Z QUASARS

We present and discuss ROSAT Position Sensitive Proportional Counter ( PSPC) spectra of seven bright low-redshift quasars obtained during poi nted observations, including in the discussion the results previously obtained on the quasar PG 1116+215. Several pointed observations are a vailable for two quasars. In two of our objects the count rate varied by a factor similar to 2 during the pointed observations and we have e xtracted spectra at different flux levels. In all, we analyzed 10 PSPC spectra, plus the survey data. The analysis leads to the following re sults: (1) In most cases, a single power law + absorption gives an ade quate fit and the derived value of N-H is consistent with the Galactic value. In these cases, the two-component models yield fits with simil ar reduced chi(2) but do not bring statistically significant improveme nt to the fit. (2) Flux variations up to a factor of 2 occurred but we re generally not accompanied by spectral shape variations. (3) The fas test variation observed was a decrease by a factor of 2 in 10 hr in PG 1404+226, which has a luminosity at high state of 10(46) ergs s(-1). This quasar also shows evidence for a strong absorption edge. (4) We s how, through simulations, that if the total counts are less than appro ximately 4000-6000 genuine two-component spectra can be adequately fit ted with a single power law. However, in the case of PG 1116+215, the very high statistics (12,000-25,000 counts) allow us to establish the lack of any significant contribution from a flat high-energy component . (5) Finally, we show that the ratio of UV to soft X-ray flux (measur ed at 0.4 keV) is not correlated with the spectral shape in the PSPC r ange. The correlation found by other authors between the ratio of UV t o soft X-ray flux (measured at 2 keV) and the spectral shape is most p robably caused by the hidden correlation between the normalization at 2 keV and the spectral slope in the PSPC range.