X‐Ray Spectrum of the High‐Polarization Quasar PKS 1510−089

We present results from the X-ray spectra of the radio-loud, high-polarization quasar PKS 1510-089, based on new data obtained using ASCA and from archival ROSAT data. We find the X-ray spectrum obtained by ASCA to be unusually hard, with the photon index Γ = 1.30 ± 0.06, while the (nonsimultaneous) ROSAT data indicate a steeper spectrum with Γ = 1.9 ± 0.3. However, we find the X-ray flux at 1 keV to be within ~10% during both observations. Thus, we suggest the most likely explanation is that there is a break (with ΔΓ ≥ 0.6) in the underlying continuum at ~0.7 keV. Although the sample of high-polarization quasars for which high-quality X-ray spectra are available is small, flat X-ray spectra seem to be characteristic of these objects, and they also appear to be harder than that of the other radio-loud but low-polarization quasars. The multiwavelength spectrum of PKS 1510-089 is similar to many other γ-ray blazars, suggesting the emission is dominated by that from a relativistic jet. A big blue bump is also seen in its multiwavelength spectrum, suggesting the presence of a strong thermal component as well.