Quasars at high redshift

Fatma Shaban and Aneta Siemiginowska


We performed a study of high redshift (z > 2) quasars, looking for the main differences between Radio Loud Quasars (RLQ) and Radio Quiet Quasars (RQQ) in the X-ray band. Our 472 RQQ and 81 RLQ samples were selected by cross-matching the SDSS DR7 quasars catalog with the Chandra Source Catalog. We computed the X-ray luminosity for the two samples and confirmed the X-ray luminosity excess of RLQ over RQQ. We fit the X-ray spectra assuming the absorbed power law model and obtained the photon index (Γ) values for all the sources in the sample. We excluded quasars with a low number of counts (< 10) and large uncertainty on the best-fit photon index (Γerr > 1), and obtained the mean values of ΓRLQ = 1.70 and ΓRQQ = 2.19 +0.46 −0.44 for the RLQ and RQQ samples, respectively, showing that the RLQ have flatter (harder) X-ray spectra than RQQ. The Kuiper-two test confirms this result with the significant difference between the RLQ and RQQ photon index distributions (Dk = 0.37 and P-value = 10−6). We also evaluated the hardness ratio distributions and confirmed that the spectra of RLQ are flatter than the spectra of the RQQ. The RLQ’s hard-to-soft ratio distribution is skewed towards the hard X-ray band, while the RQQ is towards the soft X-ray band. The hard-to-medium and medium-to soft ratios show no difference.