Chandra Detection of Three X-Ray Bright Quasars at z > 5

We report Chandra detection of three UV-bright radio-quiet quasars at z greater than or similar to 5. We have collected a sufficient number of photons to extract an X-ray spectrum of each quasar to measure their basic X-ray properties, such as the X-ray flux, power-law photon index (Gamma), and optical-to-X-ray spectral slope (alpha(OX)). J074749+115352 at z = 5.26 is the X-ray brightest radio-quiet quasar at z > 5. It may have a short timescale variation (on a timescale of similar to 3800 s in the observer's frame, or similar to 600 s in the rest frame), which is, however, largely embedded in the statistical noise. We extract phase folded spectra of this quasar. There are two distinguishable states: a high soft state with an average X-ray flux similar to 2.7 times the low hard state, and a significantly steeper X-ray spectral slope (Gamma = 2.40(-0.32)(+0.33) versus 1.78(-0.24)(+0.25)). We also compare the three quasars detected in this paper to other quasar samples. We find that J074749+115352, with an SMBH mass of M-SMBH 1.8 x 10(9) M-circle dot and an Eddington ratio of lambda(Edd) approximate to 2.3, is extraordinarily X-ray bright. It has an average alpha(OX) = -1.46 +/- 0.02 and a 2-10 keV bolometric correction factor of L-bol/L2-10 keV = 42.4 5.8, both significantly depart from some well defined scaling relations. We compare Gamma of the three quasars to other samples at different redshifts, and do not find any significant redshift evolution based on the limited sample of z > 5 quasars with reliable measurements of the X-ray spectral properties.

Automated summary

The study reports the detection of three UV-bright radio-quiet quasars at z greater than or similar to 5 with Chandra, and analyzes their X-ray properties and spectral characteristics. One quasar shows potential short timescale variation, and comparisons are made with other quasar samples.