SRG/eROSITA uncovers the most X-ray luminous quasar at z > 6

Abstract

We report the discovery of X-ray emission from CFHQS J142952+544717, the most distant known radio-loud quasar at z = 6.18, on 2019 December 10–11 with the eROSITA telescope on board the SRG satellite during its ongoing all-sky survey. The object was identified by cross-matching an intermediate SRG/eROSITA source catalogue with the Pan-STARRS1 distant quasar sample at 5.6 < z < 6.7. The measured flux ∼8 × 10−14 erg cm−2 s−1 in the 0.3–2 keV energy band corresponds to an X-ray luminosity of $2.6^{+1.7}_{-1.0}\times 10^{46}$ erg s−1 in the 2–10 keV rest-frame energy band, which renders CFHQS J142952+544717 the most X-ray luminous quasar ever observed at z > 6. Combining our X-ray measurements with archival and new photometric measurements in other wavebands (radio to optical), we estimate the bolometric luminosity of this quasar at ∼(2–3) × 1047 erg s−1. Assuming Eddington limited accretion and isotropic emission, we infer a lower limit on the mass of the supermassive black hole of ∼2 × 109 M⊙. The most salient feature of CFHQS J142952+544717 is its X-ray brightness relative to the optical/UV emission. We argue that it may be linked to its radio-loudness (although the object is not a blazar according to its radio properties), specifically to a contribution of inverse Compton scattering of cosmic microwave background photons off relativistic electrons in the jets. If so, CFHQS J142952+544717 might be the tip of the iceberg of high-z quasars with enhanced X-ray emission, and SRG/eROSITA may find many more such objects during its 4-yr all-sky survey.

First constraints on the AGN X-ray luminosity function atz~ 6 from an eROSITA-detected quasar

“…quasars known to date, together with CFHQJ14 (Willott et al 2010;Medvedev et al 2020a), SDSS J010013.02+280225.8 (Wu et al 2015;Ai et al 2016), and PSO J030947.49+271757.31, the blazar discovered by Belladitta et al (2020). This can be seen in Fig.…”

“…We note that CFHQJ14 (black point) tends towards flatter α OX values than X-raydetected quasars at similar UV luminosities. Its α OX was computed from the luminosity reported in Medvedev et al (2020b).…”

The ROSAT Raster survey in the north ecliptic pole field

“…The ROSAT NEP data reaches significantly higher X-ray luminosities of log(L X /(erg s −1 ))<46) than the both the COSMOS sample and the list of mid-infrared selected upper limits. This is only topped by the highest X-ray luminosity source CFHQS J142952+544717 discovered in the eROSITA all-sky survey (Medvedev et al 2020) and possibly one of the AGN1 in our sample (see above). The upper redshift cutoff of all samples is rather similar, with the bulk of objects at z<4, but a significant number of AGN1 and AGN2 candidates at z=4-7.…”

“…This is on one hand due to the lower Galactic latitude of the NEP (δ∼30 • ), compared to the COSMOS field (δ∼42 • ), but also due to the reduction of stars at fainter X-ray fluxes. The large orange diamond shows the position of the highestredshift radio loud quasar CFHQS J142952+544717, the most X-ray luminous quasar ever observed at z > 6, which was recently identified in the eROSITA all-sky survey (Medvedev et al 2020). Figure 11 (right) shows the same objects as in the left figure, but now with the (observed) 0.5-2 keV X-ray luminosity on the X-axis.…”

“…In addition to the ROSAT NEP raster scan catalogue objects (colours as in Figure 6 without the H06 objects indicated), at fainter fluxes the objects from the largely spectroscopically identified Chandra legacy survey of the COSMOS field (Marchesi et al 2016) have been added with small open symbols and the same colours. The large orange diamond shows the position of the highest-redshift radio loud quasar CFHQS J142952+544717 recently identified in the eROSITA all-sky survey (Medvedev et al 2020). Right: X-ray to optical flux ratio versus X-ray luminosity.…”