A powerful (and likely young) radio-loud quasar at z = 5.3

Abstract

We present the discovery of PSO J191.05696+86.43172 (hereafter PSO J191+86), a new powerful radio-loud quasar (QSO) in the early Universe (z = 5:32).We discovered it by cross-matching the NRAO VLA Sky Survey (NVSS) radio catalog at 1.4 GHz with the first data release of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS PS1) in the optical. With a NVSS flux density of 74.2 mJy, PSO J191+86 is one of the brightest radio QSO discovered at z - 5. The intensity of its radio emission is also confirmed by the very high value of radio loudness (R > 300). The observed radio spectrum of PSO J191+86 shows a possible turnover around 1 GHz (i.e., 6 GHz in the rest frame), making it a gigahertz-peaked spectrum (GPS) source. However, variability could afiect the real shape of the radio spectrum, because the data in hand were taken 25 years apart. By assuming a peak in the observed radio spectrum between 1 and 2 GHz (i.e., 6 and 13 GHz in the rest-frame) we find a linear size of the source of 10-30 pc and a corresponding kinetic age of 150-460 yr. This would make PSO J191+86 a newly born radio source. However, the large X-ray luminosity (5.31045 erg s1), the flat X-ray photon index (X = 1:32), and the optical-X-ray spectral index (ox = 1:329) are typical of blazars. This could indicate that the nonthermal emission of PSO J191+86 is Doppler boosted. Further radio observations (both on arcsec and parsec scales) are necessary to better investigate the nature of this powerful radio QSO.