Is the X-ray bright z=5.5 quasar SRGE J170245.3+130104 a blazar?

Jets may have contributed to promoting the growth of seed black holes in the early Universe, and thus observations of radio-loud high-redshift quasars are crucial to understanding the growth and evolution of the early supermassive black holes. Here we report the radio properties of an X-ray bright z = 5.5 quasar, SRGE J170245.3 + 130104 (J1702 + 1301). Our high-resolution radio images reveal the radio counterpart at the optical position of J1702 + 1301, while another radio component is also detected at similar to 23.5 arcsec to the south-west. Our analysis suggests that this south-west component is associated with a foreground galaxy at z asymptotic to 0.677, which is mixed with J1702 + 1301 in low-frequency low-resolution radio images. After removing the contamination from this foreground source, we recalculated the radio loudness of J1702 + 1301 to be R > 1100, consistent with those of blazars. J1702 + 1301 exhibits a flat radio spectrum ( alpha = -0.17 +/- 0.05, S proportional to nu(alpha)) between 0.15 and 5 GHz; abo v e 5 GHz, it shows a rising spectrum shape, and the spectral index alpha(8 . 2)(4 . 7) appears to be correlated with the variation of the flux density: in burst states, alpha(8 . 2)(4 . 7) becomes larger. J1702 + 1301 displays distinct radio variability on time-scales from weeks to years in the source's rest frame. These radio properties, including high radio loudness, rising spectrum, and rapid variability, tend to support it as a blazar.

Automated summary

Jets may have played a role in the growth of seed black holes in the early Universe, making observations of radio-loud high-redshift quasars important for understanding the growth and evolution of supermassive black holes. In this study, the radio properties of a bright X-ray quasar at z=5.5, J1702+1301, were examined. A radio counterpart was detected at the optical position of J1702+1301, as well as another radio component associated with a foreground galaxy. After removing contamination from the foreground source, the radio loudness of J1702+1301 was recalculated to be consistent with blazars.