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

TL;DR

This study investigates the radio properties of a high-redshift quasar, revealing characteristics such as high radio loudness, spectral variability, and rapid changes that support its classification as a blazar.

Contribution

The paper provides detailed high-resolution radio observations of a z=5.5 quasar, demonstrating its blazar-like properties and clarifying the impact of foreground contamination on radio measurements.

Findings

Radio loudness R > 1100 after contamination correction

Flat radio spectrum between 0.15 and 5 GHz, rising above 5 GHz

Significant radio variability on weekly to yearly timescales

Abstract

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 $\sim$23.5\arcsec\ to the southwest. Our analysis suggests that this southwest component is associated with a foreground galaxy at $z\approx 0.677$, 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 \pm 0.05$, $S \propto \nu^\alpha$) between 0.15 and 5 GHz; above 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 timescales 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.