NuSTAR detection of the blazar B2 1023+25 at redshift 5.3

TL;DR

This paper reports the first NuSTAR detection of the high-redshift blazar B2 1023+25 at z=5.3, revealing its jet properties and confirming its blazar nature through X-ray observations.

Contribution

It presents the first NuSTAR detection of a z>5 blazar, providing insights into its jet orientation, Doppler boosting, and high-energy emission at early cosmic times.

Findings

Detected high-energy X-ray emission from B2 1023+25.

Confirmed the blazar nature with a small viewing angle and high Lorentz factor.

Demonstrated NuSTAR's capability to study supermassive black holes at high redshift.

Abstract

B2 1023+25 is an extremely radio-loud quasar at z=5.3 which was first identified as a likely high-redshift blazar candidate in the SDSS+FIRST quasar catalog. Here we use the Nuclear Spectroscopic Telescope Array (NuSTAR) to investigate its non-thermal jet emission, whose high-energy component we detected in the hard X-ray energy band. The X-ray flux is ~5.5x10^(-14) erg cm^(-2)s^(-1) (5-10keV) and the photon spectral index is Gamma_X=1.3-1.6. Modeling the full spectral energy distribution, we find that the jet is oriented close to the line of sight, with a viewing angle of ~3deg, and has significant Doppler boosting, with a large bulk Lorentz factor ~13, which confirms the identification of B2 1023+25 as a blazar. B2 1023+25 is the first object at redshift larger than 5 detected by NuSTAR, demonstrating the ability of NuSTAR to investigate the early X-ray Universe and to study extremely active supermassive black holes located at very high redshift.