Parsec-Scale Localization of the Quasar SDSS J1536+0441A, a Candidate Binary Black Hole System

TL;DR

This study uses VLBA and VLA observations to investigate the radio emission and structure of the quasar SDSS J1536+0441A, exploring its potential binary black hole nature and the properties of its radio-emitting region.

Contribution

It provides high-resolution radio imaging and spectral analysis of SDSS J1536+0441A, offering insights into its emission mechanisms and the size of its radio-sphere, relevant for understanding radio-quiet quasars.

Findings

Detected a compact radio source with a size less than 8.5 pc.

Measured a rising spectral slope indicating optically thick synchrotron emission.

Estimated the radio-sphere radius to be about 800 gravitational radii.

Abstract

The radio-quiet quasar SDSS J1536+0441A shows two broad-line emission systems, recently interpreted as a binary black hole (BBH) system with a subparsec separation; as a double-peaked emitter; or as both types of systems. The NRAO VLBA was used to search for 8.4 GHz emission from SDSS J1536+0441A, focusing on the optical localization region for the broad-line emission, of area 5400 mas^2 (0.15 kpc^2). One source was detected, with a diameter of less than 1.63 mas (8.5 pc) and a brightness temperature T_b > 1.2 x 10^7 K. New NRAO VLA photometry at 22.5 GHz, and earlier photometry at 8.5 GHz, gives a rising spectral slope of alpha = 0.35+/-0.08. The slope implies an optically thick synchrotron source, with a radius of about 0.04 pc, and thus T_b ~ 5 x 10^10 K. The implied radio-sphere at rest frame 31.2 GHz has a radius of 800 gravitational radii, just below the size of the broad line region in this object. Observations at higher frequencies can probe whether or not the radio-sphere is as compact as expected from the coronal framework for the radio emission of radio-quiet quasars.