Toward Microarcsecond Astrometry for the Innermost Wobbling Jet of the BL Lacertae Object OJ 287

TL;DR

This study demonstrates the feasibility of microarcsecond-level astrometry on the innermost jet of OJ 287, enabling precise tracking of jet activity and testing models of SMBH binary or jet precession.

Contribution

We show that high-precision VLBI astrometry using a stable reference source can accurately locate the jet apex of OJ 287, advancing methods for studying jet dynamics in quasars.

Findings

Achieved 10 microarcsecond astrometric precision at 43.2 GHz.

Identified a stable reference source suitable for long-term jet tracking.

Confirmed no proper motion in the reference source over 8 years.

Abstract

The BL Lacertae object OJ 287 is a very unusual quasar producing a wobbling radio jet and some double-peaked optical outbursts with a possible period of about 12 yr for more than one century. This variability is widely explained by models of binary supermassive black hole (SMBH) or precessing jet/disk from a single SMBH. To enable an independent and nearly bias-free investigation on these possible scenarios, we explored the feasibility of extremely high-precision differential astrometry on its innermost restless jet at mm-wavelengths. Through re-visiting some existing radio surveys and very long baseline interferometry (VLBI) data at frequencies from 1.4 to 15.4 GHz and performing new Very Long Baseline Array (VLBA) observations at 43.2 GHz, we find that the radio source J0854$+$1959, 7.1 arcmin apart from OJ 287 and no clearly-seen optical and infrared counterparts, could provide a nearly ideal reference point to track the complicated jet activity of OJ 287. The source J0854$+$1959 has a stable GHz-peaked radio spectrum and shows a jet structure consisting of two discrete, mas-scale-compact and steep-spectrum components and showing no proper motion over about 8 yr. The stable VLBI structure can be interpreted by an episodic, optically thin and one-sided jet. With respect to its 4.1-mJy peak feature at 43.2 GHz, we have achieved an astrometric precision at the state-of-art level, about 10 $\mu$as. These results indicate that future VLBI astrometry on OJ 287 could allow us to accurately locate its jet apex and activity boundary, align its restless jet structure over decades without significant systematic bias, and probe various astrophysical scenarios.