# Constraining the Orbit of Supermassive Black Hole Binary 0402+379

## TL;DR

This study uses multi-frequency VLBI observations over 12 years to detect relative motion in the supermassive black hole binary 0402+379, constraining its orbit and mass, and marking it as the first resolved black hole binary system.

## Contribution

It provides the first direct measurement of relative motion in a resolved supermassive black hole binary, constraining its orbital parameters and mass.

## Key findings

- Detected significant core motion at 0.0054c
- Estimated orbital period of approximately 30,000 years
- Constrained SMBHB mass to about 15 billion solar masses

## Abstract

The radio galaxy 0402+379 is believed to host a supermassive black hole binary (SMBHB). The two compact core sources are separated by a projected distance of 7.3 pc, making it the most (spatially) compact resolved SMBHB known. We present new multi-frequency VLBI observations of 0402+379 at 5, 8, 15 and 22 GHz, and combine with previous observations spanning 12 years. A strong frequency dependent core shift is evident, which we use to infer magnetic fields near the jet base. After correcting for these shifts we detect significant relative motion of the two cores at $\beta=v/c=0.0054 \pm 0.0003$ at $PA= -34.4^\circ$. With some assumptions about the orbit, we use this measurement to constrain the orbital period $P\approx 3 \times 10^4$ y and SMBHB mass $M \approx 15 \times 10^9\ M_\odot$. While additional observations are needed to confirm this motion and obtain a precise orbit, this is apparently the first black hole system resolved as a visual binary.