Identifying the secondary jet in the RadioAstron image of OJ~287

TL;DR

The paper addresses identifying and characterizing the secondary jet in OJ 287 within a binary black hole framework by fitting a ballistic, aberration-affected jet model to the 12 μas RadioAstron map. The authors treat the secondary jet as launched perpendicular to its disk and subject to relativistic aberration, with the apparent sky speed $\beta_T$ as the sole free parameter, and they constrain it by overlaying the predicted jet line on the image. They find a slower secondary jet ($\beta_T\approx1$, $\beta\approx0.927$, $\gamma\approx2.7$, $\delta\approx5$) that shows a higher spectral turnover and a magnetic field about 20 times larger than the primary jet, consistent with a secondary origin and with the 2021 optical flare. The results support a two-jet interpretation for OJ 287 and provide a framework to test jet dynamics in a binary SMBH through future, higher-resolution, multi-epoch VLBI observations to observe the predicted wagging of the secondary jet tail. The study advances understanding of jet propagation under relativistic aberration and disk-crossing dynamics in binary SMBH systems.

Abstract

The 136 year long optical light curve of OJ~287 is explained by a binary black hole model where the secondary is in a 12 year orbit around the primary. Impacts of the secondary on the accretion disk of the primary generate a series of optical flares which follow a quasi-Keplerian relativistic mathematical model. The orientation of the binary in space is determined from the behavior of the primary jet. Here we ask how the jet of the secondary black hole projects onto the sky plane. Assuming that the jet is initially perpendicular to the disk, and that it is ballistic, we follow its evolution after the Lorentz transformation to the observer's frame. Since the orbital speed of the secondary is of the order of one-tenth of the speed of light, the result is a change in the jet direction by more than a radian during an orbital cycle. We match the theoretical jet line with the recent 12 $μ$as-resolution RadioAstron map of OJ~287, and determine the only free parameter of the problem, the apparent speed of the jet relative to speed of light. It turns out that the Doppler factor of the jet, $δ\sim5$, is much lower than in the primary jet. Besides following a unique shape of the jet path, the secondary jet is also distinguished by a different spectral shape than in the primary jet. The present result on the spectral shape agrees with the huge optical flare of 2021 November 12, also arising from the secondary jet.

Figures (4)

• Figure 1: The position angle of the radio jet in observations at 5 GHz and 8 GHz (points) from 1981 to 2010 and the theoretical line in the binary model arXiv.1208.4524 (left). Subsequent observational points at 86 GHz have continued to remain more or less on the theoretical line 2021MNRAS.503.4400D (right). Preliminary data from 2017 to 2021 indicate that the jet continues to follow the model (G.-Y. Zhao, private communication).
• Figure 2: An illustration of the proposed astrophysical system of OJ287. The primary black hole lies at the focal point of the eccentric orbit which has been determined in Paper I. The primary black hole is surrounded by an accretion disk. Due to spin-orbit interaction, the central part of the disk out to a few Schwarzschild radii is slanted relative to the main disk 1975ApJ...195L..65B. There is a smooth transition from the inner disk to the outer disk (unlike in the figure). The primary jet starts out along the axis of the disk. The time evolution of the axis of the inner disk is calculated in Paper II. The secondary black hole is shown at two different orbital phases. Its accretion disk is parallel to the main accretion disk, and its jet therefore starts out perpendicular to the main disk. However, after Lorentz transformation to the observer's frame, the secondary jet is seen tilted in the direction of the orbital motion, and thus its direction changes constantly. Here the orbital motion is counter-clockwise.
• Figure 3: Left panel: The theoretical model of Paper II, after fitting to the radio jet observations. The primary black hole is at the center and the secondary black hole is at the position angle of about 35 degrees. The blue dashed line is the initial direction of the secondary jet, while solid lines represent the primary jet at different times. The 2014 jet direction should be close to the green line. Right panel: A detail of the RadioAstron map of Paper III in the same scale and orientation as the theoretical map. If the central component corresponds to the primary black hole, then the next one upwards marks the secondary black hole, and the highest component represents a knot in its jet. The elongation of the individual components is not real, but is a reflection of the beam shape.
• Figure 4: The image of OJ 287 at the record-breaking resolution of 12 $\mu$as, achieved in space VLBI when the RadioAstron telescope was 15 Earth diameters away from the ground based telescopes (a distance of about 190,000 km, comparable to about half of the semi-major axis of the Moon orbit). The present primary jet direction is at about $-45^{\circ}$ degrees, i.e. it goes through the component J5. The path of the primary jet between the primary core C1a and J5 is not known, and has to be determined by future high resolution observations. The highest resolution image agrees with the secondary jet, starting from the knot between components C1a and C1b toward the position angle of about $+15 ^{\circ}$ degrees. The two innermost components at the beginning of the jet lie at positions expected for the two black holes at the time of the observations in 2014 (see Fig. 1). Superposed on the map is the jet line from Table 1 (star symbols). For the original RadioAstron map, see Paper III.