VLBI Diagnostics of Off-axis Jets in Radio Flares of Tidal Disruption Events

Abstract

The origin of late-time radio flares in tidal disruption events remains unclear. In particular, the peculiar radio flare observed in AT2018hyz has motivated two leading scenarios: a delayed outflow launched $\sim1000\,\rm days$ after discovery, or an off-axis relativistic jet directed far from our line of sight. Very long baseline interferometry (VLBI) imaging provides the most direct way to distinguish between these scenarios. In this paper, we calculate synthetic radio images for both models and examine their observational signatures. The motion of the emission centroid is the most powerful diagnostic for breaking the degeneracy. In the delayed-outflow scenario, the centroid motion is confined within a non-relativistic distance, whereas in the off-axis jet scenario it exhibits apparent superluminal motion. Detecting such superluminal motion would therefore provide a smoking-gun signature of the off-axis jet interpretation. We also find that the jet image exhibits characteristic features, including a non-monotonic evolution of the image aspect ratio. These results are expected to be generic and applicable to other jetted explosions, such as microquasars and gamma-ray bursts.

Figures (9)

• Figure 1: A schematic picture of the adopted coordinate system in our calculation. For a jet geometry, the jet is assumed to have a bipolar structure and each jet propagates along $z$ axis. An observer is located within the $xz$ plane, whose viewing angle $\theta_{\rm obs}$ is measured from the $z$ axis. The incoming (traveling into the positive $z$ direction) and outgoing jets are called an approaching and counter jets, respectively. The coordinate system of the sky ($XY$) plane is defined so that the approaching jet always propagates into the positive $X$ direction.
• Figure 2: The radio light curve and spectrum of AT2018hyz for the delayed outflow model. ( Top) The $5\,\rm GHz$ light curve. The black and gray curves show the model light curves of a spherical outflow launched at $T=700$ (delayed) and $0\,\rm day$ (prompt), respectively. The black dashed line represents the analytical scaling of the flux, $F_\nu\propto T^{\frac{12-k(p+5)}{4}}$. ( Bottom) The radio spectrum at each epoch. Solid curves show the model spectra corresponding to the black curve in the top panel.
• Figure 3: The same as Fig. \ref{['fig:18hyz_sph']} but for the off-axis jet model with different viewing angles. ( Top) The thick solid curves show the total radio luminosity contributed by both approaching (thin solid) and counter (dotted) jets while the former almost overlaps with the thick solid curves for $\lesssim10000\,\rm days$. The black dashed line represents the analytical scaling, $F_\nu\propto T^{\frac{3(5-p)}{2}}$. ( Bottom) The solid curves denote the model spectra corresponding to $\theta_{\rm obs}=70^\circ$.
• Figure 4: The synthetic radio intensity maps of AT2018hyz at 5 GHz for the delayed outflow (left) and off-axis jet models (right). In each panel, the left and bottom axes represent the physical distance, while the right and top ones represent the corresponding angular distance. The red crosses show the location of the emission centroid.
• Figure 5: The same as Fig. \ref{['fig:18hyz_im']} but the $X$ and $Y$-axes in both panels are set to equal scales, ensuring that circles are displayed without distortion. The right panel shows only the approaching jet. The intensity normalization in this figure is the upper 99% of the distribution.