Locating the missing large-scale emission in the jet of M87* with short EHT baselines

TL;DR

The paper develops a short-baseline expansion for trivial VLBI closure phases to extract large-scale emission information in M87*, linking them to the image centroid and higher-order moments with minimal modeling. It validates the approach on synthetic data and applies it to EHT observations from 2017, 2018, and 2021, finding a weak northwest excess in the earlier years and a robust ~1 mas northwest centroid offset in 2021, consistent with the jet direction. This provides a calibration- and imaging-friendly tool to identify large-scale jet structure that is otherwise unresolved by long-baseline data, with potential applicability to other jet-dominated sources. The method offers a principled way to separate large-scale structure from compact core emission and to diagnose systematic biases in closure quantities for VLBI arrays.

Abstract

In Very-Long Baseline Interferometric arrays, nearly co-located stations probe the largest scales and typically cannot resolve the observed source. In the absence of large-scale structure, closure phases constructed with these stations are zero and, since they are independent of station-based errors, they can be used to probe data issues. Here, we show with an expansion about co-located stations, how these trivial closure phases become non-zero with brightness distribution on smaller scales than their short baseline would suggest. When applied to sources that are made up of a bright compact and large-scale diffuse component, the trivial closure phases directly measure the centroid relative to the compact source and higher-order image moments. We present a technique to measure these image moments with minimal model assumptions and validate it on synthetic Event Horizon Telescope (EHT) data. We then apply this technique to 2017 and 2018 EHT observations of M87* and find a weak preference for extended emission in the direction of the large-scale jet. We also apply it to 2021 EHT data and measure the source centroid about 1 mas northwest of the compact ring, consistent with the jet observed at lower frequencies.

Figures (8)

• Figure 1: Synthetic dataset used for validation. The emission is composed of a bright compact ring and an extended jet. The green dot is the centroid of the image relative to the white x in the center of the ring.
• Figure 2: The top and middle panel show the visibility phases for the source model in \ref{['fig:GMVAcentroid']}. The middle panel is a zoomed in version of the top panel. Black points show the $(u,v)$ locations of all synthetic observations, where the innermost three have been highlighted. The red and orange contours show the regions where, respectively, the first- and third-order approximations to the phases (\ref{['eq:firstmoment']} and \ref{['eq:thirdmoments']}, respectively) differ from the true phases by less than 1 degree. The bottom panel shows a horizontal and vertical slice of the phases as well as the first- and third- order approximations as dashed and dotted lines, respectively.
• Figure 3: Covariance ellipses for the fits of the centroid position offset measured from synthetic data. The top panel shows triangles involving JCMT and SMA, while the bottom panel shows triangles involving ALMA and APEX. The black ellipses shows the 2-dimensional 68% and 95% confidence region from fits over the entire dataset, while all other colors split up the data into separate triangles. The orange dot is the phase reference, which is assumed to coincide with the compact ring, and the green x is the truth, with the same coordinates as in \ref{['fig:GMVAcentroid']}. Stations are colored East to West (blue to red), which somewhat corresponds to the short baseline rotating over the course of a night. $\tilde{\chi}^2$ as defined in \ref{['sec:fittingdetails']} is the reduced chi-squared statistic, and characterizes the goodness of fit. Note the bottom panel is zoomed relative to the top.
• Figure 4: Estimated 2-dimensional 68% and 95% regions of the centroid position offset in 87 for the 2017 and 2018 datasets. The black dashed line at $288^\circ$ East of North represents the direction of the mas-scale jet.
• Figure 5: Estimated 2-dimensional 68% and 95% regions of the centroid position offset in 87 from 2021 April 13 and 18 data, the latter of which significantly nonzero. The centroid is located about 1 mas Northwest of the compact source and consistent both between bands and with the direction of the large scale jet (dashed line).