2026ApJ...999...19T

The radio galaxy M87 is well known for its jet, which features a series of bright knots observable from radio to X-ray wavelengths. The most famous of these, HST-1, exhibits superluminal motion, and our analysis of Chandra data reveals a correlation between the X-ray flux of HST-1 and its separation from the core. This correlation likely arises from moving shocks in the jet, allowing measurement of the internal structure of HST-1 in the X-ray band. To follow up on these results, we use observations from the Hubble Space Telescope Advanced Camera for Surveys (ACS) High-Resolution Channel/Wide-Field Channel/Solar-Blind Channel and the Wide Field Camera 3 (WFC3)'s UVIS to analyze the image and flux variability of HST-1. Our analysis includes 245 ACS and 120 WFC3 observations from 2002-2022, with a total exposure time of ∼345 ks. We study the brightness profile of the optical jet and measure the relative separation between the core and HST-1 for comparison to the X-ray. We find that the X-ray and the UV/optical emission could arise from physically distinct regions. The measured proper motion of the knot HST-1 is 1.04 ± 0.04c from 2002-2005 and around 2.1 ± 0.05c from 2005-2022. We discuss the acceleration of the jet and the flaring synchrotron emission from HST-1 from optical to X-rays.