Chandra Study of the Proper Motion of HST-1 in the Jet of M87

The radio galaxy M87 is well known for its jet, which features a series of bright knots visible from radio to X-ray. We analyze the X-ray image and flux variability of the knot HST-1 in the jet. Our analysis includes all 112 available Chandra ACIS-S observations from 2000-2021, with a total exposure time of ∼887 ks. We use de-convolved images to study the brightness profile of the X-ray jet and measure the relative separation between the core and HST-1. From 2003-2005 (which coincides with a bright flare from HST-1), we find a correlation between the flux of HST-1 and its offset from the core. In subsequent data, we find a steady increase in this offset. We discuss possible origins for these results, including measured superluminal motion of 6.57±0.90 c or 2.01±0.27 pc yr −1 of HST-1 and changes in the knot structure.