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

TL;DR

This study uses a comprehensive multiwavelength approach to dissect the HST-1 knot in M87, combining two decades of optical data from HST with prior Chandra X-ray results to probe both the brightness evolution and the motion of the knot. The optical/UV emission appears to be dominated by a single, baseline shock near HST-1, while the X-ray emission arises from downstream, potentially multiple, shocks, indicating different particle-acceleration regions across wavelengths. Proper-motion analysis reveals a two-phase kinematic history: a slower, pre-2005 regime with $\sim$1.04$\,c$ and a faster, post-2005 regime with $\sim$2.1–2.4$\,c$, consistent with acceleration and internal shocks in a recollimation-jet nozzle. Joint fitting with Chandra and Hubble data shows that X-ray and optical emissions occupy different locations within HST-1, supporting a magnetohydrodynamic, recollimation-shock framework and highlighting the importance of simultaneous, high-resolution, multiwavelength observations for jet physics.

Abstract

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 {\it Chandra} data \citep{Thimmappa24} 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 {\it Hubble} Space Telescope Advanced Camera for Surveys HRC/WFC/SBC 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 $\sim345$ 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$\pm$0.04 c from 2002-2005 and around 2.1$\pm$0.05 c from 2005-2022. We discuss the acceleration of the jet and the flaring synchrotron emission from HST-1 from optical to X-rays.

Figures (6)

• Figure 1: Top: Drizzled HST/ACS-HRC image of the M87 jet (Program ID J8L001010, F220W filter). The color bar is flux density in units of erg s$^{-1}$ cm$^{-2}$ Å$^{-1}$ pixel$^{-1}$. Bottom: Exposure-corrected, deconvolved Chandra/ACIS image of ObsID 3975 at 0.25-pixel resolution, with the color bar is flux in photons cm$^{-2}$ s$^{-1}$ pixel$^{-1}$.
• Figure 2: The Chandra/ Hubble lightcurves of HST-1. The flux rises around 2005 for both observations and drops in 2008, then remains the same until 2022. Blue color shows Chandra flux, and other color lines show Hubble filters. The complete dataset is available in machine-readable format with accompanying metadata.
• Figure 4: Joint fits of Model 1 (i.e., Equations \ref{['eqn:flux_HST-1']} and \ref{['eqn:offset_HST-1']}) to flux (gray) and offset (black) of HST-1 for the F250W filter.
• Figure 5: Linear regression for the 2002–2006 offset vs time in different ACS filters. Left panel: F220W, F330W, and F606W. Right panel: F250W, F475W, and F814W. The model is shown as a red line.
• Figure 6: Top panel: Piecewise regression model for the offset of HST-1 vs time during the period 2002-2022 (see Section \ref{['sec:velocity_calculation3']}). The model is shown in the magenta line, the orange line shows the pre-break model (solid before the break and dashed after the break), the blue line shows the post-break model (solid after the break and dashed before the break), and the gray line shows the break point at 54137 days. Bottom panel: zoomed view of the same figure between MJD 57200 and MJD 58200.