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Optical superluminal motion measurement in the neutron-star merger GW170817


The afterglow of the binary neutron-star merger GW1708171 gave evidence for a structured relativistic jet2,3,4,5,6 and a link3,7,8 between such mergers and short gamma-ray bursts. Superluminal motion, found using radio very long baseline interferometry3 (VLBI), together with the afterglow light curve provided constraints on the viewing angle (14–28 degrees), the opening angle of the jet core (less than 5 degrees) and a modest limit on the initial Lorentz factor of the jet core (more than 4). Here we report on another superluminal motion measurement, at seven times the speed of light, leveraging Hubble Space Telescope precision astrometry and previous radio VLBI data for GW170817. We thereby obtain a measurement of the Lorentz factor of the wing of the structured jet, as well as substantially improved constraints on the viewing angle (19–25 degrees) and the initial Lorentz factor of the jet core (more than 40).

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Fig. 1: Proper motion of GW170817.
Fig. 2: Parameter estimations using the semi-analytical point-source and hydrodynamical models.
Fig. 3: Schematic of the geometric parameters derived for GW170817.
Fig. 4: Precision astrometry with the JWST.

Data availability

All HST data used in this work are available via MAST ( The minimum dataset consists of archival HST data from programmes GO-14771, GO-14804 and GO-15329.

Code availability

The hydrodynamical code is currently being prepared for public release and is available from the corresponding authors upon request. All other codes (astrometric and semi-analytical point-source model) used in this work are available at


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We thank A. Deller for pointing out the required correction for radio VLBI positions, for reading of the manuscript and for providing comments. K.P.M. thanks A. Krone-Martins for discussions, D. Frail for commenting on an early version of this manuscript and Y. Mooley for help with the Nature submission. K.P.M. is indebted to K. Gaura-Nitay for providing the impetus to execute this project. This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with HST programmes GO-14771, GO-14804 and GO-15329. K.P.M. was a Jansky Fellow of the National Radio Astronomy Observatory and his work is currently supported through the National Research Foundation Grant AST-1911199. W.L. was supported by the David and Ellen Lee Fellowship at Caltech and Lyman Spitzer, Jr Fellowship at Princeton University.

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Authors and Affiliations



J.A. led the HST analysis. W.L. set up the semi-analytical and hydrodynamical models. K.P.M. led the scientific analysis and interpretation. All authors discussed and wrote the paper.

Corresponding authors

Correspondence to Kunal P. Mooley or Jay Anderson.

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Nature thanks Elena Pian and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Selection of Gaia reference stars for the F160W analysis.

The panels (a), (b) give the positions, magnitudes and positional uncertainties (1σ) associated with the 32 Gaia stars that are within the WFC3/IR frame, which is shown in panel (c). The legend shows the marker shape and colour used for plotting these stars based on their vetted classifications. The 6 Gaia reference stars selected based on low quoted Gaia positional errors, distant location from the host galaxy nucleus (>12 arcseconds from the nucleus of NGC 4993), centroid located on the HST chip, and away from any bad pixels, are shown as black filled circles. In panels (a), (c) the blue dashed lines denote the 12 arcsecond distance constraint from the NGC 4993 nucleus, and the green dashed lines denote the extent of the WFC3/IR chip.

Extended Data Fig. 2 Residuals from the distortion correction for WFC3/IR.

The distortion residuals along each axis (image X/Y) for image slices that are 50-pixels wide in the orthogonal direction (see Methods for details). The X residuals are shown in panel (a) and the Y residuals in panel (b). The horizontal axis in each panel represents the pixel number and the vertical axis represents the residual in units of pixels. Each set of red and black curves, as well as each data point plotted on the red and black curves, represents one slice (offset of each set of curves along the vertical axis is arbitrary). The black points/curves denote the distortion residuals after the standard HST distortion correction30 and the red after our improved correction. In general, the residuals went down by a factor of two in each coordinate after the application of the improved correction. The new distortion-correction residuals lie within 0.002 pixel per coordinate (i.e. within 0.08 mas; root mean square).

Extended Data Fig. 3 HST/Gaia merger position of GW170817.

The positions of GW170817 in the individual HST F160W exposures (blue filled and red unfilled circles; mean epoch 8 d post-merger) and the combined HST position (black star), in the Gaia pixelized frame, shown along with the radio VLBI measurements3 at 75 d and 230 d. The error bars represent 1σ statistical uncertainties. The VLBI systematic uncertainties have not been included.

Extended Data Fig. 4 Full posterior from the hydrodynamic simulations.

The parameters are: peak Lorentz factor \({\rm{lg}}{u}_{0,\max }\), angular size of the jet core lgθc [rad], power-law index q for the energy distribution of the jet wing, power-law index s for the Lorentz factor distribution of the jet wind, magnetic field equipartition parameter lgϵB, power-law index p for the electron Lorentz factor distribution, lgEiso/n0[erg cm3] — ratio between the isotropic equivalent energy on the jet axis and the circumstellar medium number density, inclination angle θv [degree] between the line of sight and the jet axis, luminosity distance to the source DL. The dashed lines in the marginalized probability distributions indicate the 90% credible interval for each parameter.

Extended Data Table 1 Log of archival HST data used in this work
Extended Data Table 2 Gaia DR2/EDR3 reference stars used for the F160W analysis
Extended Data Table 3 Positional measurements and transformed positions for F160W
Extended Data Table 4 GW170817 positions and associated uncertainties at different epochs in the Gaia/ICRF3 reference frame
Extended Data Table 5 GW170817 structured jet parameter values derived from the semi-analytical point-source model

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Mooley, K.P., Anderson, J. & Lu, W. Optical superluminal motion measurement in the neutron-star merger GW170817. Nature 610, 273–276 (2022).

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