The binary neutron-star merger GW1708171 was accompanied by radiation across the electromagnetic spectrum2 and localized2 to the galaxy NGC 4993 at a distance3 of about 41 megaparsecs from Earth. The radio and X-ray afterglows of GW170817 exhibited delayed onset4,5,6,7, a gradual increase8 in the emission with time (proportional to t0.8) to a peak about 150 days after the merger event9, followed by a relatively rapid decline9,10. So far, various models have been proposed to explain the afterglow emission, including a choked-jet cocoon4,8,11,12,13 and a successful-jet cocoon4,8,11,12,13,14,15,16,17,18 (also called a structured jet). However, the observational data have remained inconclusive10,15,19,20 as to whether GW170817 launched a successful relativistic jet. Here we report radio observations using very long-baseline interferometry. We find that the compact radio source associated with GW170817 exhibits superluminal apparent motion between 75 days and 230 days after the merger event. This measurement breaks the degeneracy between the choked- and successful-jet cocoon models and indicates that, although the early-time radio emission was powered by a wide-angle outflow8 (a cocoon), the late-time emission was most probably dominated by an energetic and narrowly collimated jet (with an opening angle of less than five degrees) and observed from a viewing angle of about 20 degrees. The imaging of a collimated relativistic outflow emerging from GW170817 adds substantial weight to the evidence linking binary neutron-star mergers and short γ-ray bursts.
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All relevant (VLBI) data are available from the corresponding authors on request. The VLA data (presented in Fig. 2) are currently being readied for public release.
Abbott, B. P. et al. GW170817: observation of gravitational waves from a binary neutron star inspiral. Phys. Rev. Lett. 119, 161101 (2017).
Abbott, B. P. et al. Multi-messenger observations of a binary neutron star merger. Astrophys. J. 848, L12 (2017).
Hjorth, J. et al. The distance to NGC 4993: the host galaxy of the gravitational-wave event GW170817. Astrophys. J. 848, L31 (2017).
Hallinan, G. et al. A radio counterpart to a neutron star merger. Science 358, 1579–1583 (2017).
Troja, E. et al. The X-ray counterpart to the gravitational-wave event GW170817. Nature 551, 71–74 (2017).
Margutti, R. et al. The electromagnetic counterpart of the binary neutron star merger LIGO/Virgo GW170817. V. Rising X-ray emission from an off-axis jet. Astrophys. J. 848, L20 (2017).
Haggard, D. et al. A deep Chandra X-ray study of neutron star coalescence GW170817. Astrophys. J. 848, L25 (2017).
Mooley, K. P. et al. A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817. Nature 554, 207–210 (2018).
Dobie, D. et al. A turnover in the radio light curve of GW170817. Astrophys. J. 858, L15 (2018).
Alexander, K. D. et al. A decline in the X-ray through radio emission from GW170817 continues to support an off-axis structured jet. Astrophys. J. 863, L18 (2018).
Kasliwal, M. M. et al. Illuminating gravitational waves: A concordant picture of photons from a neutron star merger. Science 358, 1559–1565 (2017).
Troja, E. et al. The outflow structure of GW170817 from late-time broad-band observations. Mon. Not. R. Astron. Soc. 478, L18–L23 (2018).
Xie, X., Zrake, J. & MacFadyen, A. Numerical simulations of the jet dynamics and synchrotron radiation of binary neutron star merger event GW170817/GRB 170817A. Astrophys. J. 863, 58 (2018).
Lamb, G. P. & Kobayashi, S. Electromagnetic counterparts to structured jets from gravitational wave detected mergers. Mon. Not. R. Astron. Soc. 472, 4953–4964 (2017).
Lazzati, D. et al. Late time afterglow observations reveal a collimated relativistic jet in the ejecta of the binary neutron star merger GW170817. Phys. Rev. Lett. 120, 241103 (2018).
Margutti, R. et al. The binary neutron star event LIGO/Virgo GW170817 160 days after merger: synchrotron emission across the electromagnetic spectrum. Astrophys. J. 856, L18 (2018).
Lyman, J. D. et al. The optical afterglow of the short gamma-ray burst associated with GW170817. Nat. Astron. https://doi.org/10.1038/s41550-018-0511-3 (2018).
Resmi, L. et al. Low frequency view of GW 170817/GRB 170817A with the Giant Meterwave Radio Telescope. Preprint at https://arxiv.org/abs/1803.02768 (2018).
Nakar, E. & Piran, T. Implications of the radio and X-ray emission that followed GW170817. Mon. Not. R. Astron. Soc. 478, 407–415 (2018).
Troja, E., Piro, L. & Ryan, G. Chandra observations of GW170817 reveal a fading afterglow. Astron. Telegr. 11619 (2018).
Hotokezaka, K., Kiuchi, K., Shibata, M., Nakar, E. & Piran, T. Synchrotron radiation from the fast tail of dynamical ejecta of neutron star mergers. Preprint at https://arxiv.org/abs/1803.00599 (2018).
D’Avanzo, P. et al. The evolution of the X-ray afterglow emission of GW 170817/ GRB 170817A in XMM-Newton observations. Astron. Astrophys. 613, L1 (2018).
Gill, R. & Granot, J. Afterglow imaging and polarization of misaligned structured GRB jets and cocoons: breaking the degeneracy in GRB 170817A. Mon. Not. R. Astron. Soc. 478, 4128–4141 (2018).
Taylor, G., Frail, D., Berger, E. & Kulkarni, S. High resolution observations of GRB 030329. AIP Conf. Ser. 727, 324–327 (2004).
Boutelier, T., Henri, G. & Petrucci, P.-O. The influence of the jet opening angle on the appearance of relativistic jets. Mon. Not. R. Astron. Soc. 418, 1913–1922 (2011).
Lind, K. R. & Blandford, R. D. Semidynamical models of radio jets: relativistic beaming and source counts. Astrophys. J. 295, 358–367 (1985).
Hotokezaka, K. et al. A Hubble constant measurement from superluminal motion of the jet in GW170817. Preprint at https://arxiv.org/abs/1806.10596 (2018).
Nakar, E., Gottlieb, O., Piran, T., Kasliwal, M. M. & Hallinan, G. From γ to radio - the electromagnetic counterpart of GW 170817. Preprint at https://arxiv.org/abs/1803.07595 (2018).
Fong, W., Berger, E., Margutti, R. & Zauderer, B. A. A decade of short-duration gamma-ray burst broadband afterglows: energetics, circumburst densities, and jet opening angles. Astrophys. J. 815, 102 (2015).
Wanderman, D. & Piran, T. The rate, luminosity function and time delay of non-Collapsar short GRBs. Mon. Not. R. Astron. Soc. 448, 3026–3037 (2015).
Greisen, E. W. in Information Handling in Astronomy – Historical Vistas (ed. Heck, A.) 109–126 (Kluwer Academic, Dordrecht, 2003).
Shepherd, M. C. Difmap: an interactive program for synthesis imaging. ASP Conf. Ser. 125, 77–84 (1997).
Condon, J. J. Errors in elliptical Gaussian FITS. Publ. Astron. Soc. Pacif. 109, 166–172 (1997).
Pradel, N., Charlot, P. & Lestrade, J.-F. Astrometric accuracy of phase-referenced observations with the VLBA and EVN. Astron. Astrophys. 452, 1099–1106 (2006).
Tzioumis, A. K. et al. VLBI observations at 2.3 GHz of the compact galaxy 1934-638. Astron. J. 98, 36–43 (1989).
Tingay, S. J., Preston, R. A. & Jauncey, D. L. The subparsec-scale structure and evolution of Centaurus A. II. Continued very long baseline array monitoring. Astron. J. 122, 1697–1706 (2001).
Gottlieb, O., Nakar, E. & Piran, T. The cocoon emission – an electromagnetic counterpart to gravitational waves from neutron star mergers. Mon. Not. R. Astron. Soc. 473, 576–584 (2018).
Gottlieb, O., Nakar, E., Piran, T. & Hotokezaka, K. A cocoon shock breakout as the origin of the γ-ray emission in GW170817. Mon. Not. R. Astron. Soc. 479, 588–600 (2018).
Mignone, A. et al. PLUTO: a numerical code for computational astrophysics. Astrophys. J. Suppl. Ser. 170, 228–242 (2007).
Lazzati, D. et al. Off-axis prompt X-ray transients from the cocoon of short gamma-ray bursts. Astrophys. J. 848, L6 (2017).
Eichler, D. Testing the viewing angle hypothesis for short GRBs with LIGO events. Astrophys. J. 851, L32 (2017).
Kathirgamaraju, A., Barniol Duran, R. & Giannios, D. Off-axis short GRBs from structured jets as counterparts to GW events. Mon. Not. R. Astron. Soc. 473, L121–L125 (2018).
Bromberg, O., Tchekhovskoy, A., Gottlieb, O., Nakar, E. & Piran, T. The γ-rays that accompanied GW170817 and the observational signature of a magnetic jet breaking out of NS merger ejecta. Mon. Not. R. Astron. Soc. 475, 2971–2977 (2018).
Pozanenko, A. S. et al. GRB 170817A associated with GW170817: multi-frequency observations and modeling of prompt gamma-ray emission. Astrophys. J. 852, L30 (2018).
Granot, J. & Piran, T. On the lateral expansion of gamma-ray burst jets. Mon. Not. R. Astron. Soc. 421, 570–587 (2012).
Corsi, A. et al. An upper-limit on the linear polarization fraction of the GW170817 radio continuum. Astrophys. J. 861, L10 (2018).
We are grateful to the VLBA, VLA and GBT staff, especially M. Claussen, A. Mioduszewski, T. Minter, F. Ghigo, W. Brisken, K. O’Neill and M. McKinnon, for their support with the HSA observations. We thank V. Dhawan and P. Demorest for help with observational issues with the VLBI system at the VLA. K.P.M. thanks A. Mioduszewski, E. Momjian, E. Greisen, T. Pearson and S. Kulkarni for discussions. We thank M. Kasliwal for providing comments on the manuscript. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities. K.P.M. is currently a Jansky Fellow of the National Radio Astronomy Observatory. K.P.M. acknowledges support from the Oxford Centre for Astrophysical Surveys, which is funded through the Hintze Family Charitable Foundation, for some initial work presented here. E.N. acknowledges the support of an ERC starting grant (GRB/SN) and an ISF grant (1277/13). A.T.D. is the recipient of an Australian Research Council Future Fellowship (FT150100415). G.H. acknowledges the support of NSF award AST-1654815. A.H. acknowledges support by the I-Core Program of the Planning and Budgeting Committee and the Israel Science Foundation. A.C. acknowledges support from the NSF CAREER award number 1455090 titled ‘CAREER: Radio and gravitational-wave emission from the largest explosions since the Big Bang’.
K.P.M., A.T.D., S.B., G.H. and D.A.F. coordinated the VLBI observations. A.T.D. and K.P.M. performed the VLBI data processing. O.G. and E.N. carried out the theoretical study, including analytic calculations and numerical simulations, with some input from K.H. K.P.M., A.T.D., E.N., G.H. and D.A.F. wrote the paper. A.C. and A.H. compiled the references. A.H., A.D. and K.P.M. compiled Methods. O.G., A.T.D., A.H. and K.P.M. prepared the figures. All co-authors discussed the results and provided comments on the manuscript.
The authors declare no competing interests.
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Extended data figures and tables
a, b, The cleaned images (natural weighting; 0.2 mas pixel−1) from the two epochs of VLBI, 75 days (a) and 230 days (b) post-merger. The centre coordinates for these images are RA = 13 h 09 min 48.069 s, dec. = −23° 22′ 53.39′. The black contours are at 11, 22 and 44 μJy beam−1 in both images (red dashed contour is −11 μJy beam−1). The peak flux density of the sources is 58 ± 5 μJy beam−1 (a) and 48 ± 6 μJy beam−1 (b) (image root-mean-square noise quoted as the 1σ uncertainty). The ellipse in the lower left corner of each panel shows the synthesized beam: (12.4, 2.2, −7) and (9.1, 3.2, −4) for the two epochs (major axis in mas, minor axis in mas, position angle in degrees).
a, b, The VLBI positions of GW170817 (a, relative to the best-fit position at day 75) and the low-luminosity active galactic nucleus in NGC 4993 (b, relative to the previously derived position using VLBA-only observations). The individual observations of GW170817 have very low signal-to-noise ratio and hence large errors; the moderately discrepant measurement on day 72 has the lowest signal-to-noise ratio and was affected by observing issues at the GBT. The NGC 4993 positions do not show any significant systematic position shifts between the two epochs, and are consistent with our estimated systematic position uncertainties of 0.15 mas in RA and 0.5 mas in dec. The root-mean-square variation in the position of the nucleus of NGC 4993 over our seven individual observations (0.14 mas in RA and 0.49 mas in dec.) is shown as a dotted ellipse in b. All error bars and uncertainties quoted are 1σ.
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Mooley, K.P., Deller, A.T., Gottlieb, O. et al. Superluminal motion of a relativistic jet in the neutron-star merger GW170817. Nature 561, 355–359 (2018). https://doi.org/10.1038/s41586-018-0486-3
- Very Long Baseline Interferometry (VLBI)
- Binary Neutron Star Mergers
- VLBI Data
- Very Long Baseline Array (VLBA)
- Fourth Epoch
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