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The X-ray counterpart to the gravitational-wave event GW170817

Abstract

A long-standing paradigm in astrophysics is that collisions—or mergers—of two neutron stars form highly relativistic and collimated outflows (jets) that power γ-ray bursts of short (less than two seconds) duration1,2,3. The observational support for this model, however, is only indirect4,5. A hitherto outstanding prediction is that gravitational-wave events from such mergers should be associated with γ-ray bursts, and that a majority of these bursts should be seen off-axis, that is, they should point away from Earth6,7. Here we report the discovery observations of the X-ray counterpart associated with the gravitational-wave event GW170817. Although the electromagnetic counterpart at optical and infrared frequencies is dominated by the radioactive glow (known as a ‘kilonova’) from freshly synthesized rapid neutron capture (r-process) material in the merger ejecta8,9,10, observations at X-ray and, later, radio frequencies are consistent with a short γ-ray burst viewed off-axis7,11. Our detection of X-ray emission at a location coincident with the kilonova transient provides the missing observational link between short γ-ray bursts and gravitational waves from neutron-star mergers, and gives independent confirmation of the collimated nature of the γ-ray-burst emission.

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Figure 1: Optical/infrared and X-ray images of the counterpart of GW170817.
Figure 2: Optical and infrared spectra of the kilonova associated with GW170817.
Figure 3: Multi-wavelength light curves for the counterpart of GW170817.
Figure 4: Schematic diagram for the geometry of GW170817.

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Acknowledgements

We acknowledge the advice and contribution of N. Gehrels, who was co-investigator of our Chandra and Hubble Space Telescope observing programs. We thank B. Wilkes and the Chandra X-ray Center staff, N. Reid and the Space Telescope Science Institute (STScI) staff, J. Stevens and the CSIRO staff, L. Ferrarese and the Gemini support staff, in particular R. Salinas, M. Andersen, H. Kim, P. Candia and K. Silva. E. Troja thanks Bianca A. Vekstein, A. Bersich and F. Troja for help during the preparation of this manuscript. We thank V. Bajaj (STScI) and S. Hernandez for their assistance with data reduction. Work at LANL was done under the auspices of the National Nuclear Security Administration of the US Department of Energy at Los Alamos National Laboratory (LANL) under contract number DE-AC52-06NA25396. All LANL calculations were performed on LANL Institutional Computing resources. This research used resources provided by the LANL Institutional Computing Program, which is supported by the US Department of Energy National Nuclear Security Administration under contract number DE-AC52-06NA25396. M.I., S.-K.L., J.K., C.C., G.L., and Y.Y. acknowledge support from NRFK grant number 2017R1A3A3001362, funded by the Korean government. Work by C.-U.L. and S.-L.K. was supported by the KASI (Korea Astronomy and Space Science Institute) grant 2017-1-830-03. This research made use of the KMTNet system operated by KASI, and the data were obtained at three Cerro-Tololo Inter-American Observatory host sites in Chile, the South African Astronomical Observatory in South Africa, and the Siding Spring Observatory in Australia. E. Troja acknowledges support from grants GO718062A and HSTG014850001A. R.S.-R. acknowledges support by the Italian Space Agency through contract number 2015-046-R.0 and by the European Union Horizon 2020 Programme under the AHEAD project (grant agreement number 654215). T.S. acknowledges support by MEXT KAKENHI (grant numbers 17H06357 and 17H06362).

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E. Troja, L.P., H.v.E. and O.K. composed the text, with input from all co-authors. E. Troja and T.S. obtained and analysed the Chandra X-ray observations. Hubble Space Telescope observations were obtained, reduced and analysed by E. Troja, O.D.F., R.E.R. Jr and H.G.K. E. Troja, N.R.B., S.B.C., J.B.G. and R.S.-R. obtained, processed and analysed the Gemini data. M.I., C.-U.L., S.-L.K., J.K., C.C., G. L., H.M.L. led the optical imaging with KMTNet. E. Troja, L.P., R.R. and M.H.W. obtained, processed and analysed the Australia Telescope Compact Array observations. R.T.W., O.K., C.L.F. and C.J.F. led the modelling of the kilonova emission. H.v.E., L.P. and E. Troja led the modelling of the GRB and afterglow emission. A.M.W., W.H.L. and J.M.B. contributed to the spectral modelling. M.I., Y.Y. and S.-K.L. led the analysis of the host galaxy. All authors discussed the results and commented on the manuscript.

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Extended data figures and tables

Extended Data Figure 1 Spectral energy distributions of the optical/infrared counterpart.

We can empirically describe the spectral energy distribution and its temporal evolution as the superposition of two blackbody components in linear expansion. A single component provides a good fit at early times (T0 + 0.5 d), but at later times we find that two components (shown by the dashed and dotted lines) with different temperatures and expansion velocities represent a better description of the dataset. The large effective radii (R > 4 × 1014 cm at T0 + 0.5 d) inferred from the blackbody fits imply an average velocity v > 0.2c. Magnitudes are corrected for Galactic extinction along the line of sight51. Data have been shifted for plotting purposes.

Extended Data Figure 2 Models of off-axis afterglows at X-ray and radio energies.

Direct comparison between off-axis light curves for two different jet opening angles θj (15° and 28°). As long as the difference between the viewing angle and the jet angle is maintained, a continuous range of jet angles can be found that is consistent with the observations in X-rays and at radio wavelengths, mostly covering the peak. Dashed lines show light curves computed using the semi-analytic spreading top-hat jet model11 for identical input parameters. Note that the simulated angular fluid profile quickly becomes complex as the jet evolves, and the similarity in light curves to those derived from the top-hat shell illustrate that the global features do not depend strongly on this angular profile. The simulated light curves include synchrotron self-absorption, which was not found to be important for the current parameters. (GW, gravitational wave.)

Extended Data Figure 3 Afterglow modelling for different jet profiles viewed at an angle.

We consider three well known jet profiles: top-hat (dot-dashed line), Gaussian (solid line), and power law (dashed line). A power-law structured jet is not consistent with the lack of afterglow detection at early times. A top-hat jet and a Gaussian structured jet can both describe the afterglow behaviour, and imply a large off-axis angle. The Gaussian jet has the additional advantage of consistently explaining both the prompt γ-rays and the afterglow emission.

Extended Data Figure 4 Kilonova light curves as a function of the viewing angle.

Comparison of the observational data with the synthetic light curves from the two-component axisymmetric radiative transfer model at different viewing angles: 0° (on-axis view); 30°, 60° and 90° (edge-on equatorial view). Our model includes a wind with mass Mw ≈ 0.015M and velocity vw ≈ 0.08c, and dynamical ejecta with mass Mej ≈ 0.002M and velocity vej ≈ 0.2c.

Extended Data Figure 5 Illustrative example of the contamination modelling.

a, Two-dimensional dispersed image at the position of AT 2017gfo. b, Our model describing the emission from NGC 4993, smoothed with a Savitzky–Golay filter in order to remove any high-frequency structure. c, Difference between the data and the model.

Extended Data Figure 6 Broadband spectral energy distribution of NGC 4993.

The model assumes a delayed star-formation rate, standard spectral templates80 and initial mass function81. Models for three different stellar ages are shown. Fluxes are corrected for Galactic extinction along the line of sight51. Vertical error bars are 1σ. Data above 5,000 Å (open circles) are not used in the fit as they may be affected by emission from dust. The modelling of the spectral energy distribution favours a mean stellar age of 3–7 billion years (Gyr) and disfavours ages less than 2 Gyr. The mean stellar mass is found to be in the range of (5–10) × 1010M with a solar metallicity.

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Troja, E., Piro, L., van Eerten, H. et al. The X-ray counterpart to the gravitational-wave event GW170817. Nature 551, 71–74 (2017). https://doi.org/10.1038/nature24290

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