The Binary Neutron Star Event LIGO/Virgo GW170817 160 Days after Merger: Synchrotron Emission across the Electromagnetic Spectrum

By Raffaella Margutti, K. Alexander, Xiaoyi Xie, L. Sironi, B. Metzger, A. Kathirgamaraju, W. Fong, P. Blanchard, E. Berger, A. MacFadyen and 10 more. 2018

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We report deep Chandra X-ray Observatory (CXO), Hubble Space Telescope (HST), and Karl J. Jansky Very Large Array (VLA) observations of the binary neutron star event GW170817 at t < 160 days after merger. These observations show that GW170817 has been steadily brightening with time and might have now reached its peak, and constrain the emission process as non-thermal synchrotron emission where the cooling frequency νc is above the X-ray band and the synchrotron frequency νm is below the radio band. The very simple power-law spectrum extending for eight orders of magnitude in frequency enables the most precise measurement of the index p of the distribution of non-thermal relativistic electrons accelerated by a shock launched by a neutron star (NS)–NS merger to date. We find p = 2.17 ± 0.01, which indicates that radiation from ejecta with Γ ∼ 3–10 dominates the observed emission. While constraining the nature of the emission process, these observations do not constrain the nature of the relativistic ejecta. We employ simulations of explosive outflows launched in NS ejecta clouds to show that the spectral and temporal evolution of the non-thermal emission from GW170817 is consistent with both emission from radially stratified quasi-spherical ejecta traveling at mildly relativistic speeds, and emission from off-axis collimated ejecta characterized by a narrow cone of ultra-relativistic material with slower wings extending to larger angles. In the latter scenario, GW170817 harbored a normal short gamma-ray burst (SGRB) directed away from our line of sight. Observations at t ≤ 200 days are unlikely to settle the debate, as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/c392dd1e5f5389a32cbb5a821e832f29f8a962c1

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In Astronomy and Astrophysics

1. B. Metzger
2. Bing Zhang
3. E. Berger
4. E. Quataert
5. D. Kasen
6. R. Fern'andez
7. Luke F. Roberts
8. Albino Perego

Citations in the corpus

• The Karl G. Jansky Very Large Array Sky Survey (VLASS). Science Case and Survey Design (2019, background): “…with the bulk of the radio emission consistent with a wide-angle outflow of mildly relativistic material (Kasliwal et al. 2017; Mooley et al. 2018b; Margutti et al. 2018; Hotokezaka et al. 2018; Troja et al. 2018), and late-time emission dominated by a narrow energetic jet (Mooley et al. 2018c,d;…”
• The evolution of the X-ray afterglow emission of GW 170817 / GRB 170817A in XMM-Newton observations (2018, methodology): “7−0.8), with negligible spectral evolution over a broadband spectrum (Fν ∝ ν−0.6; Mooley et al. 2018; Margutti et al. 2018; Ruan et al. 2018; Troja et al. 2018a).”
• Binary Neutron Star Mergers: Mass Ejection, Electromagnetic Counterparts, and Nucleosynthesis (2018, background): “A synchrotron remnant has been detected in X-ray, optical, and radio bands for GW170817 (Haggard et al. 2017; Hallinan et al. 2017; Margutti et al. 2017, 2018; Mooley et al. 2018a; Troja et al. 2017; Alexander et al. 2018; Dobie et al. 2018; Lyman et al. 2018; Ruan et al. 2018).”
• Bayesian inference for compact binary coalescences with bilby: validation and application to the first LIGO–Virgo gravitational-wave transient catalogue (2020, background): “…et al. 2019; Dhawan et al. 2020; Chen et al. 2018), insights into the structures of gamma-ray bursts (Abbott et al. 2017f; Mooley et al. 2018; Margutti et al. 2018; Fong et al. 2019; Biscoveanu et al. 2020b), and identifying the origins of heavy elements (Abbott et al. 2017g; Chornock et al.…”
• A Decline in the X-Ray through Radio Emission from GW170817 Continues to Support an Off-axis Structured Jet (2018, methodology): “…for the production of relativistic ejecta in such a system (Alexander et al. 2017; Granot et al. 2017; Haggard et al. 2017; Hallinan et al. 2017; Kasliwal et al. 2017; Margutti et al. 2017, 2018; Murguia-Berthier et al. 2017; Troja et al. 2017; Kathirgamaraju et al. 2018; Mooley et al. 2018b).”
• Long-term GRMHD simulations of neutron star merger accretion discs: implications for electromagnetic counterparts (2018, background): “The non-thermal emission from GW170817 can also be produced by a jet viewed off-axis, which has an angle-dependent Lorentz factor (a ‘structured jet’, e.g. Haggard et al. 2017; Lazzati et al. 2018; Margutti et al. 2018).”