The Binary Neutron Star event LIGO/VIRGO GW170817 a hundred and sixty days after merger: synchrotron emission across the electromagnetic spectrum (1801.03531v2)

Abstract: We report deep Chandra, HST and VLA observations of the binary neutron star event GW170817 at $t<160$ d 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 $\nu_c$ is above the X-ray band and the synchrotron frequency $\nu_m$ 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 $N(\gamma)\propto \gamma^{-p}$ accelerated by a shock launched by a NS-NS merger to date. We find $p=2.17\pm0.01$, which indicates that radiation from ejecta with $\Gamma\sim3-10$ dominates the observed emission. While constraining the nature of the emission process, these observations do \emph{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, \emph{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 SGRB directed away from our line of sight. Observations at $t\le 200$ days are unlikely to settle the debate as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material.

• The paper presents analysis of CXO, HST, and VLA data up to 160 days post-merger, identifying steadily brightening non-thermal synchrotron emission and determining the power-law index p = 2.17 \"".01.
• The determined value of p indicates the emission originates from relativistic shocks with Lorentz factors \"\"\"\" ~ 3-10, finding consistency with both quasi-spherical and structured jet models without uniquely constraining the geometry.
• This research refines theoretical models of relativistic shocks in BNS mergers and reinforces the importance of multi-messenger astronomy by using electromagnetic observations to complement gravitational wave data.

Examination of Synchrotron Emission in the Binary Neutron Star Event GW170817

The paper "The Binary Neutron Star event LIGO/VIRGO GW170817 a hundred and sixty days after merger: synchrotron emission across the electromagnetic spectrum" by Margutti et al. offers an extensive analysis of the multi-phase observational approach towards understanding the aftermath of the GW170817 event. GW170817 was the first observed instance of a binary neutron star (BNS) merger detected through gravitational waves, complemented by a detailed electromagnetic spectrum detection.

Observational Insights and Methodology

The authors present a detailed examination of the post-merger phase of GW170817 through deep observations using Chandra X-ray Observatory (CXO), Hubble Space Telescope (HST), and the Karl J. Jansky Very Large Array (VLA) for up to 160 days post-merger. The cornerstone of this research is the identification of steadily brightening radiation attributed to non-thermal synchrotron emission, which spans eight orders of magnitude in frequency. This observational strategy involved constraining the cooling frequency, $\nu_c$, to lie above the X-ray band and the synchrotron frequency, $\nu_m$, below the radio band, enabling precise determination of the power-law index $p = 2.17 \pm 0.01$.

Numerical Findings and Implications

The paper delivers robust numerical indicators favoring synchrotron emission as the dominant process, constrained significantly by high-precision observations. The value of $p$ suggests that the synchrotron emission originates from relativistic shocks at speeds corresponding to Lorentz factors in the range of $\Gamma \sim 3-10$. Both quasi-spherical and structured jet models provide consistency with the observed spectral and temporal profiles. Of critical importance is the determination that while the emission process is clearly synchrotron, it doesn't explicitly constrain the geometry of the relativistic ejecta, leaving room for debate between a stratified ejecta mode and a structured jet scenario.

Theoretical and Practical Implications

The research carries significant implications for theoretical models of BNS mergers. The detailed synchrotron model refines our understanding of relativistic shocks and aligns with predictions from trans-relativistic shock models. Practically, the findings from GW170817 set a precedent for using electromagnetic observers to complement gravitational waves, effectively solidifying the multi-messenger approach in astrophysics for future events.

Concluding Thoughts and Future Directions

Margutti et al.'s paper elucidates the critical observation of how non-thermal synchrotron emission in GW170817 holds up as the dominant radiation mechanism post-BNS merger. The results, characterized by a non-evolving spectral index over a significant time span, underscore the need for enhanced simulation models to potentially distinguish between competing hypotheses: quasi-spherical stratified ejecta versus structured ultra-relativistic jets.

As observational capabilities evolve, especially in the domain of higher precision and wider spectral range, future studies will likely carry the torch further in dissecting the fundamental nature of BNS mergers. Additionally, the continued convergence of observational data and theoretical models will refine our grasp of the dynamics governing such massive cosmic phenomena. Subsequent research may leverage the lessons from GW170817 to explore the prospects of more directly probing the internal and external dynamics of BNS mergers in a broader astrophysical context.