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

TL;DR

This study presents deep multi-wavelength observations of GW170817 at 160 days post-merger, revealing a steady brightening and a simple synchrotron spectrum that constrains electron acceleration but not the ejecta's relativistic nature.

Contribution

It provides the most precise measurement of the electron energy distribution index and compares models of ejecta geometry, suggesting both spherical and off-axis jet scenarios fit the data.

Findings

The electron distribution index p=2.17±0.01 was measured.

The emission is consistent with both spherical and off-axis jet models.

The source has been steadily brightening, possibly reaching its peak.

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.