The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-Time Emission from the Kilonova Ejecta (1710.05457v1)

Abstract: We present Very Large Array (VLA) and Atacama Large Millimeter/sub-millimeter Array ALMA radio observations of GW\,170817, the first Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo gravitational wave (GW) event from a binary neutron star merger and the first GW event with an electromagnetic (EM) counterpart. Our data include the first observations following the discovery of the optical transient at both the centimeter ($13.7$ hours post merger) and millimeter ($2.41$ days post merger) bands. We detect faint emission at 6 GHz at 19.47 and 39.23 days after the merger, but not in an earlier observation at 2.46 d. We do not detect cm/mm emission at the position of the optical counterpart at frequencies of 10-97.5 GHz at times ranging from 0.6 to 30 days post merger, ruling out an on-axis short gamma-ray burst (SGRB) for energies $\gtrsim 10^{48}$ erg. For fiducial SGRB parameters, our limits require an observer viewer angle of $\gtrsim 20^{\circ}$. The radio and X-ray data can be jointly explained as the afterglow emission from an SGRB with a jet energy of $\sim 10^{49}-10^{50}$ erg that exploded in a uniform density environment with $n\sim 10^{-4}-10^{-2}$ cm$^{-3}$, viewed at an angle of $\sim 20^{\circ}-40^{\circ}$ from the jet axis. Using the results of our light curve and spectral modeling, in conjunction with the inference of the circumbinary density, we predict the emergence of late-time radio emission from the deceleration of the kilonova (KN) ejecta on a timescale of $\sim 5-10$ years that will remain detectable for decades with next-generation radio facilities, making GW\,170817 a compelling target for long-term radio monitoring.

• The paper demonstrates that early radio data refutes an on-axis SGRB, favoring an off-axis jet with energies of 10⁴⁹–10⁵⁰ erg and a viewing angle of 20–40°.
• It uses combined VLA, ALMA, and X-ray observations to constrain jet dynamics and ambient density in the range of 10⁻⁴–10⁻² cm⁻³.
• The study forecasts detectable radio emissions from the kilonova ejecta within 5–10 years, emphasizing the promise of next-generation radio facilities.

Overview of "The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817: Radio Constraints on a Relativistic Jet and Predictions for Late-Time Emission from the Kilonova Ejecta"

This paper presents an analysis of radio observations conducted on the binary neutron star merger event GW170817. As the first gravitational wave event detected with an electromagnetic (EM) counterpart, it offers unique insights into the behavior of such extreme cosmic occurrences. The researchers utilize the Very Large Array (VLA) and Atacama Large Millimeter/sub-millimeter Array (ALMA) to investigate the radio emission from this event, emphasizing the afterglow from a relativistic jet and forecasting the characteristics of long-term radio emissions from the kilonova ejecta.

Key Findings and Methodology

The paper focuses on radio observations taken at various frequencies following the merger event. Notably, the researchers failed to detect cm/mm emission immediately after the merger but observed faint emissions at 6 GHz at 19.47 and 39.23 days post-merger. This observational data refutes the hypothesis of an on-axis short gamma-ray burst (SGRB) with energies greater than approximately $10^{48}$ erg, indicating an observer viewing angle of at least 20 degrees. Such observations align with an off-axis jet model, characterized by a jet energy of $10^{49}-10^{50}$ erg in a uniform density environment ($n\sim 10^{-4}-10^{-2}$ cm$^3$), and a viewing angle between 20 and 40 degrees.

The authors integrate radio and X-ray data to model the afterglow, employing constraints from these observations to delineate the possible jet angles and circumbinary densities. Their analysis suggests that an on-axis jet with typical SGRB parameters is inconsistent with observed data, while off-axis models comfortably comply with both radio and X-ray observational constraints.

Predictions for Future Radio Emission

Beyond immediate post-merger emissions, the paper theorizes about future radio wave phenomena stemming from the deceleration of the kilonova ejecta. Modeling this emission suggests radio detectability from the blue kilonova component within 5-10 years post-merger. Given the kinetic energy and expected timing, this research identifies next-generation radio facilities as pivotal in monitoring these evolutionary emissions.

Implications and Speculation

The results present significant implications for understanding the physics of binary neutron star mergers. Confirming the presence of off-axis relativistic jets enriches the models of SGRB initiations and provides direct observational evidence supporting the formation of such jets in neutron star mergers. Additionally, the paper highlights the influence of low circumbinary densities, frequently associated with elliptical host galaxies, on the propagation and detectability of merger afterglows.

Future developments in radio astronomy, particularly the increased sensitivity of next-generation instruments, promise substantial advancements in EM-GW astrophysics. Persistent monitoring of GW170817 and similar events will likely refine our understanding of post-merger dynamics, bridge observational gaps across EM spectra, and further elucidate the cosmic roles of neutron stars.

Conclusion

Alexander et al. systematically address the electromagnetic implications arising from the landmark GW170817 event, offering comprehensive observations and modeling to chart the expected behavior of neutron star mergers in radio frequencies. Their predictive framework for long-term kilonova ejecta emissions positions the current and forthcoming radio facilities to play a crucial role in the continued exploration of multi-messenger astronomy.