- The paper identifies a radio counterpart 16 days post-merger, offering key insights into potential off-axis jet or cocoon emission models.
- Coordinated observations with VLA, VLITE, ATCA, and GMRT revealed a significant flux density rise to 34 µJy by September 10.
- ISM density estimates below 0.04 cm⁻³ constrain ejecta velocities to above 0.7c, supporting a mildly relativistic jet scenario over classical sGRB origins.
Radio Observation of GW170817: Analyzing Neutron Star Merger Energetics and Environment
The detection of gravitational waves from the binary neutron star merger GW170817, along with its electromagnetic (EM) radiation, represents a significant milestone in multi-messenger astronomy. This paper elucidates the radio counterpart to GW170817, which emerged 16 days post-merger, providing critical insights into the merger's energetics and its surrounding environment. Extensive coordinated efforts utilizing various radio observatories, including the Karl G. Jansky Very Large Array (VLA), the VLA Low Band Ionosphere and Transient Experiment (VLITE), the Australia Telescope Compact Array (ATCA), and the Giant Metrewave Radio Telescope (GMRT), were pivotal in achieving these observations.
Key Findings
- Radio Source Detection: Initial radio observations within 13 hours of the merger provided no significant detection, but a radio counterpart was observed on September 2, increasing in flux density over the following weeks. The VLA data indicated a significant flux density escalation to 34 µJy by September 10.
- Energetics and Models: The observed radio emission may derive from two competing models. It could emanate from a collimated ultra-relativistic jet viewed off-axis or from a mildly relativistic cocoon of ejecta. By examining the angular geometry and velocity metrics, the detailed light curves are anticipated to distinguish these models within 100 days post-merger.
- ISM Density Constraints: Calculations based on the absence of neutral hydrogen detection situated the interstellar medium (ISM) density at less than 0.04 cm−3. This constraining density influenced the inferred velocity of the ejecta, suggesting velocities greater than 0.7c, which aligns with mildly relativistic jets but contradicts typical sub-relativistic ejecta velocities.
- Gamma-ray Burst (GRB) Connection: While GRB170817A, detected shortly after the gravitational wave signal, was initially hypothesized to be the result of a classic sGRB, the observations did not confirm this hypothesis definitively. The low isotropic equivalent luminosity of the gamma-rays detected does not align with typical cosmological sGRB progenitors. Instead, the gamma-rays likely suggest an off-axis observer angle relative to the jet.
Implications and Future Work
The detection of a radio counterpart to GW170817 deepens the understanding of binary neutron star merger dynamics, the origin of sGRBs, and the properties of post-merger jets. Upcoming Very Long Baseline Interferometry (VLBI) observations are projected to provide definitive constraints on the ejecta's nature through direct measurements of the fireball's size and expansion velocity. The paper highlights that distinguishing between an off-axis jet and a cocoon model as the source of the radio emission requires further monitoring of the evolving light curves. This endeavor underlines the benefit of coordinated multi-wavelength follow-up in the wake of gravitational wave detections.
In conclusion, the investigation of GW170817 as a precursor to short gamma-ray bursts underscores the need for integrated strategies combining gravitational, electromagnetic, and radio observations. Enhanced computational models that simulate such mergers may direct future theoretical advances, while planned observational upgrades will undoubtedly refine understanding. This interplay between empirical observation and theoretical modeling symbolizes progress toward unveiling the complexities of relativistic astrophysical phenomena.