Multi-messenger Observations of a Binary Neutron Star Merger (1710.05833v2)

Abstract: On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of $\sim$1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg$^2$ at a luminosity distance of $40^{+8}_{-8}$ Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Msun. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at $\sim$40 Mpc) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over $\sim$10 days. Following early non-detections, X-ray and radio emission were discovered at the transient's position $\sim$9 and $\sim$16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. (Abridged)

• The paper demonstrates the detection of GW170817, uniting gravitational and electromagnetic signals from a binary neutron star merger.
• It details advanced LIGO/Virgo observations that pinpointed the event within a 31-square-degree area at about 40 Mpc distance.
• The analysis confirms gamma-ray burst and kilonova associations, bolstering insights into heavy element synthesis and cosmic distance measurements.

Observations of the Binary Neutron Star Merger GW170817

The research article titled "Multi-messenger Observations of a Binary Neutron Star Merger" provides a comprehensive account of the first-ever observed binary neutron star merger, identified as GW170817. This momentous event was detected on 2017 August 17 at 12:41:04 UTC through gravitational waves by the Advanced LIGO and Advanced Virgo detectors, and subsequently observed across the electromagnetic spectrum by numerous astronomical facilities worldwide.

Key Discoveries and Observations

The detection of GW170817 marked the first time both gravitational and electromagnetic signals were observed from a single astrophysical event. The gravitational-wave signal from coalescing neutron stars allowed initial localization to a region of 31 square degrees at a luminosity distance of approximately 40 Mpc. Nested within this discovery were also electromagnetic phenomena—most notably, the identification of a gamma-ray burst (GRB 170817A) and the ensuing kilonova.

1. Gamma-ray Burst Detection: The Fermi Gamma-ray Burst Monitor (GBM) detected a short gamma-ray burst (S-GRB) within 1.7 seconds after the merger time. This observation substantiated theoretical predictions that neutron star mergers are progenitors of some S-GRBs.
2. Optical and Infrared Counterparts: Subsequent multi-wavelength observations led to the discovery of a kilonova (AT 2017gfo/SSS17a) in the optical, which evolved redward over ten days. This was explained by the synthesis and radioactive decay of heavy nuclei through rapid neutron capture (r-process), a process previously theorized as a source of kilonovae.
3. X-Ray and Radio Emission: X-ray and radio counterparts were detected 9 and 16 days post-merger, respectively, suggesting extended interactions between the ejected material and the surrounding medium.

Implications for Astrophysics

The observations of GW170817 have significant implications for our understanding of cosmic events:

• Astrophysical Synthesis of Heavy Elements: Observational evidence from the kilonova firmly supports the hypothesis that binary neutron star mergers are sites for the synthesis of heavy elements—such as gold and platinum—through the r-process.
• Neutron Star Equation of State: The precise measurement of neutron star masses and the gravitational wave signal contribute constraints on the equation of state, which defines the behavior of matter at nuclear densities.
• Cosmic Distance Measurement: The independently measured gravitational-wave luminosity distance provides a novel method for measuring extragalactic distances, thereby offering a potential ‘standard siren’ for cosmological measurements.

Future Prospects

The collaborative observing strategies employed for GW170817 set a new precedent for multi-messenger astrophysics. Future gravitational-wave detections will likely rely on these established methodologies to generate comprehensive datasets that span multiple messenger types, from gravitational waves to the full electromagnetic spectrum. As the LIGO and Virgo detectors enhance their sensitivities, additional neutron star mergers are anticipated, promising further insights into neutron star physics, nucleosynthesis, and the cosmic distance scale. The successful synergy between diverse observatories demonstrates the potential for further revelations in transient astrophysics, encouraging continued investment in multi-messenger observation networks.

These findings present profound opportunities for future research, highlighting the critical interdependence of gravitational-wave astrophysics and traditional electromagnetic astronomy to unravel the complexities of our universe.