The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. I. Dark Energy Camera Discovery of the Optical Counterpart (1710.05459v1)

Abstract: We present the Dark Energy Camera (DECam) discovery of the optical counterpart of the first binary neutron star merger detected through gravitational wave emission, GW170817. Our observations commenced 10.5 hours post-merger, as soon as the localization region became accessible from Chile. We imaged 70 deg$^2$ in the $i$ and $z$ bands, covering 93\% of the initial integrated localization probability, to a depth necessary to identify likely optical counterparts (e.g., a kilonova). At 11.4 hours post-merger we detected a bright optical transient located $10.6''$ from the nucleus of NGC\,4993 at redshift $z=0.0098$, consistent (for $H_0 = 70$\, km s$^{-1}$ Mpc$^{-1}$) with the distance of $40 \pm 8$\, Mpc reported by the LIGO Scientific Collaboration and the Virgo Collaboration (LVC). At detection the transient had magnitudes $i\approx 17.30$ and $z\approx 17.45$, and thus an absolute magnitude of $M_i = -15.7$, in the luminosity range expected for a kilonova. We identified 1,500 potential transient candidates. Applying simple selection criteria aimed at rejecting background events such as supernovae, we find the transient associated with NGC\,4993 as the only remaining plausible counterpart, and reject chance coincidence at the 99.5\% confidence level. We therefore conclude that the optical counterpart we have identified near NGC\,4993 is associated with GW170817. This discovery ushers in the era of multi-messenger astronomy with gravitational waves, and demonstrates the power of DECam to identify the optical counterparts of gravitational-wave sources.

• The paper demonstrates the rapid identification of the first optical counterpart to the binary neutron star merger GW170817 within 10.5 hours using DECam.
• It details a comprehensive imaging strategy covering 70.4 square degrees and 93% of the localization area in i- and z-bands to detect the kilonova event.
• The findings enable a direct distance measurement via the standard siren approach and significantly advance multi-messenger observations in cosmology.

Electromagnetic Counterpart Discovery of the Binary Neutron Star Merger GW170817

The research paper by Soares-Santos et al. reports on the successful identification of the optical counterpart to the binary neutron star merger event GW170817 using the Dark Energy Camera (DECam). This event stands as a significant observational milestone as it represents the first joint detection of electromagnetic emission alongside gravitational waves for a binary neutron star merger, opening new avenues in multi-messenger astronomy.

Observational Strategy and Findings

Upon receiving the gravitational-wave alert for GW170817 on August 17, 2017, DECam initiated a comprehensive optical search across 70.4 square degrees of the sky. The team achieved this with rapid imaging just 10.5 hours post-merger. This strategy was designed to cover a substantial portion (93%) of the gravitational wave localization area in the i- and z-bands, reaching depths suitable for detecting kilonova events.

Within a matter of hours, the team identified a distinct optical transient in close proximity to the NGC 4993 galaxy. At 11.4 hours after the merger, the transient displayed magnitudes of approximately 17.30 in the i-band and 17.45 in the z-band. The distance to NGC 4993 ($z=0.0098$) aligns with the 40 ± 8 Mpc distance range estimated by the LIGO and Virgo Collaborations for the merger, providing strong support for the association between the optical transient and the gravitational wave event. The authors report a 99.5% confidence level that this discovery is not a chance coincidence.

Implications and Future Directions

The authors detail the importance of this detection as it enables multi-messenger observations—offering unprecedented insights into the physics of such violent cosmological events. Analysis of data from both electromagnetic and gravitational wave detections provides a more complete understanding of the dynamics and energy mechanisms in play during a neutron star merger. This includes insights into the kilonova phenomenon, expected from the radioactive decay of heavy elements formed during the merger, and its light curve characterization, which could influence future kilonova models.

In the broader context of cosmology, the detection of this electromagnetic counterpart establishes a method for direct distance measurement through standard siren approaches, offering an innovative tool for the determination of the Hubble constant independent of traditional electromagnetic distance indicators.

Conclusion

This paper foreshadows the shift towards multi-probe investigations of gravitational wave sources. As LIGO and Virgo detectors undergo advancements and future observing runs, combined with the capabilities of wide-field instruments like DECam, the astronomy community can anticipate a rich collection of data facilitating advancements in both astrophysical insight and cosmological measurements. This discovery marks a pivotal moment in observing cosmic events through multiple channels, underscoring the robustness of collaborative, multi-institutional efforts in unraveling the complexities of the universe.