Swope Supernova Survey 2017a (SSS17a), the Optical Counterpart to a Gravitational Wave Source

Abstract: On 2017 August 17, the Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo interferometer detected gravitational waves emanating from a binary neutron star merger, GW170817. Nearly simultaneously, the Fermi and INTEGRAL telescopes detected a gamma-ray transient, GRB 170817A. 10.9 hours after the gravitational wave trigger, we discovered a transient and fading optical source, Swope Supernova Survey 2017a (SSS17a), coincident with GW170817. SSS17a is located in NGC 4993, an S0 galaxy at a distance of 40 megaparsecs. The precise location of GW170817 provides an opportunity to probe the nature of these cataclysmic events by combining electromagnetic and gravitational-wave observations.

• The paper establishes SSS17a as the fading optical transient linked to GW170817 through timely, targeted galaxy observations.
• It details the use of prioritized galaxy catalogs and optimized imaging strategies to efficiently identify electromagnetic counterparts.
• The analysis supports neutron star merger models and r-process nucleosynthesis, advancing the integration of multimessenger astronomy.

Analysis of the Optical Counterpart to a Gravitational Wave Source: SSS17a

The paper presents a detailed examination of the discovery and analysis of SSS17a, the optical counterpart to the gravitational wave event GW170817. This detection was significant as it marked a new era in multimessenger astronomy by combining gravitational wave and electromagnetic (EM) observations.

On August 17, 2017, LIGO and Virgo detected gravitational waves from a binary neutron star (BNS) merger, designated GW170817. Concurrently, the Fermi and INTEGRAL telescopes registered a gamma-ray burst labeled GRB170817A, indicating a potential association with the GW event. Approximately 10.9 hours following the gravitational wave trigger, a fading optical transient was identified using the Swope telescope at Las Campanas Observatory. This source, named Swope Supernova Survey 2017a (SSS17a), was located within the galaxy NGC 4993, approximately 40 Mpc away.

Research Methodology and Observational Strategy

The research team employed a targeted galaxy approach based on gravitational wave localization. Their strategy prioritized observations of galaxies cataloged as potential hosts for EM counterparts of GW events within the localization region, favoring those with significant stellar mass and star-formation rates. The team utilized the Swope telescope, with its capability to cover specific sky regions efficiently, to maximize observational yield.

Initial challenges included the southern hemisphere location and nearness to the Sun of the GW170817 event. The Swope telescope initiated observations after nautical twilight, about ten hours post trigger, using an $i$-band filter due to expected red kilonova emission. The localization process employed galaxy prioritization algorithms, accounting for factors such as distance and potential overlap of galaxies within the field of view. This method proved efficient, allowing for the concurrent imaging of multiple candidate galaxies, optimizing search efforts in limited time windows dictated by celestial mechanics.

Discovery of SSS17a

SSS17a was identified as a transient, rapidly fading optical source in images obtained from the Swope telescope. Located at an offset of 10.6" from NGC 4993's core, the data suggest an association with the merger event. Contrary to expectations from BBH mergers which lack EM signatures, BNS mergers can produce observable kilonova emissions resulting from radioactive decay in ejected matter. The detected $i$-band magnitude was $17.476 \pm 0.018$, which, at the calculated distance, provided critical data to infer the properties of the kilonova emission.

Implications and Further Observations

The establishment of SSS17a's connection to GW170817 has far-reaching implications. It supports models predicting that BNS mergers can generate large quantities of neutron-rich material which synthesize heavy elements, contributing to our understanding of r-process nucleosynthesis pathways. Moreover, the event allowed for the constraining of the neutron star equation of state, crucial for understanding these dense objects' interior structure.

The research team's methodology and analysis demonstrated the feasibility of targeted galaxy searches with existing, even relatively modest, astronomical resources, underlining the value of strategic planning and prioritization in transient astronomy. This balance between aperture, field of view, and informed priority setting is necessary to capitalize on forthcoming GW events, especially considering future detections with potentially broader localization regions or less luminous sources.

Conclusion

The interplay of gravitational wave and optical observations, exemplified by the discovery of SSS17a, represents an important milestone in astrophysics. It opens new possibilities for comprehensive analyses of cosmic phenomena, fostering a more integrated approach to understanding the fundamental processes governing the universe. Future developments in interferometer sensitivity and rapid-response telescopic capabilities will further enhance our capability to explore these cataclysmic events, providing insight into the dynamic and interconnected nature of astronomical phenomena.