Fast-transient Searches in Real Time with ZTFReST: Identification of Three Optically-discovered Gamma-ray Burst Afterglows and New Constraints on the Kilonova Rate

Abstract: While optical surveys regularly discover slow transients like supernovae on their own, the most common way to discover extragalactic fast transients, fading away in a few nights, is via follow-up observations of gamma-ray burst and gravitational-wave triggers. However, wide-field surveys have the potential to also identify rapidly fading transients independently of such external triggers. The volumetric survey speed of the Zwicky Transient Facility (ZTF) makes it sensitive to faint and fast-fading objects as kilonovae, the optical counterparts to binary neutron stars and neutron star-black hole mergers, out to almost 200Mpc. We introduce an open-source software infrastructure, the ZTF REaltime Search and Triggering, ZTFReST, designed to identify kilonovae and fast optical transients in ZTF data. Using the ZTF alert stream combined with forced photometry, we have implemented automated candidate ranking based on their photometric evolution and fitting to kilonova models. Automated triggering of follow-up systems, such as Las Cumbres Observatory, has also been implemented. In 13 months of science validation, we found several extragalactic fast transients independent of any external trigger (though some counterparts were identified later), including at least one supernova with post-shock cooling emission, two known afterglows with an associated gamma-ray burst, two known afterglows without any known gamma-ray counterpart, and three new fast-declining sources (ZTF20abtxwfx, ZTF20acozryr, and ZTF21aagwbjr) that are likely associated with GRB200817A, GRB201103B, and GRB210204A. However, we have not found any objects which appear to be kilonovae; therefore, we constrain the rate of GW170817-like kilonovae to $R < 900$Gpc$^{-3}$yr$^{-1}$. A framework such as ZTFReST could become a prime tool for kilonova and fast transient discovery with the Vera C. Rubin Observatory.

• The paper introduces ZTFReST as a real-time detection tool for fast optical transients using automated photometric analysis and galaxy catalog cross-matching.
• The study successfully identified three optically-discovered GRB afterglows, showcasing robust performance in detecting rapidly fading extragalactic signals.
• The authors set an upper limit of less than 900 Gpc⁻³ yr⁻¹ for GW170817-like kilonovae, providing new constraints on merger event rates in the local universe.

Overview of Fast-transient Searches with ZTFReST: GRB Afterglow Identification and Kilonova Constraints

The paper explores the search for fast-transient phenomena in real-time using the ZTF Realtime Search and Triggering (ZTFReST) infrastructure. This work primarily focuses on identifying optical counterparts of gamma-ray bursts (GRBs) and constraining the occurrence rate of kilonovae using data from the Zwicky Transient Facility (ZTF). Applying a multi-faceted approach, the authors aim to detect rapidly fading extragalactic transients, particularly GRB afterglows, and establish a boundary on the frequency of kilonova events based on their optically observable traits.

Key Methodologies and Findings

The research introduces ZTFReST, an open-source software suite, designed to pinpoint kilonovae and swift optical transients within ZTF's data stream by leveraging real-time alerts. The software's innovative algorithm assesses candidate transients using photometric evolution, cross-matches with known galaxy catalogs, and triggers further observations based on predefined thresholds. Implemented automated responses allow seamless follow-up observations using facilities like the Las Cumbres Observatory network, advancing the identification process for rapidly declining transients.

Significant results from this systematic search include the independent identification of GRB afterglows, some of which were verified with or without preceding GRB triggers. The authors disclose the discovery of three optically detected GRB afterglows during the study period alongside several fast-declining objects. Notably, the operations have not uncovered any definitive kilonovae, although the methodologies afford a constraint on the rate of GW170817-like kilonovae, estimating an occurrence rate cap at less than 900 Gpc$^{-3}$ yr$^{-1}$.

Implications and Future Directions

The outcomes of ZTFReST's deployment emphasize the potential of wide-field surveys in autonomously identifying fast extragalactic transients without reliance on external triggers. The refined detection capabilities offer new prospects to enhance the understanding of transient astrophysical phenomena, contributing insights into the temporal dynamics and distinct nature of GRB afterglows. Additionally, the lack of confirmed kilonovae, with subsequent upper limit estimations, informs theoretical models on binary neutron star and neutron-star-black hole merger frequencies within the local universe.

The demonstrated efficacy of the software infrastructure paves the way for its potential application with the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). The capability to manage vast data streams, conduct detailed query-based analysis, and initiate immediate follow-up evaluations marks a pivotal advance for the swift-transient discovery framework in upcoming astronomical surveys.

Conclusion

The comprehensive integration of ZTFReST represents a significant stride in transient identification and characterization within time-domain astronomy. This concerted effort broadcasts the feasibility of discovering compelling extragalactic transients systematically, undertaking rigorous steps towards demystifying their astronomical realities. Through the advancement of automated discovery and observational support, the paper delivers a tangible contribution to ongoing explorations within the field of fast optical transients.