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, which fade within a few nights in the optical, 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, including counterparts to multimessenger sources, independently of such external triggers. The volumetric survey speed of the Zwicky Transient Facility (ZTF), in particular, makes the survey sensitive to objects that are as faint and fast-fading as kilonovae, the optical counterparts to binary neutron stars and neutron star-black hole mergers, out to almost 200 Mpc. In this paper, we introduce an open-source software infrastructure, the ZTF REaltime Search and Triggering, ZTFReST, which is designed to identify kilonovae and fast optical transients in ZTF data. Using the ZTF alert stream combined with forced point spread function 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, for sources that pass user-defined thresholds, 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-

Cosmological Fast Optical Transients with the Zwicky Transient Facility: A Search for Dirty Fireballs

“…Due to the rapid fade rate of 2.15 ± 0.14 mag d −1 in r-band, the extinction-corrected red color (g − r = 0.42 ± 0.19 mag), and the presence of a faint (g = 24.6 mag, r = 23.7 mag) counterpart (putative host galaxy) at the transient position in Legacy Survey imaging, the transient was flagged by the daily scanner on February 05.7. This source was independently discovered by ZTFReST, and the ZTF photometry was presented in Andreoni et al (2021).…”

“…Every night, the ZTF alert stream is filtered by several independent pipelines to identify young or fast-evolving transients, such as ZTFReST (Andreoni et al 2020(Andreoni et al , 2021. In this paper we focus on events discovered via the approach described in Ho et al (2020b) and Perley et al (2021c).…”

“…) was identified in follow-up observations to a gravitational-wave trigger (Kasliwal et al 2020). Two events (AT 2020sev and AT 2020yxz) were identified exclusively by ZTFReST and published in Andreoni et al (2021). Table 1 includes events with either a likely detected GRB counterpart, or confirmation of relativistic ejecta from a redshift measurement or radio observations; it does not include some candidate extragalactic fast transients such as ZTF19aanhtzz (AT 2019aacu; Andreoni et al 2020).…”

“…The fast rise, rapid intra-night fading, and lack of detected host in deep imaging led the transient to be flagged by the daily scanner on 2020 May 24.8. ZTF 20abbiixp also passed the ZTFReST search criteria during their science validation (Andreoni et al 2021).…”

“…As we discuss in this paper, ten afterglows have been discovered to date using ZTF data, five from the year 2021 alone. Most were identified by human scanners within 12 hours of the first ZTF detection, through dedicated searches for fast optical transients that make use of the transient's rise rate, fade rate, color, and contextual information such as the host galaxy (Ho et al 2020b;Andreoni et al 2020Andreoni et al , 2021. Fast-turnaround spectroscopy, X-ray, and radio observations established their resemblance to GRB afterglows, with redshift measurements for eight events spanning z = 0.876 to z = 2.9.…”