Constraining the Kilonova Rate with Zwicky Transient Facility Searches Independent of Gravitational Wave and Short Gamma-Ray Burst Triggers

Abstract

The first binary neutron star merger, GW170817, was accompanied by a radioactivity-powered optical/infrared transient called a kilonova. To date, no compelling kilonova has been found during optical surveys of the sky, independent of gravitational-wave triggers. In this work, we searched the first 23 months of the Zwicky Transient Facility (ZTF) data stream for candidate kilonovae in the form of rapidly evolving transients. We combined ZTF alert queries with forced point-spread-function photometry and nightly flux stacking to increase our sensitivity to faint and fast transients. Automatic queries yielded > 11, 200 candidates, 24 of which passed quality checks and strict selection criteria based on a grid of kilonova models tailored for both binary neutron star and neutron star-black hole mergers. None of the candidates in our sample was deemed a possible kilonova after thorough vetting, catalog cross-matching, and study of their color evolution. The sources that passed our selection criteria are dominated by Galactic cataclysmic variables. In addition, we identified two fast transients at high Galactic latitude, one of which is the confirmed afterglow of long-duration GRB 190106A, and the other is a possible cosmological afterglow. Using a survey simulation code, we constrained the kilonova rate for a range of models including top-hat and linearly decaying light curves and synthetic light curves obtained with radiative transfer simulations. For prototypical GW170817-like kilonovae, we constrain the rate to be R < 1775 Gpc −3 yr −1 at 95% confidence level by requiring at least 2 high-significance detections. By assuming a population of kilonovae with the same geometry and com

Prospects for kilonova signals in the gravitational-wave era

“…Recently, an archival study searching for kilonovae in 23 months of ZTF data was carried out. Down to a limiting magnitude of r lim ∼ 20.5 for source detection, no transient consistent with being a kilonova was identified (Andreoni et al 2020). Considering that a kilonova can be safely detected and characterized only if its peak magnitude is at least one magnitude brighter than the limit of the survey, Fig.…”

“…Beyond the kilonova model uncertainties, an overestimated rate of mergers or the limitations of the kilonova identification algorithm as discussed in Andreoni et al (2020) could also contribute to the non-detection. Future surveys and archival studies by other optical facilities (Almualla et al 2021;Setzer et al 2019) should clarify which of these options is the most likely.…”

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

“…Survey data also provide us with a much larger number of images to mine, which is technically challenging, but at the same time could offer a broad range of discovery opportunities. Serendipitous searches of this type have already been successful in the cases of GRB afterglows (Cenko et al 2015;Stalder et al 2017;Ho et al 2018;Andreoni et al 2020b;Kasliwal et al 2020) and afterglows with no GRB detected (Cenko et al 2013;Ho et al 2020c). Multi-facility programs such as the "Deeper, Wider, Faster" program (Andreoni et al 2020c, Cooke et al, in preparation) aim discovering counterparts to fast radio bursts and other elusive transients via simultaneous multi-facility observations at many different wavelengths.…”

“…In Andreoni et al (2020b), we presented kilonova rate constraints from archival searches of serendipitous observations from March 2018 to February 2020. These observations were during ZTF Phase I, which covered March 2018 to September 2020; ZTF Phase II has been ongoing since then.…”

The potential role of binary neutron star merger afterglows in multimessenger cosmology

“…Magnitude and localization are, however, not the only conditions for detection. As recent searches for kilonova signals both during GW follow-up campaigns (Kasliwal et al 2020) and in archival data (Andreoni et al 2020) have shown, difficulties can arise in recognizing kilonovae among a myriad of optical transients, even with quality spectroscopic or color evolution observations. Detection of the kilonova counterpart and thereby acquiring the system's redshift through its host galaxy is the minimal scenario required for a multimessenger measurement of H 0 .…”