ASAS-SN Rates IV: Constraints on the Kilonova Rate

TL;DR

This study uses 11 years of ASAS-SN data to constrain the local rate of bright kilonovae, setting an upper limit that informs astrophysical models despite no detections.

Contribution

First to place a stringent upper limit on bright kilonova rates using a comprehensive, high-cadence all-sky survey over a decade.

Findings

No kilonovae detected in the survey period.

Established a 2σ upper limit of 4400 Gpc⁻³ yr⁻¹ on kilonova rate.

Constraint is competitive with other electromagnetic surveys.

Abstract

Kilonovae (KNe) are the electromagnetic signatures of neutron star mergers and are likely the dominant site of cosmic $r$-process nucleosynthesis. However, their intrinsic rate remains poorly constrained due to a paucity of confirmed events. We use the All-Sky Automated Survey for Supernovae (ASAS-SN) to place limits on the rate of bright, nearby KNe over an 11-year baseline ranging from 2014 to 2024. To evaluate the survey's completeness for KNe, we employ an injection-recovery simulation using a shock-cooling cocoon model calibrated to the early blue emission of the only well-sampled KN, SSS17a (AT 2017gfo). Finding no KNe within the survey, we calculate a $2\sigma$ ($\sim95\%$) upper limit on the local volumetric KN rate of $R_{\mathrm{KN}} < 4400\,\mathrm{yr}^{-1}\,\mathrm{Gpc}^{-3}$. Despite ASAS-SN's shallower limiting magnitude compared to other time-domain searches, its continuous, high-cadence, all-sky monitoring yields a constraint that is competitive with the strongest results from electromagnetic surveys but remains a factor of 18 higher than the LIGO-Virgo-KAGRA GWTC-4 estimate of the binary neutron star merger rate.