Can we constrain the binary neutron star merger time from kilonova light curves?

Abstract

The joint observation of gravitational waves (GWs) from a binary neutron star (BNS) merger (GW170817), together with a short Gamma-Ray Burst (GRB 170817A) and a kilonova (KN, AT2017gfo), marked the birth of multimessenger astronomy with GWs, and demonstrated the informative power of multimessenger observations. More BNS merger and KN detections are expected , thanks to the increasing sensitivity of GW detectors and the advent of new electromagnetic facilities such as the Vera C. Rubin Observatory. However, the association between a BNS merger and a KN might not be trivial: the detection of a KN might not coincide with the onset of the EM emission, making it difficult to define a temporal window to assess the coincidence with a GW event. In this work we investigate the possibility to constrain the BNS merger time from KN light curves, exploring different observing conditions and with focus on the Vera C. Rubin Observatory. We used different KN models and different inference frameworks to simulate and fit KN light curves. We explored different cadences and wavelength coverages for observations, to investigate how the different observing conditions can affect the capability to recover the BNS merger time. The tests performed also include real data related to AT2017gfo, plus a few GRBs that have been associated to possible KN candidates. We find that, under realistic observing conditions with the Vera C. Rubin Observatory, the BNS merger time can be recovered with a precision of 5.5 days (90\% CI) with a best-case scenario with an accuracy of 0.18 day and a precision of 0.74 day (90 \% CI). Results can slightly improve (by $\sim$ 5 \%) if the source redshift is available. Upper limits near the merger time can also improve the results, reducing the width of the posterior by $\sim60$ \%. The BNS merger time estimate could also benefit from EM follow-up observations with other telescopes.