Exploring the nature of ambiguous merging systems: GW190425 in low latency

GW190425 is a recently discovered gravitational wave (GW) source whose individual binary components are consistent with being neutron stars (NSs). However, the source-frame chirp mass 1.44 ± 0.02 M· is larger than that of any double NS system known as yet, and it falls in the 'ambiguous' interval for which the presence of a black hole (BH) cannot be ruled out from the GW signal analysis alone. GW190425 might host an NS and a light BH, with a mass in the so-called lower mass gap. No electromagnetic (EM) counterpart has been associated with this event, due to the poorly informative sky localisation and larger distance compared to GW170817. We construct kilonova (KN) light curve models for GW190425, in both the double NS and BH-NS scenarios, considering two equations of state (EoSs) consistent with current constraints from GW170817 and the NICER results, including BH spin effects, and testing different fitting formulae for the ejecta mass. According to our models, the putative presence of a light BH in GW190425 would have produced a brighter KN emission compared to the double NS case, ideally leading to the possibility of distinguishing the nature of the binary. However, depending on the adopted fitting formula for the ejecta, the feasibility of this distinction might depend on the EoS and on the BH spin. Concerning candidate counterparts of GW190425, classified later on as supernovae, our models could have been used to discard two transients detected in their early r-band evolution, as these fall outside the phase space encompassed by our models. We conclude that combining the chirp mass and distance information from the GW signal with a library of KN light curves can help in identifying the EM counterpart early on, and we stress that the low-latency release of the chirp mass in this interval of ambiguous values can be vital for successful EM follow-ups.