Distinguishing the nature of ''ambiguous'' merging systems hosting a neutron star: GW190425 in low-latency

Abstract

GW190425 is the newly discovered gravitational wave (GW) source consistent with a neutron star-neutron star merger with chirp mass of $1.44\pm 0.02M_{\odot }.$ This value falls in the $ambiguous$ interval as from the GW signal alone we can not rule out the presence of a black hole in the binary. In this case, the system would host a neutron star and a very light stellar black hole, with mass close to the maximum value for neutron stars, filling the $mass$ $gap$. No electromagnetic counterpart is firmly associated with this event, due to the poorly informative sky localisation and larger distance, compared to GW/GRB170817. We construct here kilonova light curve models for GW190425, for both double neutron star and black hole-neutron star systems, considering two equations of state consistent with current constraints from the signals of GW170817/GW190425 and the NICER results, including black hole spin effects and assuming a new formula for the mass of the ejecta. The putative presence of a light black hole in GW190425 would have produced a brighter kilonova emission compared to the double neutron star case, letting us to distinguish the nature of the companion to the neutron star. Concerning candidate counterparts of GW190425, classified later on as supernovae, our models could have discarded two transients detected in their early $r$-band evolution. Combining the chirp mass and luminosity distance information from the GW signal with a library of kilonova light curves helps identifying the electromagnetic counterpart early on. We remark that the release in low latency of the chirp mass in this interval of $ambiguous$ values appears to be vital for successful electromagnetic follow-ups.

14 pages, 13 figures, submitted to A&A, version 2

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