SIMBAD references

The LIGO/Virgo collaborations recently announced the detection of a binary neutron star merger, GW190425. The mass of GW190425 is significantly larger than the masses of Galactic double neutron stars known through radio astronomy. We hypothesize that GW190425 formed differently from Galactic double neutron stars, via unstable 'case BB' mass transfer. According to this hypothesis, the progenitor of GW190425 was a binary consisting of a neutron star and a ∼4-5 M_☉ helium star, which underwent common-envelope evolution. Following the supernova of the helium star, an eccentric double neutron star was formed, which merged in ≲10 Myr. The helium star progenitor may explain the unusually large mass of GW190425, while the short time to merger may explain why similar systems are not observed in radio. To test this hypothesis, we measure the eccentricity of GW190425 using publicly available LIGO/Virgo data. We constrain the eccentricity at 10 Hz to be e <= 0.007 with 90 per cent confidence. This provides no evidence for or against the unstable mass transfer scenario, because the binary is likely to have circularized to e <= 10^–4 by the time it was detected. Future detectors will help to reveal the formation channel of mergers similar to GW190425 using eccentricity measurements.