What is the current understanding of the results of the merger associated with GW190425? Black hole? Neutron star? Something else?

Question

In an exhibit on gravitational waves I saw a display of events. Each event was in a box, two circles at the top represent the orbiting objects before merger, a single circle below them represent the "best guess" resulting object.

The sizes (and numbers next to them) represent the masses of each, and the color (black or blue/gray) indicate assignment as a black hole or a neutron star. In cases where there is substantial uncertainty or near-even probability, the circle is divided in half, one side black the other blue/gray.

I noticed that for the GW190425 event the two initial objects are 2.0 and 1.4 solar masses and shown as neutron stars, and the final object at 3.2 solar masses is shown with both black and blue/gray halves, indicating that the assignment wasn't made.

I looked at Abbott et al. (2020) GW190425: Observation of a Compact Binary Coalescence with Total Mass ∼ 3.4 M⊙ which as far as I can tell doesn't say anything about the nature of the final object or even show a strain waveform.

However this Max Planck Institute for Gravitational Physics (Albert Einstein Institute) page Simulation of the neutron star coalescence GW190425 says:

Due to the large total mass of GW190425, the two stars form a black hole shortly after the merger, in contrast to GW170817. In addition, the mass of the ejecta matter and the debris disk mass is significantly smaller than for GW170817, which makes a detection of an electromagnetic counterpart very unlikely and might explain that no electromagnetic follow-up observation has been successfully detected an electromagnetic transient.

which seems fairly conclusive, though I can't yet figure out why they think so or find a source.

Question: What is the current understanding of the results of the merger associated with GW190425? Black hole? Neutron star? Something else?

Wasn't there some way to analyze the ringdown in order to identify whether the result was a neutron star or a black hole?

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For reference only: this is what got me curious enough to ask the question.

Answer 1

GW190425 was the first event to be confidently detected based on data from a single detector. This means that the localization on the Earth's sky was likely not as good as if multiple detectors observed it. As such, no electromagnetic counterpart was observed for this merger.

Although LIGO/Virgo could not rule out that one or both components were black holes, it is unlikely, theoretically speaking, that the components were black holes because their masses are well below the expected lower limit of the mass gap between neutron stars and black holes. However, the existence of such a mass gap is uncertain (e.g., see this).

Officially, the remnant of this merger is considered to be a black hole, as the mass of the remnant is too high to likely be a long-lived neutron star. More technically, the merger formed a neutron star very briefly which collapsed promptly into a black hole. The original analysis by LIGO/Virgo found a >95% probability for a prompt collapse into a black hole (see section F.6 of this).

The source-frame chirp mass and total mass reported by LIGO/Virgo are larger than those already known of binary neutron stars in the galaxy. If one considers many possible formation scenarios involving neutron-star binaries and black-hole neutron-star binaries, then it's most likely that this event resulted in a black hole remnant, and such models imply that if there was an electromagnetic counterpart it would have been redder and fainter than the counterpart of GW170817 meaning that electromagnetic observations may have missed the counterpart. However, high mass neutron star binaries are not generally expected to produce observable EM counterparts.

Assuming that the sky was searched for an EM counterpart with sufficient accuracy and there was no kilonova to be seen, numerical relativity simulations can be useful for constraining the mass ratio of binary neutron star mergers.

Regarding your question about using ringdown to constrain the type of compact remnant, the ringdown signal for BNS mergers is at frequencies of several kHz and is not easily measurable by current LIGO/Virgo detectors. The original analysis of LIGO/Virgo attempted an unmodeled search (meaning without assuming a particular type of source) for the postmerger and found no statistically significant signal (see section F.7 of this). Future detectors, such as 3G detectors, will more easily access the kHz regime, where tidal deformability in the ringdown may be observed for neutron star remnants and may be used to determine the type of remnant.

Another, very uncertain and not commonly accepted, alternative is that the compact remnant of this merger was a strange quark star, but this depends on the very uncertain QCD models for the equations of state of different types of strange quark stars, and depends on how one converts b/w baryonic and gravitational mass for these uncertain stars. See this review for other exotic explanations.