Does GW190425 require an alternative formation pathway than a fast-merging channel?

Abstract

The LIGO/Virgo Scientific Collaboration (LVC) recently announced the detection of a compact object binary merger, GW190425, with a total mass of $3.4_{-0.1}^{+0.3}$ M${}_{\odot }$, and individual component masses in the range of about 1.1 to 2.5 $M_{\odot }$. If the constituent compact objects are neutron stars, then the total mass is five standard deviations higher than the mean of $2.66\pm 0.12$ M${}_{\odot }$ for Galactic binary neutron stars (BNSs). LVC suggests such massive BNS systems are born from a fast-merging channel, and therefore, their non-detection in the Galaxy to be due to a selection effect. However, we are unable to reconcile the inferred formation efficiency from the reported merger rate, ${\mathcal{R}}_{GW190425}=460_{-390}^{+1050}$ yr${}^{-1}$ Gpc${}^{-3}$, with predictions from our own study for fast-merging BNS systems. Moreover, the comparable merger rates of GW190425, and GW170817 are possibly in tension with our results for two reasons: (i) more massive systems are expected to have a lower formation rate, and (ii) fast merging channels should constitute $\lesssim 10\%$ of the total BNS systems if case BB unstable mass transfer is permitted to take place as a formation pathway. We argue that to account for the high merger rate of GW190425 as a BNS system requires: (i) revisiting our understanding of NS formation in supernova explosions, or (ii): that more massive NSs need to be preferentially born with either very weak or very high magnetic fields so that they would be undetectable in the radio surveys. Perhaps massive NSs detected in NS-white dwarf binaries are our clues to the formation path of GW190425 systems.

Accepted for publication in ApJ

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