According to Capano and colleagues, you cant distinguish between neutron stars and black holes in compact binary mergers with the help of current LIGO/ Virgo detectors and the one which were claimed by A+ and Voyager is dubious. Their study recently appeared in Arxiv.
In August 2017, the first detection of a binary neutron star merger, GW170817, made it possible to study neutron stars in compact binary systems using gravitational waves. Despite being the loudest (in terms of signal-to-noise ratio) gravitational wave detected to date, it was not possible to unequivocally determine that GW170817 was caused by the merger of two neutron stars instead of two black holes from the gravitationalwave data alone. That distinction was largely due to the accompanying electromagnetic counterpart. This raises the question: under what circumstances can gravitational-wave data alone, in the absence of an electromagnetic signal, be used to distinguish between different types of mergers?
Now, Capano and colleagues studied whether a neutron-star– black-hole binary merger can be distinguished from a binary black hole merger using gravitational-wave data alone. They build on earlier results using chiral effective field theory to explore whether the data from LIGO and Virgo, LIGO A+, LIGO Voyager, or Cosmic Explorer could lead to such a distinction.
The results suggested that the present LIGO-Virgo detector network will most likely be unable to distinguish between these systems even with the planned near-term upgrades. They also mentioned that, the success of A+ and Voyager is “dubious”.
They also look at the ln Bayes factor (loge B) between two models and found that, for decisive evidence, loge B ≥ 10 is required. In addition, they noted that the cases with the highest ln Bayes factor always occur with the stiff equation of state, the 5M black hole companion, and at 40Mpc.
“The highest ln Bayes factor occurs for black hole mass of 5M, even though systems with mass of black holes 10, 15, 20M have higher signal-to-noise ratios. The tidal effects decrease as mass increases and this effect is clearly of greater importance than the increase in signal strength.”
Moreover, it has been shown that, when the companion mass increases to even 10M, the Bayes factor drops rapidly regardless of distance or equation of state for the current detectors. If the equation of state is as soft as the analysis of GW170817 suggests, then LIGO Voyager will certainly be unable to distinguish neutron-star–black-hole systems from binary black holes no matter how close or loud the signal is.
Finally, they concluded that in order to obtain decisive evidence of neutron star-black hole from Gravitational wave data, third-generation instruments such as Cosmic Explorer will be required.
Reference: Stephanie M. Brown, Collin D. Capano, Badri Krishnan, “Using gravitational waves to distinguish between neutron stars and black holes in compact binary mergers”, Arxiv, pp. 1-9, 2021. https://arxiv.org/abs/2105.03485
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