Distinguishing Neutron-Star Mergers from Black Hole Mergers
The gravitational waves launched during the merger of two neutron stars or black holes encode the pair’s masses. Usually, inferred masses suffice to distinguish neutron stars from black holes in mergers, but their respective theoretical mass ranges, while not overlapping, are uncertain and close enough for ambiguity to arise. In a merger, a potentially disambiguating signal comes from the neutron stars’ finite sizes, which alter gravitational waves through tidal effects. Unfortunately, tidal signatures are often too weak to be detected directly. Now Michael Müller of the University of Greifswald in Germany and Reed Essick of the University of Toronto have shown that tidal effects can be inferred statistically from a large enough sample of merger events [1].
Tidal effects are determined by a neutron star’s mass and radius, which are interrelated via the equation of state (EOS). Müller and Essick’s starting point was an expression for a merger’s tidal signature, which depends on the two masses and, ultimately, on the EOS. To demonstrate the efficacy of the approach, the researchers took as given the populations of neutron stars and black holes as well as their merger rates. The adjustable parameters included the number of events in the catalog and the sensitivity of the gravitational-wave detector.
By analyzing the statistics of their synthetic merger catalogs, Müller and Essick concluded that LIGO and other current facilities should be able to determine the fraction of mergers that include at least one neutron star—once 200 such events have been logged. But compiling such a large catalog will take several centuries. New, more sensitive detectors will be needed to reach the threshold sooner.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- M. Müller and R. Essick, “Distinguishing between black holes and neutron stars within a population of weak tidal measurements,” Phys. Rev. D 113, 064062 (2026).