Both neutron-star and black-hole binaries produce a rising gravitational-wave chirp as their orbits tighten. The clearest potential difference is that neutron stars are made of deformable matter: tides can subtly alter a neutron-star system’s late-inspiral signal. A merger may also leave a high-frequency remnant signal or produce observable light. None of these clues is guaranteed, and a waveform feature alone may not identify the objects.
What the two mergers have in common: the inspiral chirp
As two compact objects orbit one another, they lose energy to gravitational waves. Their orbit shrinks, their orbital speed rises, and the waves grow stronger and increase in frequency. In a spectrogram—a plot with time on the horizontal axis and frequency on the vertical—a signal can appear as a track sweeping upward.
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This basic chirp occurs for both neutron-star and black-hole binaries. Its duration in a detector depends on the system’s properties and on which part of the signal the instrument can observe; it is not a fixed signature of one object type.
GW170817 provides a specific neutron-star example, not a universal template: LIGO reported that its signal could be visible for a minute or more. About 100 seconds before the merger, the stars were roughly 400 kilometers apart and completing about 12 orbits per second. LIGO’s GW170817 summary describes these measurements.
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Where neutron-star matter can change the signal
Neutron stars have physical surfaces and dense matter inside them; black holes do not contain neutron-star matter that can be probed in this way. Late in a close neutron-star inspiral, each star’s companion can deform it through tidal gravity. That deformation can leave a subtle imprint on the gravitational-wave signal, potentially revealing information about the stars’ internal matter.
LIGO’s overview gives 1–2 solar masses as the generally expected neutron-star mass range. Its comparison with stellar-mass black holes observed by LIGO and Virgo—10 or even 50 times the Sun’s mass—describes observations reported at the time of that page, not a universal boundary separating the two classes. LIGO’s explanation of testing general relativity in the presence of matter discusses the tidal effect.
Why a tidal imprint is not a definitive label
The tidal effect can be small compared with detector noise. Whether it can be measured depends on the signal’s strength, the source’s properties, and detector sensitivity. In its model-selection analysis of GW170817, LIGO found that the remaining plausible equations of state produced tidal effects too small to distinguish. Using gravitational waves alone, that analysis could not tell whether GW170817 involved two neutron stars, two black holes, or a neutron star and a black hole. LIGO’s analysis of what GW170817 could reveal about ultra-dense matter explains this limit.
That result is a caution against treating any one waveform feature as a universal classifier. Masses, spins, signal-to-noise, detector sensitivity, and the part of the signal that is observable all matter. A missing or unresolved tidal imprint does not, by itself, prove that the objects were black holes.
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What may happen at and after a neutron-star merger
A neutron-star merger can have different outcomes. The merged object may collapse promptly into a black hole or survive as a short- or long-lived neutron-star remnant. If a remnant produces a detectable post-merger gravitational-wave signal, it is expected at higher frequencies than the inspiral chirp: LIGO gives an approximate range of 1,000–4,000 Hz, depending on the remnant’s mass and compactness. These are expected frequencies, not a signal detected from GW170817; the search described by LIGO found no post-merger signal. LIGO’s post-merger search summary covers the possible outcomes and search.
How light can help—and why its absence proves little
Gravitational waves are not the only possible evidence. Material ejected in a neutron-star merger or a gamma-ray burst can produce electromagnetic observations that support the presence of neutron-star matter. In GW170817, a gamma-ray burst followed the merger by two seconds, and telescopes observed the aftermath across multiple wavelengths. LIGO reported at the time that GW170817 was observable for more than 30 times longer than any previous gravitational-wave signal—a historical comparison, not a claim about all later detections. LIGO’s GW170817 event page describes the event and its counterpart observations.
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Light is not guaranteed, even when a neutron star is involved. LIGO reported no electromagnetic counterparts for the neutron-star–black-hole events GW200105 and GW200115. For the illustrated parameter choice for GW200115, the black hole swallowed the neutron star without tidally disrupting it. So an observed counterpart can add an important clue, but its absence alone cannot establish that a merger involved two black holes. LIGO’s GW200105 and GW200115 event page gives the details.
How to read the evidence together
- Rising chirp: evidence of an inspiraling compact binary, but shared by neutron-star and black-hole systems.
- Late-inspiral tidal imprint: can support the presence of neutron-star matter if the effect is measurable; it may be too subtle to resolve.
- Post-merger signal: may reveal a neutron-star merger remnant, but the outcome, expected frequency, and detectability vary.
- Electromagnetic counterpart: can provide independent evidence of matter and merger aftermath; no counterpart does not rule out a neutron star.
The most reliable interpretation comes from combining these clues with inferred masses and spins, signal strength, detector sensitivity, and any observations of light. The waveform can constrain what happened without always delivering a simple object-by-object label.
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