| Astrophysics (Index) | About |
A GW detection (gravitational wave detection) is the reception of a gravitational wave (GW) pattern that is recognized as the signal of an astronomical event (a gravitational wave event or GW event) by a gravitational-wave detector such as LIGO. Detections so far have mostly been GWs from black hole mergers, but a few from mergers including neutron stars. Detecting these merger signals has been the aim of these first ground-based GW detectors: the merger process had been analyzed regarding the signal they would produce and the successful detections confirm current science of black holes and neutron stars as well as the general relativity model of gravity.
The merger signal is a chirp (waves rising in frequency, from quicker and quicker orbits preceding the impact), followed by a short period of lesser waves, the ringdown (waves from some further repositioning of mass just after the impact, i.e., the recoil). To be recognized, the signal must show a pattern distinct from the gravitational wave background. The signal reveals information analytically (e.g., the chirp mass), and more through numerical simulation of mergers of pairs of objects of various sizes and rotations. There is an inclination-distance degeneracy because the signal is to some degree directional: if viewed from edge-on to the orbit, the signal is twice as strong as if viewed from perpendicular to the orbit. The ringdown signal helps reduce this ambiguity. Any determination of the angle between the merging objects' rotational axes provides evidence regarding whether the two objects were born as a binary star versus some sort of capture.
The first six detections accepted:
| GWyymmdd | what merged | detector(s) | |
| GW150914 | black holes | LIGO | first detection |
| GW151226 | black holes | LIGO | |
| GW170104 | black holes | LIGO | |
| GW170608 | black holes | LIGO | |
| GW170814 | black holes | LIGO/Virgo | |
| GW170817 | neutron stars | LIGO/Virgo | location and source spotted |
Subsequently, some additional candidate detections within the above time period have been accepted as real. As of 2024 there have been over 100 detections and candidate events, including additional neutron star mergers and some neutron-star black-hole mergers. The four detectors (including Virgo and KAGRA) resumed operation in May 2023; LIGO running until January 2024, with current plans to resume after scheduled maintenance in March 2024. Virgo and KAGRA ran only briefly, and are undergoing maintenance and repair.
The term sub-threshold gravitational wave (sub-threshold GW) is used for apparent detections that don't meet an agreed-upon detection threshold. Such a near-detection might remain an un-confirmable candidate, but subsequent analysis of data may lead to it being dismissed or confirmed. (The term sub-threshold GW might also be taken to include the GWs passing through Earth that are so weak that they don't even raise such a suspicion.)
|