Gravity-wave everyday life

Gravitational waves from black hole mergers were first detected in September 2015 1 . At first, every detection of gravitational waves was a noisy event, but then we got used to them. The Nobel Prize for the discovery was awarded very quickly - in 2017. In the fall of 2017, the experiment, in which, in addition to the American LIGO detectors, the Italian VIRGO participated, was temporarily suspended to improve the installations. The result at the time of stopping is 11 events, the masses of merging objects and an estimate of the distances to them are shown in the graph below. One of these events is the merger of two neutron stars.

Since then, LIGO has doubled the power of the laser beam, reduced quantum noise, and improved some of the mirrors. As a result, sensitivity was increased by 40%. On VIRGO, the sensitivity was doubled, although starting from a lower level. Since the beginning of April 2019, the joint session has resumed. Events, as promised, took place at an average pace once a week, and few people paid attention to them. But this is interesting - real statistics appear.

Here is a table of gravitational wave events with the date of registration, preliminary classification of events, distance estimates and statistical significance - with what frequency such an event can be simulated by noise.

* Frequency of random simulation of such an event by noise: once every N years.

Data source - gracedb.ligo.org/latest/ . Where the identification of an event is not absolutely reliable, an estimate of the probability of that nature of the event is provided. The distance estimate (megaparsecs) is apparently given in terms of the “luminosity distance”: R2 ~ Labs/Lobs (at small redshifts this value coincides with the usual distance). Otherwise, it is difficult to explain the value of 5725 Mpc (06/07) - in light years this will be greater than the age of the Universe.

So, we have 20 more compact object merging events. Mostly black holes, but there is one reliable merger event between a pair of neutron stars (April 25). Another 75% candidate, two 50% candidates, and one 30% candidate for neutron star pairs. There is only a small chance that some of the merging objects fall into the mass gap between neutron stars and black holes. The maximum mass of a neutron star is determined by the Oppenheimer-Volkov limit (more than two Mʘ, the exact value is not determined). The minimum mass of a black hole is 5–6 Mʘ, this follows from the statistics of observed black holes.

Unfortunately, mass estimates have not yet been published. The list of events is still “breathing” - the interpretation of some events is sometimes revised. Apparently, all data will be published after detailed processing in the form of a new catalog of events. We are looking forward to the replenishment. It would be especially interesting to look at the mass estimate for the 06/07 event; apparently, there are very large masses there, otherwise it would be difficult to register the event from such a luminosity distance.

Boris Stern

1 See trv-science.ru/2016/02/12/chto-uvideli-detektory-ligo/