During a press conference at the American Astronomical Society meeting in San Diego, the second detection of gravitational waves from the merging of two black holes by the LIGO experiment was announced. It is a different event from the first, historic, one announced on February 11, 2016: this time the merger took place about 1.4 billion years ago and was detected on Earth on December 26, 2015. The event is also described in a article published in the journal “Physical Review Letters”.
The event recorded on September 14, 2015, the one announced on February 11, 2016, by the LIGO (Laser Interferometer Gravitational-Wave Observatory) experiment was a historic moment because it provided the first experimental evidence of the existence of the gravitational waves predicted by Albert Einstein’s theory of general relativity. The validity of that detection was confirmed by the December 26, 2015 event, dubbed GW151226.
Again, the gravitational waves were produced by a merger between two black holes but this time their masses are much lower than those that generated the first event detected by the LIGO experiment. In this case, their masses were estimated to 14 and 8 solar masses and the resulting mass is about 21 solar masses. About one solar mass was converted into the gravitational waves energy.
The second event signal is weaker than the first one but in the moment in which the black holes merged the signal frequency became higher getting into the range detectable by LIGO earlier than the first event. This allowed to observe about 27 orbits of the two black holes, which occurred in about one second while the first event had a duration of about 0.2 seconds.
In the second event, one of the black holes that merged was spinning and it’s the first time that it was possible to determine that. This suggests that some event triggered that spin such as the stealing of the mass from a companion star. That might have happened before the collapse which led to the birth of that black hole.
A third event was also announced, detected on October 30, 2015. The signal is weaker than even the second event, which makes greater the possibility that this is a false detection. The exam of the third event haven’t yet provided a conclusive answer but even if it a different source than gravitational waves should be found it will still be useful to learn how to better handle this type of observations.
The research on the December 26, 2015 event are not over: first of all, the origin area was identified with great approximation only. The Virgo collaboration, which runs a similar experiment finished in 2003 in Santo Stefano a Macerata, in the municipality of Cascina, in Italy, is also participating in the research. We’re still at the beginning of a new way of observing the cosmos thanks to gravitational waves instead of the electromagnetic waves detected by normal astronomical observatories.
