In the month of April, there were a total of five candidates of LIGO detected. With just the first month of O3, it yielded 5 GW events. Here is an article about how are these candidates are identified.
Past Observing Runs
Since April 1, 2019, LIGO situated in Washington and Louisiana, and its Italian partner, VIRGO located at the European Gravitational Observatory (EGO) in Italy began a search for gravitational waves. This was the third observing run of the GWs and the search run was therefore called O3. The previous two searches for GWs, O1 and O2, could not give results as big as the O3.
The first observing run called O1 was for about 4 months spanning across September 14, 2015 to January 19, 2016. In that time, two gravitational wave detections were made, including the history-making first on September 14, 2015.
The second observing run called O2 covered a period of about 9 months from November 30, 2016 to August 25, 2017. During this run, there was another historic observation of GWs with the paradigm shifting discovery of merging neutron stars.
Our 3rd Observing Run (O3) is due to start in early 2019. With improved sensitivity we may detect 1–50 #NeutronStars across the year-long joint @LIGO @ego_virgo run! We also hope @KAGRA_PR will join before O3 ends, growing the global network. Read more at https://t.co/PlM7eOeX8G pic.twitter.com/WpRzrfY0Lt
— LIGO (@LIGO) October 17, 2018
The O3 search was expected to reach deeper into the universe than the previous two searches because of the addition of the advanced detectors that O3 had, and it even did, with 5 additional GW candidates. Since LIGO’s historic second observing run ended on August 25th, 2017, the LIGO team was gearing up for another round of maintenance and installing significant upgrades with its next observation run O3, which they wanted to improve greatly in terms of their expected Design Sensitivity. After these updates, each detector surveyed larger volumes of the universe than before, leading to being able to identify extreme events like the smash ups between black holes and neutron stars. The O3 run is the longest in the history of LIGO advanced detectors and is a complete 12 months of continuous observing. Factors like noise hunting, optical alignment, and modifications of control system softwares were greatly improved, all to improve the detector sensitivity.
In the first month of O3 of the LIGO-Virgo detectors, five binary merger candidates have been announced. This is the highest detected in a short span of 30 days. Three of these five GW candidates are black hole mergers, one of them is a binary neutron star merger and one, according to LIGO, could be a merger event of a neutron star and a black hole, which has never been observed before.
There has previously been a binary neutron star merger candidate detected earlier called the GW170817 in 2018. This new candidate is named S190425z and is estimated to have occurred about 500 million light years away from the Earth. Only one of the twin LIGO facilities picked up its signal along with Virgo. This candidate was observed by a large number of telescopes worldwide. Because only two of the three detectors registered the signal, estimates of the location in the sky from which it originated were not precise, leaving astronomers to survey nearly one-quarter of the sky for the source.
The one neutron star and black hole collision is named as S190426c and is estimated to have taken place roughly 1.2 billion light-years away. It was seen by all three LIGO-Virgo facilities, which helped better narrow its location to regions covering about 1,100 square degrees, which spans about 3 percent of the total sky.
The detectors have identified a relatively large area in the sky as the probable region from which the gravitational waves were emitted. LIGO-India, which is the GW wave detector being built in Maharashtra, India aims to dramatically reduce this localisation uncertainty.
How To Identify LIGO Candidates
Gravitational waves detections are basically distortions detected in space-time. In other words, any object with mass that accelerates produces gravitational waves. Using the very sensitive interferometers, tiny variations measured on the basis of how lasers of the interferometers of GW detectors are able to traverse parts of the giant L-shaped interferometer structure is detected. The sensitivity of all the GW detectors is such that each of them can sense massive collisions millions or billions of light-years from Earth. By combining all the detector measurements,, potential GW sources are detected.
There are four categories of gravitational waves based on how they are produced. Each one generates a unique vibrational signature that can be sensed by detectors. These waves could be continuous gravitational waves, Compact Binary Inspiral Gravitational Waves, Stochastic Gravitational Waves or burst gravitational waves.
Several space scientists, including GW groups from India, are searching this whole area of the sky with telescopes for the electromagnetic radiation that these merger candidates would emit, but no EM counterpart has been detected so far. Several Indian researchers from different institutions are part of this mission through the LIGO-India Scientific Collaboration, or LISC, which includes activities related to the LIGO-India observatory and participation in the various working groups of the LSC.
So far, the network of active detectors worldwide has spotted evidence for 2 neutron star mergers, 13 black hole mergers and 1 possible black hole-neutron star merger. With these new improvements in the LIGO network to detect GWs and the success that they have made, researchers are even more focused and optimistic about detecting more number of gravitational wave candidates.