Even before the excitement over the discovery of the first binary black hole merger can ring down, LIGO scientists working with gravitational wave detectors at Livingston, Louisiana, and Hanford, Washington, U.S., have announced witnessing the merging of a second black hole pair. They infer this from the observation of a gravitational wave (GW151226) detected on December 26, 2015. Being a much weaker signal than the first, the detection of this merger required the use of sophisticated noise filtering and mathematical analyses, some of which has been developed by Indian scientists.
Weak signal
The LIGO Scientific Collaboration (LSC) and Virgo Collaboration have made public the set of observations. The entire signal, coming from 1.4 billion light years away, lasted just one second. Having approximately 14 and 8 times the mass of the sun, the two black holes, during their cataclysmic merging, released approximately one solar mass within this short interval. They then merged to form a black hole of approximately 21 solar mass.
The study was published on June 15 in the journal Physical Review Letters. Speaking to The Hindu, Bala Iyer, one of the authors of the paper and principal investigator for the Indian team in the LSC, said, “The fact that we had not seen such massive black holes earlier led some people to wonder if the first discovery was a freak chance. This discovery shows clearly that a population of binary black holes does exist.”
Arising from the merger of smaller black holes, this signal was a lot weaker than the one announced on February 11. The signal-to-noise ratio of the second merger was much lower than its massive predecessor, causing the detection to be more challenging. A special technique called matched filtering invented in 1949 by Wiener had to be adapted for gravitational wave data analysis. In developing this technique and other important areas, such as theoretical modelling of the expected signals, the Indian contribution is significant.
Astrophysicist Jayant Narlikar said, “When the wave hits the detector, the signal is mixed with noise (disturbances), to separate the two you need a mathematical method. This was developed mainly in India, by Sanjeev Dhurandhar, Bala Iyer and others. Early work by C.V. Vishveshwara … is important.”
Detection of the wave takes place in two stages: The first stage, online detection, is a matched filtering process, in which a bank of a few hundred thousand template signals are slid across the incoming data stream. If a correlation between the template and the received signal is seen, it creates a trigger, and further analysis is carried out offline. In the second stage, a comparison between triggers received from the two stations (at Livingstone and Hanford) are matched and checked for consistency.
The adaptation of matched filtering to detect gravitational waves was developed at IUCAA, Pune, under the leadership of Sanjeev Dhurandhar. Matched filtering also required the accurate modelling of expected signals — work that was carried out by Bala Iyer at RRI, Bengaluru, and co-workers from France. Indians contributed significantly in the handling of instrumental artefacts and for including multiple detectors for a sensitive search.
Currently, a number of scientists are engaged in building a LIGO detector in India.
Prof. Narlikar compared this “revolutionary” work to the discovery of the telescope: “After Galileo’s discovery, astrophysics went into a new phase. The same can be said about the discovery of gravitational waves.”
