Scientists working at the twin LIGO instruments detected a second gravitational wave. The signal was recorded on December 26, 2015 the LIGO. It originates from a pair of merging black holes of about 14 and 8 solar masses – smaller than the ones detected on September 14 of last year. Researchers from the Max Planck Institute for Gravitational Physics in Potsdam and Hannover and the Leibniz Universität Hannover made significant contributions to the discovery in several key areas: the development of highly accurate gravitational-wave models, search methods to detect faint signals, determining their astrophysical parameters, and advanced detector technology. This second discovery proves that a new era of gravitational-wave astronomy has begun.
The signal was detected in LIGO’s first observation run “O1” on December 26, 2015 at 4:38:54 Central European Time (CET) by both of the LIGO detectors, and was named GW151226. The wave arrived 1.1 ms earlier at the Livingston detector than at the Hanford detector.
The event was much weaker than the first detection on September 14 and was buried in the detector noise. A so-called “matched-filter” search was essential for the detection. In such searches, the data are compared to or filtered with many predicted signals in order to find the best match. The predicted signals are based on highly accurate gravitational-wave models developed by scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute). It was thanks to these models that the LIGO science team was able to show that the signal was caused by the merger of two black holes.
“It’s fabulous that our waveform models have pulled out from the noise such a weak but incredibly valuable gravitational-wave signal,” says Alessandra Buonanno, director at the AEI in Potsdam and professor at the University of Maryland. “GW151226 perfectly matches our theoretical predictions for how two black holes move around each other for several tens of orbits and ultimately merge. Remarkably, we could also infer that at least one of the two black holes in the binary was spinning!”
After the initial detection of the signal, subsequent analyses, for which the AEI provided half of the computing power, revealed the astrophysical properties of the observed binary system. Most of the data analysis was performed on the Atlas supercomputer, the most powerful computer cluster in the world designed for gravitational-wave data analysis, which has contributed significantly more than any other system used by the LIGO and Virgo Collaborations.
The results show that the observed system GW151226 consists of one black hole with about 14 times the mass of our Sun and one with about 8 solar masses. The two gravity traps merged at a distance of some 1.4 billion light-years from Earth. The researchers also found that at least one of the black holes spins on its axis. The merger emitted the equivalent of about 1 solar mass in gravitational wave energy and left behind a rotating 21 solar mass black hole.
Scientists within the Simulating eXtreme Spacetime collaboration have corroborated this scenario by running numerical-relativity simulations with parameters close to GW151226. These were in excellent agreement over the entire signal with the waveform models mentioned above used to infer the astrophysical properties of the source, further verifying that GW151226 was generated by the collision of two stellar-mass black holes in General Relativity.
The signal, extracted from the detector noise, differs in several key aspects from the first detected signal. Because of the smaller masses, the signal was registered by the instruments for a longer time, about 1 second – 27 orbits of the black holes before merger. For the first detection on September 14, 2015, only the last five orbits were observable. During the one second mentioned, the gravitational wave increased in frequency from 35Hz to 430 Hz. The signal’s peak strain amplitude of about 3 x 10 22 made it about three times weaker than the first detection.
“Now even the skeptics have to admit that our first detection was not a fluke,” says Bruce Allen, Managing Director of the Max Planck Institute for Gravitational Physics in Hannover.“I am now totally confident that in the next few years we will detect dozens of similar black hole mergers, and learn a lot about the universe. It is very satisfying to see that the data analysis methods we have invented in the past twenty years work as well as we had hoped.”
“With this second observation we truly are on the path to genuine gravitational-wave astronomy. We can start to explore the variety of sources on the unknown dark side of the Universe,” says Karsten Danzmann, director at the AEI in Hannover and director of the Institute for Gravitational Physics at LUH. “After so many years of research, development, and preparation it is very satisfying to see our vision finally come true.”
Advanced LIGO’s next data-taking run “O2” will begin this fall and is expected to last for about six months. By then, further improvements in detector sensitivity should allow LIGO to reach as much as 1.5 to 2 times more of the volume of the universe. The GEO600 gravitational-wave detector will also take part in the observation run. The Virgo detector is expected to join in the latter half of the O2 run.
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