Posted by PB on June 15, 2016
On September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first-ever direct detection of gravitational waves, a signal dubbed GW150914. Now LIGO has done it again! A second confirmed gravitational-wave event, called GW151226, was detected on December 26, 2015 at 03:38:53 UTC (Christmas evening in the United States). Like GW150914, GW151226 was generated by a pair of stellar-mass black holes with masses 14.2 and 7.5 times that of the sun. The two black holes revolved rapidly around each other until finally merging to form a single 20.8 solar mass black hole. The remaining one solar mass was released as energy in the form of gravitational waves.
GW151226 arrived first at LIGO Livingston and 1.1 ms later at LIGO Hanford. It was picked up within 70 seconds by a real-time matched-filtering algorithm. It was detected with a signal-to-noise ratio (SNR) of 13 and a significance greater than 5 sigma. By contrast, GW150914 had an SNR of 24. Despite the weaker signal, the search algorithm efficiently picked out the signal buried in the noise. The signal persisted in LIGO's sensitive band for a period of 1 second, considerably longer than GW150914 (which lasted about 0.2 seconds). The wave frequency increased from 35 Hz to 450 Hz over about 55 cycles before the black holes merged into one. Checks for environmental disturbances, such as stray electromagnetic fields, found that such disturbances could not have caused more than 6% of the peak strain amplitude. Additionally, the sensors monitoring the environment did not record any signal which matched the time and frequency evolution of GW151226.
Gravitational-wave phase evolution during binary inspiral is primarily dictated by a combination of the two component masses called the chirp mass. The long inspiral of GW151226 allows for a good estimate of the chirp mass (accurate to within 3%). The uncertainties in the measurements of the individual masses are higher. However, it is unlikely that either component has a mass lower than 4.5 solar masses, which eliminates the possibility of one being a neutron star. Thus, GW151226 is confirmed as a second binary black hole system. The black holes which generated GW151226 are less massive than those which generated the first signal, GW150914. They are more similar to the black holes found in x-ray binaries; however, their masses are measured in a completely independent way. With these first gravitational-wave events in hand, we can now start to build the mass distribution of stellar-mass black holes in the universe and understand how binary black holes might form.
Careful analysis of the data allows for determination of additional system parameters. For instance, we can say that at least one of the initial black holes had a spin component greater than 0.2 (where the maximum possible value is 1.0). This is a stunning new result from GW151226; the original detection, GW150914, did not provide enough information to say if either black hole was spinning. Measuring spin is of particular importance to determining the method by which binary black holes form. We can also measure the final black hole spin to be 0.74 and the luminosity distance to the source to be 440 Mpc. The position of the source on the sky was determined to be within a region of size 850 square degrees. Astronomers were alerted and searched this region for coincident electromagnetic signals, but, as expected for a binary black hole system, none were found.
Gravitational waves are predicted by general relativity, the theory of gravity developed by Albert Einstein about a century ago. We can carefully study our measured gravitational waves to see if they really match the predictions from Einstein's theory or if they differ in tiny ways, providing evidence for a slightly modified theory. All of the analysis to date suggests strong consistency with general relativity. The constraints only become stronger when the data from both GW150914 and GW151226 are combined.
The current LIGO detectors, called Advanced LIGO, are the most sensitive instruments ever built, with the ability to measure length changes 10,000 times smaller than the nucleus of an atom. The detection of the first gravitational-wave signal in September was the culmination of decades of work by scientists around the world. The detection of this second signal, GW151226, has truly begun the era of gravitational-wave astronomy. We also detected a third candidate signal, named LVT151012, in October. The probability that this signal is real and caused by another binary black hole merger is about 87% -- large, but not nearly large enough to call it a confirmed gravitational wave. No other stellar-mass binary black holes were detected in Advanced LIGO's first observing run, which lasted from September 2015 to January 2016, but searches for other types of sources are ongoing. The second observing run starts this fall with more sensitive detectors. We expect roughly 10 new binary black hole mergers to be detected in that time. We also expect that the Virgo detector in Italy will join the gravitational-wave detector network during this new observing run. With three detectors, we can better determine the position of the source on the sky, making it easier for astronomers to find electromagnetic counterpart signals.
University of Wisconsin-Milwaukee faculty, students, and research staff are actively involved with the analysis of the data collected by LIGO. In particular, UWM scientists play a large role in developing and operating the computer code which first detected GW151226 in real time. UWM researchers also have made crucial contributions to the development of theoretical waveform models and to the calibration of the detector data. Additionally, UWM provides critical computing facilities for carrying out data analysis in the LIGO Scientific Collaboration.
The original paper can be found here: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.241103 or here: https://dcc.ligo.org/public/0124/P151226/013/LIGO-P151226_Detection_of_GW151226.pdf. A second paper providing more details on both confirmed detections and the October candidate can be found here: https://dcc.ligo.org/public/0124/P1600088/015/bbh-o1.pdf. All of our data, along with tutorials to help use them, can be found at: https://losc.ligo.org/events/GW151226/.