Using Radio to Detect the Gravitational Waves of Merging Black Holes

The detection of gravitational waves topped nearly every chart highlighting the most important science stories of 2016. LIGO, the Laser Interferometer Gravitational-Wave Observatory, made headlines by detecting direct evidence of ripples in spacetime caused by two merging black holes. Historic and exciting, this discovery will probably be the first of many gravitational wave signals we see over the coming years—and not all of them will come from gravitational wave observatories.

Supermassive black holes at the hearts of galaxies are thought to form through the merging of smaller, yet still massive black holes, such as the ones depicted here.
Image Credit: NASA.

The upcoming Square Kilometre Array (SKA) radio telescope, which will be the most sensitive radio telescope in existence, is likely to detect gravitational waves from supermassive black hole binary systems according to research published recently in the American Physical Society’s journal Physical Review Letters.

The researchers, Yan Wang from Huazhong University of Science and Technology in China and Soumya Mohanty from The University of Texas Rio Grande Valley, came to this conclusion based on an analysis of simulated data from millisecond pulsars that SKA and similar telescopes are likely to detect. Pulsars are neutron stars that have stable rotation rates and emit beams of light at very regular intervals.

“When we are in the line of sight of the beam, our radio telescopes pick up the emission. Much like observing a rotating searchlight, we see pulses of radio waves whenever the beam sweeps across our telescopes,” explain the researchers.

The SKA will be a massive array of radio telescopes, spanning two continents, on which construction will begin in 2018. Given the population of pulsars, telescopes like SKA are expected to detect around 6,000 pulsars that emit radio waves at millisecond intervals. Because their emissions are so regular, millisecond pulsars are like extremely precise celestial clocks that emit radio waves instead of ticks.

This artist’s rendition of the SKA-mid dishes in Africa shows how they may eventually look once completed. The 15m wide dish telescopes will provide the SKA with some of its highest-resolution imaging capability, working towards the upper range of radio frequencies that the SKA will cover.
Image Credit: SKA Organisation.

Almost all galaxies, including ours, have supermassive black holes at the center. When two galaxies merge, their black holes come together to form binary systems and emit gravitational waves in the process. “Detecting these gravitational wave signals will give astronomers very important clues about how galaxies were formed, how often they merge, etc.,” say the researchers.

Gravitational waves from supermassive black hole binary systems should cause slight changes in the arrival time of pulsar emissions on Earth. One method for detecting gravitational waves, therefore, is to look for disruptions in the arrival time of radio waves from millisecond pulsars. In order to detect a gravitational wave with any degree of certainty though, you need see correlate changes in emission arrival times for many different pulsars. This technique is called a pulsar timing array.

Pulsar timing arrays exist now, but most include only a few dozen pulsars. The researchers estimate that SKA should be able to detect thousands of millisecond pulsars due to its large collection area (one square kilometer) and excellent data processing system. This would enable timing arrays with hundreds of pulsars, making them much more sensitive to gravitational waves.

In this new research, Wang and Mohanty created a realistic simulation of the kind of timing array that is likely to emerge from SKA-era telescopes. Since an array with hundreds of pulsars will present serious data processing challenges, the researchers explored and optimized a mathematical approach to sorting through data and identifying the signal of a supermassive black hole binary. In addition, they analyzed how accurately such a system could identify the location of supermassive black hole binaries in the sky.

The team’s results indicate that this kind of pulsar timing array could confidently detect gravitational wave signals from a wide range of supermassive black hole binaries. In addition, it seems that this technique may be able to pinpoint the sources of these gravitational waves in the sky accurately enough that researchers could turn optical telescopes on that patch of the sky and perhaps identify a pair of merging galaxies.

Managing a pulsar timing array this large will be complicated. For example, there are technical challenges related to monitoring many pulsars at once that haven’t been worked out yet. However, the researchers say, “The most exciting part of this work is the prospect of being able to use the thousands of pulsars that the upcoming radio telescopes will discover as a giant galaxy-sized gravitational wave detector!”

Currently, astronomers have identified hundreds of objects that may contain supermassive black hole binaries. However, the only way to really know if they do is to detect gravitational waves coming from the objects. As these research results indicate, a pulsar timing array may be able provide this confirmation once next generation radio telescopes come online. Such an array could also enable an all-sky search for black hole binaries that could turn up sources other telescopes haven’t yet identified. Instead of celebrating the detection of gravitational waves, in the not-too-distance future we may be celebrating the countless new discoveries made possible by gravitational wave detections.

Kendra Redmond

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