Gravitational waves have two major limitations of modern observatories. For one, they can only observe powerful gravitational bursts, such as the merging of black holes and neutron stars. Second, they can only observe these mergers at wavelengths of hundreds to thousands of kilometers. This means that we can only observe the merging of stellar masses. Of course, there is a lot of interesting gravitational astronomy occurring at other wavelengths and noise levels that encourages astronomers to be smart. One of these clever ideas is to use pulsars as telescopes.

The concept is known as the pulsar clocked array (PTA). Pulsars are rotating neutron stars with a strong magnetic field directed to direct a burst of radio energy toward Earth with each spin. We perceive them as ordinary radio bursts. Some pulsars, known as millisecond pulsars, spin so fast that they emit hundreds of radio pulses per second. Because the rotation of a neutron star is almost as regular as clockwork, pulsars can be used as a kind of cosmic clock.

Therefore, if the pulsar is moving in any way (for example, around a star), the relative motion of the pulsar will cause the pulses to shift slightly. We can measure these changes very precisely. Our observations are so precise that long before we observed them directly, pulsars were used to measure the orbital decay of binary stars as indirect evidence of gravitational waves.

Even though pulsars are not part of a binary system, small gravitational forces cause them to shift slightly. Therefore, their momentum shifts a small amount when a gravitational wave passes through them. These shifts actually occur at the level of random fluctuations of the pulses, so we cannot see the effect of the gravitational wave from a single pulsar. We need observations of many pulsars to see statistical fluctuations. So we need a set of pulsar time frequencies.

Earlier this year, astronomers at NANOGrav used an array of 67 pulsars together with 15 years of data and were able to measure the background gravitational hum of the universe. Supermassive binary black holes (SMBHs) are likely sources of this background, but the results are inconclusive. One problem with the data was that although the team was able to measure gravitational waves, they were not able to pinpoint their starting point.

There are several ongoing PTA projects, which means we will soon have a large amount of observational data. In the new study, the team proposes how this data can be used to identify the source of background gravitational waves. His ideas focus on accurate measurement of the distance to the pulsars in the array. Although we currently know the distances of some pulsars very precisely, the distances of many pulsars are uncertain. Detailed observations of PTA pulsars by observatories such as the Very Long Baseline Array can give us the precision we need. Knowing both the distance and time variation of the pulsar will give us a range for the source. With a pulsar array, the ranges will overlap to triangulate the source.

As the article shows, good accuracy can be achieved with PTA from only a dozen pulsars. This initial work only focused on a 2-dimensional array, but a more 3-dimensional array should be reasonably accurate as well. It’s certainly accurate enough to prove whether these background waves are coming from supermassive binary black holes or something we don’t fully understand yet. Source