My first Magazine Sky & Telescope - 01.2019

Detecting Individual Sources Detecting one or more individual black hole binaries remains the ultimate goal of those who use pulsar timing arrays. “It would be like detecting a continuous wave, or tone,” JPL’s Lazio says. “Much like if you are standing close to somebody at a party, you can hear that person’s voice.” Based on infrared survey data and cosmological simula- tions, Chiara Mingarelli (Flatiron Institute) and colleagues estimate that there should be several dozen nHz sources within 730 million light-years of Earth. These inspiraling black holes will be detectable for a long time in human terms. (a) Ongoing individual source 60 40 20 0 -20 -40 -60 0 500 1000 1500 2000 2500 Days since observing start 3000 3500 3000 3500 3000 3500 (b) Stochastic background 20 10 0 -10 -20 0 In order to ﬁ nd the background signal, astronomers need a lot of pulsar pairs, Sesana explains. When a gravitational wave passes, it stretches space in one direction and squeezes it perpendicular to that direction. “So if two pulsars are observed at a 90° angle, the pulse will arrive later from one pulsar and earlier from the other,” he says. By making many such correlations between pulsar pairs, and also seeing sig- nals from pulsars close together on the sky being affected the same way, the PTA teams will eventually be able to tease out the stochastic background signal. Adding to the complexity, team members have to disen- tangle subtle gravitational-wave signals from myriad sources of noise. For example, despite the fact that millisecond pul- sars beat with a precise regularity similar to humanity’s best atomic clocks, individual pulsars exhibit slight jitters that must be accounted for. Electrons in interstellar space also slightly delay the arrival of low-frequency radio waves. Another source of noise stems from the fact that astrono- mers’ reference point for pulsar timing is not Earth’s posi- tion but the solar system’s center of mass, called the bary- center. The barycenter’s exact location has to be known to incredibly high precision for this work, because an error of just a few dozen meters changes a pulsar’s timing by several nanoseconds. Slight errors in the barycenter’s position can thus partially mimic the stochastic background’s signal. “Our knowledge of the planetary motions in the solar system is now effectively a limiting factor for us, which is quite astonishing,” says Ransom (see “The Solar System Barycen- ter,” page 27). Astronomers debate whether they’re already seeing hints of the stochastic background in their data, and one or more of the PTAs will probably detect it within the next ﬁ ve years. But the real payoff will come after scientists watch the signal build up over time. By disentangling all the complexities in the signal, scientists will learn about the distribution of black hole masses and the eccentricity of binary orbits. Perhaps more important, astrophysicists should be able to discern a great deal about the rate of black hole mergers as a function of redshift, which in turn will be a proxy for how the galaxy merger rate has changed over cosmic history. In fact, the failure to detect the background by now seems to be ruling out the most optimistic models in which the collision of two large galaxies always produces a black hole merger. 500 1000 1500 2000 2500 Days since observing start (c) One-time event 20 10 0 -10 -20 0 500 1000 1500 2000 2500 Days since observing start p EXAMPLES OF SOURCES Different gravitational-wave sources will create distinct signals in pulsar data, and each pulsar gives a slightly different view of the signal. Shown here are three simulated examples using the periods of three real pulsars (three colors): an ongoing signal from a pair of billion-solar-mass black holes lying about 140 million light-years away (a); a background signal combin- ing many sources (b); and the signal from a single event, such as a black hole merger, passing Earth on day 1,500 (c). The graphs show what the data look like after astronomers remove the effects of each pulsar’s spin and other things that affect the signal along its path from the pulsar to Earth. “A typical binary will spend about 25 million years in the PTA band, which is in stark contrast to LIGO sources!” she says. How long a binary is detectable depends on its mass, Min- garelli explains. More massive binaries produce stronger (and thus more easily detectable) gravitational waves. But more powerful waves drive a faster inspiral rate, so the systems are visible for shorter time periods. For example, more massive galaxies have more massive black holes, so a black hole inspi- sk yandtele scope.com • JA N UA RY 2 019 25

My first Magazine Sky & Telescope - 01.2019 | Page 27