My first Magazine Sky & Telescope - 01.2019

The Next Gravitational-Wave Revolution Lovell CHIME Westerbork Effelsberg Nançay VLA Green Bank Sardinia FAST Arecibo Parkes MeerKAT p WORLDWIDE EFFORT Astronomers use 12 radio observatories to track roughly 75 millisecond pulsars. Of these facilities, three — CHIME in Canada, FAST in China, and MeerKAT in South Africa — are new participants; the others have been involved for at least 10 years. ral in a galaxy such as the gigantic elliptical M87 would emit detectable gravitational waves for 4 million years. But a hum- bler black hole pair in a smaller galaxy, such as the Sombrero (M104), would offer a 160-million-year window. The odds that a very massive binary is sending out gravitational waves during the time that PTAs are observing is thus lower. Unless we’re really lucky, it will probably take 10 to 15 years to build up enough data to see deep enough into space to detect an individual binary. Theorists expect the gravitational waveform from a single binary to be a very sim- ple sinusoid, and because the binary system will likely have an orbital period of several decades, the signal will change very little over many years. The longer baseline could make electromagnetic follow-up easier. If the inspiraling and merging black holes are embed- ded in disks of gas — and recent work suggests that this could be true for supermassive binaries, unlike the smaller ones that LIGO and Virgo detect — then they would also produce light. “If a loud individual source is detected, PTAs will give its sky localization, opening the possibility to identify the host galaxy and carry out multimessenger observations of the system, pretty much as it happened with the LIGO neutron star binary GW170817,” Sesana says (S&T: Feb. 2018, p. 32). “The differ- ence is that with PTAs, everything builds up slowly over the course of the years, and you don’t hit the jackpot in a snap.” to shorter-wavelength gravitational waves. All of this works to increase sensitivity to both the stochastic background and individual binaries. But progress has been slow for a variety of reasons, from planning across time zones and work cultures to accounting for each telescope’s individual quirks. There’s also the all-too- human tension of each team wanting for itself the glory of making the ﬁ rst conﬁ rmed detection. 2025 2035 2050 2000 2040 2005 Vernal equinox 2015 2030 Sun 2010 Going Global Adding to the growing optimism is the fact that scientists from all three teams are now combining their data sets to form an even more powerful network: the International Pul- sar Timing Array (IPTA). The IPTA enables scientists to ana- lyze data from most of the observed pulsars, including from facilities in both hemispheres. Having more observations means shorter time gaps in data, which increases sensitivity 26 JA N UA RY 2 019 • SK Y & TELESCOPE 2045 Now 2020 EPTA NANOGrav PPTA New members

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