A North-American team of astronomers is collecting data from neutron stars across the Milky Way, weaving a galactic-sized web that will tingle when traversed by gravitational waves from the largest black holes in nature. Surprisingly, the success of this effort hinges on the robotic exploration of our own solar system.
This team of astronomers is united by NANOGrav (the North-American Nanohertz Observatory for Gravitational Waves) a National Science Foundation Physics Frontiers Center. The Green Bank Telescope (GBT) is a fundamental tool in many NANOGrav projects, and collected data used in this research.
“In the fall of 2016, looking at my computer screen felt like riding on a rollercoaster,” says Stephen Taylor, a member of NANOGrav, then an astronomer at NASA’s Jet Propulsion Laboratory, and now an assistant professor of physics and astronomy at Vanderbilt University. Just a few months earlier, LIGO had announced the historic detection of gravitational waves from merging stellar-sized black holes. Gravitational waves are propagating ripples in spacetime, predicted in Einstein’s theory of gravity. Their 2015 detection opened new vistas on the most extreme objects in the universe. “And now our data showed early signs of waves from the gargantuan black-hole binaries at centers of galaxies—a discovery to rival LIGO’s.”
Whereas LIGO and its French–Italian counterpart Virgo observe gravitational waves by monitoring the stretching and squeezing of 4-km-long laser interferometers, NANOGrav looks for changes in the shape of our entire galaxy, seeking signs of black holes up to a billion times more massive than LIGO’s. To do so, NANOGrav has collected 15 years of data from galactic pulsars, neutron stars that emit radio pulses as they rotate. Their rotation is so regular that they can be used as geometric references: as passing gravitational waves stretch and squeeze the space between the pulsars and radio observatories on Earth, the waves can be identified as minuscule anomalies in sequence of pulses received at Earth.
Fast forward to the January 2017 meeting of the American Astronomical Society in Texas. JPL astronomer Joe Simon felt very nervous about the NANOGrav presentation, which would announce the first hints of supermassive black holes in pulsar data. “Our collaboration was still frantically investigating unforeseen sources of error that may be impersonating gravitational waves. Only recently, our Australian colleagues had reported the total absence of these signals, to the point that they suggested astronomers rethink their notion of galactic centers. And yet there was definitely something in our data. But what?” A few weeks later, at a workshop in Colorado, JPL theoretical physicist Michele Vallisneri grinned broadly while sharing that “We too in NANOGrav have made a discovery. So far, we have found Jupiter!” What had happened? The story is told in a newly published ApJ article authored by Vallisneri, Taylor, Simon, and their NANOGrav colleagues.
To identify GWs unequivocally in pulsar signals, we need to listen from a place of absolute stillness at the true center of the solar system – a location known as the solar-system barycenter, which lies close to the surface of the sun. To find it we need to consider also the masses and positions of all planets, and even asteroids. Indeed, the barycenter is the location where the masses of all planets, moons, and asteroids balance out; it’s the pivot of the solar system seesaw (see illustration). When we observe pulsars with telescopes on Earth, the movement of our planet with respect to the barycenter creates modulations in the spacing of the radio pulses, just like an ambulance siren will alter pitch as it moves toward or away from us. It is only at the barycenter that all these artifacts disappear. “There is no way that we could build a radio telescope at the solar-system barycenter,” says Simon, “but we use our knowledge of the masses and orbits of the planets, as measured from Earth and from spacecraft, to locate the barycenter and reconstruct the pulsar timings as they would have been from that privileged vantage point. The catch is that errors in the masses and orbits will translate to pulsar-timing artifacts that may well look like gravitational waves.”
“Nobody in our business had really worried about the accuracy of those orbits,” continues Taylor. “JPL computes them for the astronomical community, and they are good enough to land a spacecraft on Mars within 100 feet of the intended target. But when we used slightly different versions of the JPL orbit tables to analyze the NANOGrav data, we came to divergent conclusions regarding the presence of GWs.” While befuddling, says NANOGrav Physics Frontier Center co-director Maura McLaughlin (the Eberly distinguished professor of physics and astronomy at West Virginia University), this should be seen as a reason for pride in our collaboration: “We had not realized that our pulsar data had become so precise and abundant, and therefore so sensitive to tiny perturbations, that even 100-foot fluctuating error in the barycenter would be enough to throw us off.”
Vallisneri and the NANOGrav team members at JPL set to work to create a statistical treatment of orbit errors that could be folded into the NANOGrav analysis. They quickly concluded that the fault lay with Jupiter – or rather with our imperfect knowledge of its orbit. The stormy planet is by far the heaviest in the solar system, even if it is only a thousandth of the solar mass, and we have limited radio measurements (the most precise kind) of its orbit, because NASA’s Galileo, which circled Jupiter between 1995 and 2003, failed to deploy its main antenna. To make things worse, Jupiter’s orbital period is close to the timespan of NANOGrav’s data, so errors in its orbit create most confusing artifacts for the analysis. By accounting for the possibility of these errors, the JPL team managed to reconcile the gravitational-wave results obtained with all recent orbit tables issued by JPL. Doing so, however, made the NANOGrav observations less sensitive to the waves, and watered down the hints of supermassive black holes.
As NANOGrav continues to collect ever more abundant and precise pulsar timings, NANOGrav astronomers are confident that massive black holes will show up soon and unequivocally in the data. Even Jupiter seems to be lifting its curse: NASA’s new Jupiter orbiter Juno is enabling much more faithful orbit estimates, even under NANOGrav’s exacting standards. “The story of how we discovered Jupiter helps us stay modest, but it also emphasizes the oneness of astronomical research,” concludes Vallisneri. “To probe extragalactic mysteries we had to rely on NASA’s pioneering exploration of the solar system. We are so lucky to have colleagues and institutions that pursue both with equal impetus.”
More information and media contacts for this story can be found at Research News at Vanderbilt University.
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