While earlier NANOGrav results showed evidence for an unexplained signal detected in the pulsars they observed, it was too faint to determine the cause. Additional data, published today in the Astrophysical Journal Letters, show that the signal is consistent with gravitational waves passing through our galaxy, likely generated by supermassive black holes merging in distant galaxies. This is the first evidence for gravitational waves at these low frequencies, opening a new way to study the most massive objects in the universe
 
The signal was revealed by looking for tiny perturbations in the arrival time of radio pulses from pulsars, which are rapidly spinning neutron stars. The fastest pulsars, called millisecond pulsars, spin on their axis up to 700 times per second. These pulsars are essentially cosmic clocks that are as stable as some of the best laboratory atomic clocks. “Our research group at NRL has led an effort to discover and time millisecond pulsars, many of which have been added to the NANOGrav pulsar timing array,” said Dr. Paul Ray, head of the High Energy Astrophysics and Applications Section at NRL. This stability allows tiny deviations, of less than 1 millionth of a second, to be measured, tracing the stretching and squeezing of spacetime as gravitational waves pass the Earth and pulsars. Those deviations, and the pattern of their correlations on the sky, are the telltale signature of gravitational waves, allowing scientists to conclude that gravitational waves are the most likely cause of the observed signal.
 
Another use of highly stable pulsars is a natural analog to GPS, giving a spacecraft the ability to navigate and tell time autonomously, even far out in the solar system where GPS is not available. “Any pulsar-based navigation system will ultimately be limited by the random sloshing around caused by gravitational waves, so the characterization of these signals is of keen interest to NRL and the Navy,” said Dr. Paul Ray, “We must fully understand and characterize these clocks if we are going to use them in precise navigation and timekeeping applications.”

The current NANOGrav results are based on radio observations using some of the largest radio telescopes in the world. These radio signals can be perturbed by the tenuous plasma of ionized gas in interstellar space, requiring a careful procedure to correct for those perturbations. At NRL, Research Physicist Dr. Matthew Kerr is taking a different approach by timing many of the same pulsars using gamma-rays instead of radio waves.
 
“Although the gamma-ray measurements are less precise, they are unaffected by interstellar plasma, and we have a consistent 15 year dataset from the Fermi Gamma-Ray Space Telescope,” said Kerr, “and it is important to ensure that radio propagation effects are not masquerading as the gravitational wave signal we are seeing." Kerr’s analysis was recently published in the journal Science.
 
Astrophysicists around the globe have been busy chasing this gravitational-wave signal. Several papers released today by collaborations in Europe, China, and Australia also see a consistent signal in their data. As part of a broader collaboration called the International Pulsar Timing Array (IPTA), various groups are combining their data in order to better characterize the signal and search for new types of sources.
 
“The NANOGrav result is strengthened by international collaboration with the IPTA,” said Dr. Megan DeCesar, an astronomer at NRL. “Other pulsar timing arrays also see this signal in independent data, which increases our confidence that it is real. By combining our data into a single larger data set, we will be able to characterize the signal more precisely and perhaps confirm its origin.”
 
 
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