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Title: The NANOGrav Nine-year Data Set: Observations, Arrival Time Measurements, and Analysis of 37 Millisecond Pulsars
Authors: NANOGrav Collaboration
Paper Reference: Astrophysical Journal, 2015, 813, 65
The expected gravitational wave spectrum at nanohertz frequencies from various supermassive black-hole merger models (color) along with upper limits of the spectrum measured from the NANOGrav nine-year data set (black). The black-dashed line represents the experimental upper limit of the gravitational wave strength when assuming that the signal is entirely due to super massive black hole binary mergers (i.e., power-law); the solid line represents the upper limit when allowing for the derived spectrum to have any shape. The colored areas correspond predictions of three different models. At large frequencies, the free-shape spectrum is dominated by white-noise (i.e. non-astrophysical) signals due to pulsars with small data sets.
Until this year, astronomers have only been able to indirectly determine the presence of gravitational waves -- tiny, wave-like shifts of space and time -- through the measurements of decaying orbits of neutron stars. In January 2016, the LIGO collaboration announced the first direct detection of gravitational waves from a system of black holes orbiting and colliding together.
The discovery by LIGO has ushered in the era of gravitational-wave astronomy, showing that direct measurements of spacetime ripples are possible.
Along with experiments like LIGO, an international collaboration of students and researchers study an array of radio pulsars in order to hunt for gravitational waves with nanohertz frequencies emitted by merging supermassive black-hole binary systems. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) “pulsar timing array” collaboration relies heavily on the Arecibo Observatory for making the most precise pulsar measurements possible. While each pulsar has a unique set of properties, gravitational waves passing through the Earth will imprint the same spacetime ripples into all pulsars in the array. These signals can be measured directly once the array has sufficient sensitivity (the more bright pulsars in the array, the greater the sensitivity). The figure shows NANOGrav's sensitivity to gravitational waves originating from supermassive black-hole binary mergers. The data are from the NANOGrav nine-year data set and include observations, arrival time measurements, and analysis of 37 millisecond pulsars. Even though NANOGrav has not yet detected a strong gravitational wave signal, its current sensitivity can start to rule out or constrain proposed models of the supermassive black-hole binary population in the Universe. NANOGrav continues to collect high-precision data with the Arecibo telescope and will likely directly detect nanohertz-frequency gravitational waves within the next 5 to 10 years.
The Arecibo Observatory and pulsar astronomy share an important history in gravitational wave science. Indeed, the first evidence for the existence of gravitational waves came from long-term Arecibo observations of a pulsar in a decaying orbit with another neutron star, where the rate of orbital shrinkage matched the rate expected from the loss of energy carried away by emitted gravitational waves. The NANOGrav effort, along with the international pulsar-timing-array community, will soon allow us to directly see gravitational waves from distant black-hole pairs. The Arecibo Observatory therefore continues to play a decisive role in gravitational wave astronomy.