Introduction to NANOGrav

The NANOGrav (North American Nanohertz Observatory for Gravitational Waves) Physics Frontiers Center is a collaboration of scientists working to detect and study very low frequency (about nanohertz) gravitational waves. Using an array of high-precision millisecond pulsars, we seek to measure the influence of gravitational waves on the Earth. At these frequencies, we expect to detect a stochastic background of merging supermassive black holes, individual inspiral and merger events, and possibly cosmic strings and inflationary gravitational waves. NANOGrav consists of astrophysicists at over a dozen institutions throughout the United States and Canada. We count among our ranks senior faculty and researchers, postdoctoral scholars, graduate and undergraduate students, and even high school students through our educational and outreach partnerships.

There are several ways to get involved in NANOGrav. We invite any interested scientists and students to give a talk at one of our monthly science seminars, and we welcome collaboration with scientists in fields relevant to NANOGrav. Interested scientists should consult the NANOGrav Membership Policy and Bylaws for more information.

How it Works

Millisecond pulsars rotate with incredible stability, allowing them to be used as precise clocks. The time of arrival (TOA) of a pulse can usually be measured to δ/(S/N), where δ is the pulse width (typically a few hundred microseconds) and S/N is the signal-to-noise. For many millisecond pulsars, this results in TOAs measured with ≲ few μs precision. TOAs are used to construct a timing model that is coherent in pulse phase, i.e. it accounts for every single rotation of the pulsar by modeling the rotational, astrometric, and (when applicable) binary parameters of the system. The residuals in these timing models (the difference between the predicted and measured TOAs) can have an RMS scatter as little as 100 nanoseconds over timescales of many years.

The Hellings-Downs curve, which shows the expected angular correlation in pulsar timing residuals due to gravitational waves.

NANOGrav seeks to detect and study gravitational waves by looking for their influence in timing residuals from an array of ultra-precise millisecond pulsars. Gravitational waves passing through the Solar system will lead to correlations in the timing residuals between pairs of pulsars, even though the influence on the pulsars themselves will not be correlated across the array. This "Earth term" depends only on the baseline between pulsar pairs and is given by the Hellings-Downs curve. We are sensitive to gravitational waves with periods between the cadence of pulsar timing observations (weeks) to the span of our dataset (years), which corresponds to nanohertz frequencies. This allows NANOGrav to probe a unique phase space that is complementary to interferometric gravitational wave detectors. The sensitive of our array increases as we add more pulsars, improve TOA precision and the RMS of our timing residuals (which depend on instrumentation and our understanding of the pulsars and propagation effects through the interstellar medium), and time. The latter factor is thanks to the expected nature of the gravitational wave spectrum, which increases in amplitude as we move to lower frequencies.

Primary Science Goals

Our primary science goal is to directly detect the influence of gravitational waves on space-time within the next decade, thus ushering in a new era of low-frequency gravitational wave astronomy. We are already placing stringent constraints on the stochastic supermassive black hole background, which in turn constrains the black hole merger rate out to redshift z~1. After detection, our increasing sensitivity will allow us to:

  • fully determine the black hole merger rate from 0 < z < 1
  • determine or constrain the solution to the "last parsec problem," thereby bridging the evolutionary gap between dynamical friction and gravitational wave emission
  • map the history of structure formation over cosmic time
  • identify and characterize individual supermassive black hole binaries
  • probe the highly nonlinear regime of strong-field general relativity through the observation of gravitational wave bursts with memory

NANOGrav will also be able to explore large portions of parameter space for cosmic strings. These topological defects are predicted by a large class of cosmological models, from symmetry breaking models to string-theory-inspired models. NANOGrav will either confirm their existence or place severe limits on properties such as the energy scale at which they form. A cosmic string detection would reveal information about high energy physics unattainable via accelerator experiments.

Secondary Science Goals

NANOGrav uses high-precision millisecond pulsars as gravitational wave detectors, observing over two dozen sources at regular intervals. This produces valuable secondary science including a greater understanding of the dynamic interstellar medium, the stability of millisecond pulsar rotation and emission mechanisms, the discovery of new pulsars, and a detailed characterization of individual binary pulsar systems. The latter may include precise neutron star mass measurements that constrain the equation of state of ultra-dense matter and allow us to study general relativity in ever more diverse and extreme environments. Our data set can also be used as an independent check of Solar System ephemerides and universal time standards.

Technical Requirements

NANOGrav uses millisecond pulsars as clocks whose signals respond to the minuscule changes in space-time caused by gravitational waves. This deviation is expected to be less than ~100 nanoseconds, which drives our technical requirements—radio telescopes and backends capable of determining pulse times of arrival to nanosecond precision for an array of dozens of high-precision millisecond pulsars distributed across the sky. NANOGrav's key instruments are the William E. Gordon Telescope at the Arecibo Observatory and the Robert C. Byrd Green Bank Telescope. Both telescopes are absolutely vital; Arecibo because of its unparalleled sensitivity and the GBT because of its own excellent sensitivity and ability to observe over 85% of the sky. NANOGrav members have equipped both telescopes with state-of-the-art GPU-based backends. We currently observe 79 pulsars every two weeks at both telescopes and cooperate closely with two large area pulsar surveys, the PALFA survey at Arecibo and the GBNCC survey at the GBT. The NSF divestment from the GBT is a serious threat to our science goals because there is no other North American telescope that can replace it. NANOGrav is working with our partners to secure the future of the GBT.


The NANOGrav 12.5-year Data Set

The NANOGrav 12.5-year Data Set

We have recently released our latest, 12.5-year data set, which consists of observations from the Arecibo Observatory and the Green Bank Telescope on 47 millisecond pulsars. We have introduced two pulsars into this data set, J1946+3417 and J2322+2057, for which we have timing baselines over 2 years; however, our longest baselines are nearly 13 years in length and make our pulsar timing array sensitive to low-frequency, nanohertz gravitational waves. This data set has two varieties: a “narrowband” version, which is very similar in its form and construction to our previous data sets (the 11-, 9-, and 5-year data sets), and a “wideband” version, which is the first data set of its kind. The timing analyses of the data sets are mutually consistent, and we are in the process of analyzing these data for the presence of gravitational waves.

The data are available on our website, here!

The narrowband and wideband data set papers are available on the ArXiv.

Meet “Spikey,” a Possible Pair of Merging Supermassive Black Holes

“Spikey”, an exciting new supermassive black hole binary candidate, was featured in Scientific American. Spikey was observed by the Kepler space telescope, and shows an unusual symmetric flare. This flare is well explained by relativistic self-lensing in a binary system, when the smaller supermassive BH passes behind the bigger one. If the flare repeats in the upcoming months, the binary nature of the candidate will be confirmed. The paper was led by graduate student Betty Hu, and co-authored by NANOGrav post-doctoral fellow Maria Charisi.

Read more at Scientific American or read the full paper on the ArXiv.

NANOGrav Data Searched for Gravitational Wave Memory

Newly published results! NANOGrav searches 11-year data set for unique gravitational wave signature – gravitational wave memory.

Read more in The Astrophysical Journal.

Special Session: “New Results From The North American Nanohertz Observatory for Gravitational Waves”

Special Session approved, “New Results From The North American Nanohertz Observatory for Gravitational Waves”

235th meeting of the American Astronomical Society Honolulu, HI 5 January 2020

This Special Session will highlight advancements in the search for nanohertz gravitational waves using pulsar-timing arrays, and the exciting multi-messenger opportunities to probe supermassive binary black holes. The session will include three invited talks followed by a panel discussion.

Most Massive Neutron Star Ever Detected Found in NANOGrav Data

Summary: Astronomers using the GBT have discovered the most massive neutron star to date, a rapidly spinning pulsar approximately 4,600 light-years from Earth. This record-breaking object is teetering on the edge of existence, approaching the theoretical maximum mass possible for a neutron star. “Neutron stars are as mysterious as they are fascinating,” said Thankful Cromartie, a graduate student at the University of Virginia and Grote Reber doctoral fellow at the National Radio Astronomy Observatory in Charlottesville, Virginia. “These city-sized objects are essentially ginormous atomic nuclei. They are so massive that their interiors take on weird properties. Finding the maximum mass that physics and nature will allow can teach us a great deal about this otherwise inaccessible realm in astrophysics.”

Read more at the NRAO website.

Event Horizon Telescope

The NANOGrav Collaboration congratulates the Event Horizon Telescope team for their success in creating a spectacular first direct image of the supermassive black hole at the center of galaxy M87. NANOGrav members James Cordes and Shami Chatterjee are also members of the EHT collaboration pulsar working group, which focuses on finding pulsars around the supermassive black hole at the center of the Milky Way galaxy. This measurement represents the culmination of a 10-year effort and a number of technological and scientific advancements, not least of which is the proof that supermassive black holes of millions to billions of times the mass of the sun are in fact the engines of intense gravity and light in the centers of many galaxies. For years astronomers have seen the “smoking gun” from these compact titans in the form of large-scale radio jets and intensely glowing X-rays, but the EHT result has delivered the first direct image of the heart of one of these objects. NANOGrav is involved in a long-standing effort to directly detect not just one, but two supermassive black holes in a tight orbit. This detection will be made not through their light, but through the effect of their gravitational waves on radio pulses from celestial clocks called pulsars.

Astronomers see galaxies merging throughout the universe, some of which should result in binary supermassive black holes. Credit: NASA
Pulsar Watchers Close In On Galaxy Merger History

For the past twelve years, a group of astronomers have been watching the sky carefully, timing pulses of radio waves being emitted by rapidly spinning stars called pulsars, first discovered 50 years ago. These astronomers are interested in understanding pulsars, but their true goal is much more profound; the detection of a new kind of gravitational waves. With a new, more sophisticated analysis, they are much closer than ever before.

The Dawn of Multi-Messenger Astronomy

NANOGrav congratulates our LIGO colleagues and their collaborators across the electromagnetic spectrum on another milestone of modern astronomy: the first detection of a merger of two neutron stars. This first detection of an object in both light and gravitational waves is a remarkable feat and demonstrates the unique power of uniting these two methods to explore our Universe.

Merging Galaxies and Gravitational Waves: From Mpc to mpc

Special Session approved, and contributed papers welcome “Merging Galaxies and Gravitational Waves: From Mpc to mpc”

229th meeting of the American Astronomical Society
Grapevine, TX
6 January 2017

This Special Session will highlight advancements in astrophysics in the low frequency gravitational waveband and will feature a mix of invited and contributed oral presentations and posters.

Gravitational Waves Pass through Pulsar Timing Array ( Credit: NRAO )
Gravitational Wave Search Provides Insights into Galaxy Evolution and Mergers

New results from NANOGrav – the North American Nanohertz Observatory for Gravitational Waves – establish astrophysically significant limits in the search for low-frequency gravitational waves.

Congratulations to our LIGO Colleagues

NANOGrav congratulates our LIGO colleagues on their discovery of gravitational waves from a binary black hole system. This result is a major milestone, not only in the field of gravitational-wave astronomy, but in the history of science!

Gravitational Wave Astronomy Advances with New NSF-funded NANOGrav Physics Frontiers Center

The National Science Foundation (NSF) has awarded the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) $14.5 million over 5 years to create and operate a Physics Frontiers Center (PFC).