Pulsar timing arrays are on track to detect long-period gravitational waves by measuring their effects on the light-travel times of pulses from rotating neutron stars (pulsars). NANOGrav monitors a set of pulsars that together form a Galactic scale gravitational-wave observatory. Our detector is used to study supermassive black hole binaries in order to understand the morphology, kinematics, gas content, and feedback mechanisms of galaxies. Pulsars can also be used to detect gravitational waves from topological defects in space time called cosmic strings, which are predicted by some high energy physics models.

### Upcoming Talks

Statistical Methods For Nanohertz Gravitational Wave Detection

16 November 2020 Statistical Methods for the Detection, Classification, and Inference of Relativistic Objects, ICERM, Brown University

### News

Damage to the Arecibo Telescope

The damage to the NSF’s Arecibo telescope from the falling support cable earlier this month will result in significant disruption to telescope operations, dealing a blow to radio astronomy, planetary science, and aeronomy. Arecibo has been the most sensitive radio telescope in the world for most of the last half century, and a continuing series of upgrades have kept it at the cutting-edge in multiple science areas.

Multi-Messenger Searches for Gravitational Waves in Galaxy 3C66B

When galaxies merge, the supermassive black holes that reside in those galaxies’ centers may form binaries. After inspiralling for millions of years, those supermassive pairs should eventually coalesce. During that inspiral process, they will emit some of the brightest gravitational waves in the universe. NANOGrav’s first multi-messenger search—that is, a search that uses both light and gravitational-wave information to seek a detection—was undertaken on the candidate binary supermassive black hole in galaxy 3C66B. Previous radio observations had led to the hypothesis that this galaxy contains a supermassive black hole binary. The apparent 1.05-year orbital period for the binary in 3C66B makes it an excellent candidate to analyze with pulsar timing arrays.

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.

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 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

## Understanding Gravity

When most people think of gravity, they think of a force that keeps keeps things together: it keeps people on the surface of the Earth, it keeps the Earth in orbit around the Sun, and it even keeps entire galaxies together. This way of thinking about gravity — as a long range force of attraction — was firmly established in the 17th century by Isaac Newton. Newton's law of gravity is a spectacular example of how some simple mathematical rules can accurately explain what we observe in nature, but it isn't the end of the story. By the end of the 19th century, people had found several situations in which the classical physical laws, such as Newton's law of gravity, didn't quite work. Newton's theory isn't totally wrong, but it is incomplete. Few people realized just how profoundly a more complete law of gravity would change our view of the Universe, but that is exactly what happened after Albert Einstein weighed in.

In 1916, Einstein published his general theory of relativity, a completely new way of thinking about gravity. In general relativity, or GR, we think of gravity as a distortion, or curvature, of the fabric of space and time itself (called space-time). In this context, space means the distance between two objects, or the shortest path you could take to get between point A and point B. This is not the same thing as "outer space" — every thing in the Universe exists in space-time, including the Earth and everything on (and in) it. In the concept of space-time, time refers to that which is measured by clocks.

What does this all mean? You can think of space-time as a sheet of fabric that is pulled tight (it isn't a perfect analogy, but it is the simplest one). Now imagine placing a heavy object like a bowling ball on this sheet. The sheet curves around the bowling ball, and according to GR, this curvature is gravity. A smaller object like a golf ball placed on the sheet will naturally fall towards the larger object, and if you give the golf ball a little push, it will circle the bowling ball just like a planet orbiting a star (friction eventually causes the golf ball to hit the bowling ball, but in space this doesn't happen). Anything and everything with mass, from stars to actual golf balls, will cause space-time to curve, and hence create a gravitational field. This may all sound pretty wild, but GR is the most elegant and complete description of gravity ever, and it has passed every test that scientists have ever put it through. In other words, it works, and it works well.

## Understanding Gravitational Waves

One of the predictions of GR is the existence of gravitational waves. Gravitational waves are ripples in space-time that are caused when massive objects move in a certain way. These ripples actually cause objects to shrink and stretch as the wave passes through them, but the effect is tiny — even a very strong gravitational wave will cause an object to shrink and stretch by one part in a quadrillion! (1 quadrillion = 1,000,000,000,000,000)

This animation demonstrates the effect that a passing gravitational wave has on a circular ring of particles. The particles are stretched and pulled in different directions as the wave passes through them. Gravitational waves have two polarizations—"plus" (top) and "cross" (bottom).

We have very good circumstantial evidence that gravitational waves exist as predicted by GR. Observations of neutron stars (massive, dead stars) that are in binary star systems with another neutron star or white dwarf star show that the stars are slowly getting closer. This happens because these binary systems emit gravitational waves that carry away orbital energy. However, we have never seen the actual shrinking and stretching of space-time caused by a passing gravitational wave because the effect is so tiny and difficult to measure. The goal of NANOGrav is to make just such a detection and to use gravitational waves as a tool for studying the Universe.

Not all gravitational waves are identical. Like light waves, we can characterize a gravitational wave by its frequency (the number of waves that pass by us in one second). NANOGrav is sensitive to very, very low frequency gravitational waves, hence the term "nanohertz" in our name. Even though gravitational waves travel at the speed of light, the waves NANOGrav can detect take a billion seconds to go from one peak to the next (in other words, they have very long wavelengths). NANOGrav and other similar experiments are the only way of studying these types of gravitational waves.

## A New Frontier

For the whole of human history, almost everything we have learned about the distant Universe (everything outside our Solar System) has come from studying light. In this sense, light includes all parts of the electromagnetic spectrum: radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma-ray. Some very important contributions have also been made by studying sub-atomic particles, but for the most part light is the main messenger that carries information about the Universe to us. Of course, not everything emits light, or at least light that we can easily detect. Some examples are black holes, white dwarfs, and neutron stars, as well as hypothetical objects called cosmic strings. It is also impossible for us to detect light from when the Universe was younger than about 379,000 years old. But all of these things are predicted to emit gravitational waves.

The direct detection of gravitational waves by the LIGO collaboration has opened an entirely new window on our Universe. Detecting gravitational waves at the much lower frequencies that NANOGrav is sensitive to will allow us to gain unique and complimentary knowledge about our universe. NANOGrav will be able to answer questions about how the most massive black holes in the Universe form, how galaxies merge and grow throughout cosmic history, and how gravity behaves at the limit of our understanding. These are just a few of the discoveries we expect to make, but the unexpected and unpredictable may be even more exciting. In the past, whenever we have opened up new frontiers in astronomy, we have discovered things we never even imagined, and the same will almost certainly be true as we enter the new era of gravitational wave astronomy.

## Pulsar Timing Arrays

Neutron stars are the remains of a dead star that was more massive than our Sun, but not massive enough to form a black hole. Pulsars, which form a special class of neutron stars, are extreme, fascinating, and just plain cool objects in and of themselves — a teaspoon of neutron star material on Earth would weigh as much as the entire human race! And pulsars have magnetic fields so strong that they would erase every credit card and computer hard drive on Earth...even if the pulsar was as far away as the Moon! But from the point of view of NANOGrav, what makes a pulsar so interesting is that it can be used as a very precise clock. This is because pulsars send out a beam of radio waves and because they rapidly spin. Each time the beam of a pulsar points towards the Earth, we see a pulse of radio waves, hence the name pulsar. These pulses can be used just like the tick of a clock. The most precise pulsars, known as millisecond pulsars, spin hundreds of times a second and approach the stability of the best atomic clocks on Earth.

NANOGrav uses pulsars to form a type of cosmic global positioning system that is capable of detecting the minute effects of passing gravitational waves. The GPS that you use in your car or on your phone works by communicating with satellites in orbit around the Earth, and can determine your position very accurately. NANOGrav is using millisecond pulsars to do the same basic thing: to look for tiny changes in the position of the Earth that are due to the shrinking and stretching effect of passing gravitational waves. Just like GPS uses several satellites spread throughout the sky, NANOGrav uses an array of pulsars, forming a pulsar timing array. And since gravitational waves have such a small effect, only the most stable millisecond pulsars that make the best clocks will work.

Since pulsars are distributed throughout our Milky Way galaxy, in a very real sense we are using the Galaxy itself as a gravitational wave detector.