Conceptual Illustration of Multi-Messenger Astrophysics

The supermassive black hole binaries at the cores of galaxies produce electromagnetic waves at radio to gamma-ray wavelengths that can be detected by telescopes on Earth and in space. They also produce gravitational waves that can be studied through their effects on an array of radio pulsars. These dual electromagnetic and gravitational wave messengers provide extremely valuable insights that cannot be gleaned from either type of observation alone. Illustration: Olena Shmahalo

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Multimessenger Astrophysics

The gravitational waves that NANOGrav will detect are produced by binaries of supermassive black holes.  While black holes by themselves are very dark, they can be the brightest objects in the universe when they are actively accreting large amounts of gas, which heats up as plasma and produces electromagnetic emission observable with traditional astronomical instruments. Accreting supermassive black holes that we can observe electromagnetically are called active galactic nuclei.  We have learned a huge amount about supermassive black holes by observing active galactic nuclei, for example that their properties are closely related to those of their host galaxies, and that the energy emitted by the brightest active galactic nuclei (called quasars) is enough to halt the formation of stars across their entire host galaxies.  

Spectrum Snapshots of the Whirlpool Galaxy

Shown above is several snapshots through the electromagnetic (light) spectrum of the Spiral Galaxy, M51, which is also known as the Whirlpool Galaxy. Under each image is a description of what process produced the specific form of light that we see through our telescopes. Courtesy of NASA/University of Chicago.

While there are now millions of known active galactic nuclei and quasars, there has yet to be an example with a confirmed binary supermassive black hole.  Nonetheless, a huge number of candidate binaries have been identified in active galactic nuclei that exhibit different types of unusual characteristics that might hint at the presence of a binary companion.  Indeed, there are a very large number of such electromagnetic characteristics that have been proposed as signaling the presence of a supermassive black hole binary.  The detection of gravitational waves will likely be necessary to definitely determine which of these candidates, and what electromagnetic signatures, are indeed from true binaries.

More broadly, gravitational wave and electromagnetic observations are extremely complementary.  By combining both types of signatures, in what’s called multi messenger astronomy, we’ll be able to learn vastly more about these incredible systems than we could with either type of observation alone.  Electromagnetic signals are produced by the hottest, high energy plasma, while gravitationally waves are produced by the innermost, highest-mass regions that are often dark to traditional telescopes.  Similarly, telescope observations offer us information about the accretion rates and environments of supermassive black hole binaries, while pulsar timing arrays can unveil the evolving distribution of masses and forces.

While traditional astronomy is conducted using electromagnetic signals, gravitational wave astronomy unveils a different side of astrophysical phenomena — how moving masses deform spacetime. By combining the gravitational wave signals that NANOGrav studies with electromagnetic observations of the same sources, we can glean even more information about the way galaxies merge and grow over cosmic time.

More Topics

Conceptual Illustration of Radio Observations of Pulsar Timing Arrays

Radio Astronomy

We observe with the largest telescopes in the world in order to detect electromagnetic waves with the very longest wavelengths. Radio astronomy allows us to probe energetic processes which are invisible to optical telescopes.

Pulsar Conceptual Illustration

Pulsars as Cosmic Clocks

Neutron stars are the collapsed cores of massive stars which have ended their lives in cataclysmic supernova explosions. Pulsars are rapidly rotating, highly magnetic neutron stars which emit beamed radio emission, like cosmic lighthouses. They are unique laboratories for a variety of fundamental physics experiments.

Gravitational Waves Conceptual Illustration

Low-Frequency Gravitational Waves

Gravitational waves are ripples in space-time predicted by Einstein's theory of general relativity. They are produced by accelerating masses and can be used to study objects like black hole binaries which do not emit visible light. Our experiment is sensitive to low-frequency gravitational waves with periods of years to decades.

Abstract Conceptual Illustration of Galactic Evolution

Galaxies and Supermassive Black Holes

Galaxies have evolved through cosmic time through consecutive mergers with other galaxies. We will gain unique insights into this important process by detecting the gravitational waves produced by extremely massive pairs of black holes at the cores of merged galaxies.