Are astronomers seeing a signal from giant black holes?

An international team of astronomers has discovered what could be the early sign of a background signal arising from supermassive black holes, observed through low-frequency gravitational waves. These scientists are comparing data collected from several instruments, including the National Science Foundation's Green Bank Telescope (GBT.)

Gravitational Waves ripple through spacetime at a light-year-scale, and could originate from mergers of the most massive black holes in the Universe-or from events occurring soon after the formation of the Universe in the Big Bang.

The International Pulsar Timing Array (IPTA) joins the work of several astrophysics collaborations from around the world, including independent data sets of the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array in Australia (PPTA).

The IPTA has shared a new data release, known as Data Release 2 (DR2) consisting of precision timing data from 65 millisecond pulsars-stellar remnants which spin hundreds of times per second, sweeping narrow beams of radio waves that appear as pulses due to the spinning. 20 of these pulsars were observed by the Green Bank Telescope.

"The GBT contributes to the IPTA as one of the most important telescopes used by NANOGrav. The combination of the GBT's excellent sensitivity, instruments, and ability to see so much of the sky make it a critical part of the IPTA's efforts," shares Dr. Ryan Lynch, a Green Bank Observatory scientist and NANOGrav member.

Research of the combined IPTA DR2, and other independent data sets from the three constituent collaborations, has revealed strong evidence for this new low-frequency gravitational wave background signal, correlated many of the pulsars. The characteristics of this common-among-pulsars signal are in broad agreement with those expected from a GW "background" (GWB).

This background is formed by many different overlapping GW signals emitted from the cosmic population of supermassive binary black holes (i.e., two supermassive black holes orbiting each other and eventually merging), analogous to background noise from the many overlapping voices in a crowded hall. This result further strengthens the gradual emergence of similar signals that have been found in the individual data sets of the participating collaborations over the past few years.

But scientists caution they do not yet have definitive evidence for the GWB, and are still looking into what else this signal could be, and gathering more information to strengthen their findings. The "smoking gun" for a gravitational wave detection is a unique relationship in the strength of the signal between pulsars in different parts of the sky.

While these "spatial correlations" have not yet been detected, the existing signal is consistent with what scientists expect to see at first. The IPTA is working diligently to analyze more recent data, which could confirm the nature of the new signal. In addition, contributions from new telescopes such as MeerKAT and from other collaborations, such as the India Pulsar Timing Array, will be important in the future.

Dr. Maura McLaughlin of West Virginia University, who uses the GBT for data collection for NANOGrav, says that, "If the signal we are currently seeing is the first hint of a GWB, then based on our simulations, it is possible we will have more definite measurements of the spatial correlations necessary to conclusively identify the origin of the common signal in the near future."

"The IPTA is a great example of scientists and instruments from around the world coming together to advance our understanding of the cosmos," shares Lynch. The Green Bank Observatory is developing new technology to enhance the GBT's capabilities for this research, "New instruments, like our upcoming ultrawideband receiver [funded by the Moore Foundation], will ensure that the GBT continues to make essential contributions to NANOGrav and the IPTA. If what we are seeing here is indeed the signature of gravitational waves, then the next few years are going to be really exciting."

Related Links
Green Bank Telescope
Understanding Time and Space

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