North American Nanohertz Observatory for Gravitational Waves

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NANOGrav
Alternative namesNANOGrav
Website https://nanograv.org

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is a consortium of astronomers [1] who share a common goal of detecting gravitational waves via regular observations of an ensemble of millisecond pulsars using the Green Bank Telescope, Arecibo Observatory, the Very Large Array, and the Canadian Hydrogen Intensity Mapping Experiment (CHIME). Future observing plans include up to 25% total time of the Deep Synoptic Array 2000 (DSA2000). This project is being carried out in collaboration with international partners in the Parkes Pulsar Timing Array in Australia, the European Pulsar Timing Array, and the Indian Pulsar Timing Array as part of the International Pulsar Timing Array.

Contents

Gravitational wave detection using pulsar timing

Plot of correlation between pulsars observed by NANOGrav vs angular separation between pulsars, compared with a theoretical model (dashed purple, or Hellings-Downs curve) and if there were no gravitational wave background (solid green) Correlation vs angular separation between pulsars.svg
Plot of correlation between pulsars observed by NANOGrav vs angular separation between pulsars, compared with a theoretical model (dashed purple, or Hellings–Downs curve) and if there were no gravitational wave background (solid green)

Gravitational waves are an important prediction from Einstein's general theory of relativity and result from the bulk motion of matter, fluctuations during the early universe, and the dynamics of space-time itself. Pulsars are rapidly rotating, highly magnetized neutron stars formed during the supernova explosions of massive stars. They act as highly accurate clocks with a wealth of physical applications ranging from celestial mechanics, neutron star seismology, tests of strong-field gravity, and Galactic astronomy.

The idea to use pulsars as gravitational wave detectors was originally proposed by Sazhin [4] and Detweiler [5] in the late 1970s. The idea is to treat the solar system barycenter and a distant pulsar as opposite ends of an imaginary arm in space. The pulsar acts as the reference clock at one end of the arm sending out regular signals which are monitored by an observer on the Earth. The effect of a passing gravitational wave would be to perturb the local space-time metric and cause a change in the observed rotational frequency of the pulsar.

Hellings and Downs [6] extended this idea in 1983 to an array of pulsars and found that a stochastic background of gravitational waves would produce a correlated signal for different angular separations on the sky, now known as the Hellings–Downs curve. This work was limited in sensitivity by the precision and stability of the pulsar clocks in the array. Following the discovery of the first millisecond pulsar in 1982, Foster and Donald C. Backer [7] were among the first astronomers to seriously improve the sensitivity to gravitational waves by applying the Hellings-Downs analysis to an array of highly stable millisecond pulsars.

The advent of state-of-the-art digital data acquisition systems, new radio telescopes and receiver systems and the discoveries of many new pulsars advanced the sensitivity of the pulsar timing array to gravitational waves. The 2010 paper by Hobbs et al. [8] summarizes the early state of the international effort. The 2013 Demorest et al. [9] paper describes the five-year data release, analysis, and first NANOGrav limit on the stochastic gravitational wave background. It was followed by the nine-year and 11-year data releases in 2015 and 2018, respectively. Each further limited the gravitational wave background and, in the second case, techniques to precisely determine the barycenter of the solar system were refined.

In 2020, the collaboration presented the first evidence of gravitational wave background within the 12.5-year data release, taking the shape of a noise consistent with the expectations; however, it could not be definitely attributed to gravitational waves. [10] [11]

In the 2020 Decadal Survey of Astronomy and Astrophysics, the National Academies of Science named NANOGrav as one of eight mid-scale astrophysics projects recommended as high priorities for funding in the next decade.

In June 2023, NANOGrav published further evidence for a stochastic gravitational wave background using the 15-year data release. In particular, it provides a measurement of the Hellings–Downs curve, [12] the unique sign of the gravitational wave origin of the observations. [13] [14]

Funding sources


The NSF first funded researchers within NANOGrav as part of the Partnerships for International Research and Education (PIRE) program from 2010 to 2015; the Physics Frontiers Center (PFC) program from 2015 to 2021; and from a second PFC grant starting in 2021. NANOGrav as a NSF PFC has been supported by the NSF Divisions of Physics and Astronomical Sciences and the Windows on the Universe program. The NSF has also contributed to supporting International Pulsar Timing Array through the AccelNet program. NANOGrav has additionally been supported by The Gordon and Betty Moore Foundation, the Natural Sciences and Engineering Research Council of Canada, and the Canadian Institute for Advanced Research.

The research activities of NANOGrav have also been supported by single-investigator grants awarded through the Natural Sciences and Engineering Research Council (NSERC) in Canada, the National Science Foundation (NSF) and the Research Corporation for Scientific Advancement in the USA.

Related Research Articles

Cosmic strings are hypothetical 1-dimensional topological defects which may have formed during a symmetry-breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking was not simply connected. Their existence was first contemplated by the theoretical physicist Tom Kibble in the 1970s.

<span class="mw-page-title-main">Pulsar</span> Rapidly rotating neutron star

A pulsar is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Earth, and is responsible for the pulsed appearance of emission. Neutron stars are very dense and have short, regular rotational periods. This produces a very precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar. Pulsars are one of the candidates for the source of ultra-high-energy cosmic rays.

<span class="mw-page-title-main">Millisecond pulsar</span> Pulsar with a rotational period less than about 10 milliseconds

A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio, X-ray, and gamma ray portions of the electromagnetic spectrum. The leading hypothesis for the origin of millisecond pulsars is that they are old, rapidly rotating neutron stars that have been spun up or "recycled" through accretion of matter from a companion star in a close binary system. For this reason, millisecond pulsars are sometimes called recycled pulsars.

<span class="mw-page-title-main">Gravitational wave background</span> Random background of gravitational waves permeating the Universe

The gravitational wave background is a random background of gravitational waves permeating the Universe, which is detectable by gravitational-wave experiments, like pulsar timing arrays. The signal may be intrinsically random, like from stochastic processes in the early Universe, or may be produced by an incoherent superposition of a large number of weak independent unresolved gravitational-wave sources, like supermassive black-hole binaries. Detecting the gravitational wave background can provide information that is inaccessible by any other means about astrophysical source population, like hypothetical ancient supermassive black-hole binaries, and early Universe processes, like hypothetical primordial inflation and cosmic strings.

The Tolman–Oppenheimer–Volkoff limit is an upper bound to the mass of cold, non-rotating neutron stars, analogous to the Chandrasekhar limit for white dwarf stars. Stars more massive than the TOV limit collapse into a black hole. The original calculation in 1939, which neglected complications such as nuclear forces between neutrons, placed this limit at approximately 0.7 solar masses (M). Later, more refined analyses have resulted in larger values.

<span class="mw-page-title-main">Gravitational wave</span> Propagating spacetime ripple

Gravitational waves are transient displacements in a gravitational field—generated by the motion or acceleration of gravitating masses—that radiate outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1893 and then later by Henri Poincaré in 1905 as the gravitational equivalent of electromagnetic waves. In 1916, Albert Einstein demonstrated that gravitational waves result from his general theory of relativity as ripples in spacetime.

<span class="mw-page-title-main">Gravitational-wave observatory</span> Device used to measure gravitational waves

A gravitational-wave detector is any device designed to measure tiny distortions of spacetime called gravitational waves. Since the 1960s, various kinds of gravitational-wave detectors have been built and constantly improved. The present-day generation of laser interferometers has reached the necessary sensitivity to detect gravitational waves from astronomical sources, thus forming the primary tool of gravitational-wave astronomy.

<span class="mw-page-title-main">Gravitational-wave astronomy</span> Branch of astronomy using gravitational waves

Gravitational-wave astronomy is a subfield of astronomy concerned with the detection and study of gravitational waves emitted by astrophysical sources.

A pulsar timing array (PTA) is a set of galactic pulsars that is monitored and analyzed to search for correlated signatures in the pulse arrival times on Earth. As such, they are galactic-sized detectors. Although there are many applications for pulsar timing arrays, the best known is the use of an array of millisecond pulsars to detect and analyse long-wavelength gravitational wave background. Such a detection would entail a detailed measurement of a gravitational wave (GW) signature, like the GW-induced quadrupolar correlation between arrival times of pulses emitted by different millisecond pulsar pairings that depends only on the pairings' angular separations in the sky. Larger arrays may be better for GW detection because the quadrupolar spatial correlations induced by GWs can be better sampled by many more pulsar pairings. With such a GW detection, millisecond pulsar timing arrays would open a new low-frequency window in gravitational-wave astronomy to peer into potential ancient astrophysical sources and early Universe processes, inaccessible by any other means.

<span class="mw-page-title-main">European Pulsar Timing Array</span> Five-radio telescope collaboration to track stellar remnants gravitational waves

The European Pulsar Timing Array (EPTA) is a European collaboration to combine five 100-m class radio-telescopes to observe an array of pulsars with the specific goal of detecting gravitational waves. It is one of several pulsar timing array projects in operation, and one of the four projects comprising the International Pulsar Timing Array, the others being the Parkes Pulsar Timing Array, the North American Nanohertz Observatory for Gravitational Waves, and the Indian Pulsar Timing Array.

PSR J1614–2230 is a pulsar in a binary system with a white dwarf in the constellation Scorpius. It was discovered in 2006 with the Parkes telescope in a survey of unidentified gamma ray sources in the Energetic Gamma Ray Experiment Telescope catalog. PSR J1614–2230 is a millisecond pulsar, a type of neutron star, that spins on its axis roughly 317 times per second, corresponding to a period of 3.15 milliseconds. Like all pulsars, it emits radiation in a beam, similar to a lighthouse. Emission from PSR J1614–2230 is observed as pulses at the spin period of PSR J1614–2230. The pulsed nature of its emission allows for the arrival of individual pulses to be timed. By measuring the arrival time of pulses, astronomers observed the delay of pulse arrivals from PSR J1614–2230 when it was passing behind its companion from the vantage point of Earth. By measuring this delay, known as the Shapiro delay, astronomers determined the mass of PSR J1614–2230 and its companion. The team performing the observations found that the mass of PSR J1614–2230 is 1.97 ± 0.04 M. This mass made PSR J1614–2230 the most massive known neutron star at the time of discovery, and rules out many neutron star equations of state that include exotic matter such as hyperons and kaon condensates.

The International Pulsar Timing Array (IPTA) is a multi-institutional, multi-telescope collaboration comprising the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the Parkes Pulsar Timing Array (PPTA) in Australia, and the Indian Pulsar Timing Array Project (InPTA). The goal of the IPTA is to detect ultra-low-frequency gravitational waves, such as from mergers of supermassive black holes, using an array of approximately 30 pulsars. This goal is shared by each of the participating institutions, but they have all recognized that their goal will be achieved more quickly by combining their respective efforts and resources.

<span class="mw-page-title-main">PSR J0348+0432</span> Pulsar–white dwarf binary system in Taurus constellation

PSR J0348+0432 is a pulsar–white dwarf binary system in the constellation Taurus. It was discovered in 2007 with the National Radio Astronomy Observatory's Robert C. Byrd Green Bank Telescope in a drift-scan survey.

Maura Ann McLaughlin is an astrophysics professor at West Virginia University in Morgantown, West Virginia known for her work on fast radio bursts (FRBs).

<span class="mw-page-title-main">NGC 7019</span> Galaxy in the constellation Capricornus

NGC 7019 is a spiral galaxy located about 480 million light-years away in the constellation of Capricornus. It was discovered by American astronomer Francis Leavenworth in 1886. It is host to a supermassive black hole with an estimated mass of with an estimated mass of 7.5 × 107M.

<span class="mw-page-title-main">Ingrid Stairs</span> Canadian astronomer

Ingrid Stairs is a Canadian astronomer currently based at the University of British Columbia. She studies pulsars and their companions as a way to study binary pulsar evolution, pulsar instrumentation and polarimetry, and Fast Radio Bursts (FRBs). She was awarded the 2017 Rutherford Memorial Medal for physics of the Royal Society of Canada, and was elected as a Fellow of the American Physical Society in 2018.

Steven L. Detweiler was a theoretical physicist and professor of physics at the University of Florida best known for proposing pulsar timing arrays as a means to detect gravitational waves, an idea that led to the discovery of a stochastic gravitational wave background in 2023.

<span class="mw-page-title-main">Hellings-Downs curve</span> Gravitational wave detection tool

The Hellings-Downs curve is a theoretical tool used to establish the telltale signature that a galactic-scale pulsar timing array has detected gravitational waves, typically of wavelengths . The method entails searching for spatial correlations of the timing residuals from pairs of pulsars and comparing the data with the Hellings-Downs curve. When the data fit exceeds the standard 5 sigma threshold, the pulsar timing array can declare detection of gravitational waves. More precisely, the Hellings-Downs curve is the expected correlations of the timing residuals from pairs of pulsars as a function of their angular separation on the sky as seen from Earth. This theoretical correlation function assumes Einstein's general relativity and a gravitational wave background that is isotropic.

<span class="mw-page-title-main">NGC 1419</span> Galaxy in the constellation Eridanus

NGC 1419 is an elliptical galaxy located 62 millon light years away in the constellation of Eridanus. The galaxy was discovered by astronomer John Herschel on October 22, 1835, and is a member of the Fornax Cluster. NGC 1419 is a host to a supermassive black hole with an estimated mass of 25 millon solar masses.

<span class="mw-page-title-main">NGC 5419</span> Galaxy in the constellation Centaurus

NGC 5419 is a large elliptical galaxy in the constellation of Centaurus. Its velocity with respect to the cosmic microwave background is 4,375 ± 23 km/s, which corresponds to a Hubble distance of 64.5 ± 4.5 Mpc. It was discovered by British astronomer John Herschel on 1 May 1834.

References

  1. Jenet, F.; et al. (2009). "The North American Nanohertz Observatory for Gravitational Waves". arXiv: 0909.1058 [astro-ph.IM].
  2. "IOPscience - Focus on NANOGrav's 15 yr Data Set and the Gravitational Wave Background".
  3. "After 15 years, pulsar timing yields evidence of cosmic gravitational wave background". 2022.
  4. Sazhin, M.V. (1978). "Opportunities for detecting ultralong gravitational waves". Sov. Astron. 22: 36–38. Bibcode:1978SvA....22...36S.
  5. Detweiler, S.L. (1979). "Pulsar timing measurements and the search for gravitational waves". Astrophysical Journal . 234: 1100–1104. Bibcode:1979ApJ...234.1100D. doi:10.1086/157593.
  6. Hellings, R.W.; Downs, G.S. (1983). "Upper limits on the isotropic gravitational radiation background from pulsar timing analysis". Astrophysical Journal Letters . 265: L39–L42. Bibcode:1983ApJ...265L..39H. doi: 10.1086/183954 .
  7. Foster, R.S.; Backer, D.C. (1990). "Constructing a pulsar timing array". Astrophysical Journal . 361: 300–308. Bibcode:1990ApJ...361..300F. doi:10.1086/169195.
  8. Hobbs, G.; et al. (2010). "The International Pulsar Timing Array project: using pulsars as a gravitational wave detector". Classical and Quantum Gravity . 27 (8): 084013. arXiv: 0911.5206 . Bibcode:2010CQGra..27h4013H. doi:10.1088/0264-9381/27/8/084013. S2CID   56073764.
  9. Demorest, P.; et al. (2013). "Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves". Astrophysical Journal . 762 (2): 94–118. arXiv: 1201.6641 . Bibcode:2013ApJ...762...94D. doi:10.1088/0004-637X/762/2/94. S2CID   13883914.
  10. Arzoumanian, Zaven; Baker, Paul T.; Blumer, Harsha; Bécsy, Bence; Brazier, Adam; Brook, Paul R.; Burke-Spolaor, Sarah; Chatterjee, Shami; Chen, Siyuan; Cordes, James M.; Cornish, Neil J.; Crawford, Fronefield; Cromartie, H. Thankful; Decesar, Megan E.; Demorest, Paul B. (2020-12-01). "The NANOGrav 12.5 yr Data Set: Search for an Isotropic Stochastic Gravitational-wave Background". The Astrophysical Journal. 905 (2): L34. arXiv: 2009.04496 . Bibcode:2020ApJ...905L..34A. doi: 10.3847/2041-8213/abd401 . ISSN   0004-637X.
  11. O'Neill, Ian; Cofield, Calla (11 January 2021). "Gravitational Wave Search Finds Tantalizing New Clue". NASA . Retrieved 11 January 2021.
  12. "Hellings and Downs curve". astro.vaporia.com. Retrieved 29 June 2023.
  13. Agazie, Gabriella; Anumarlapudi, Akash; Archibald, Anne M.; Arzoumanian, Zaven; Baker, Paul T.; Bécsy, Bence; Blecha, Laura; Brazier, Adam; Brook, Paul R.; Burke-Spolaor, Sarah; Burnette, Rand; Case, Robin; Charisi, Maria; Chatterjee, Shami; Chatziioannou, Katerina (2023-07-01). "The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background". The Astrophysical Journal Letters. 951 (1): L8. arXiv: 2306.16213 . Bibcode:2023ApJ...951L...8A. doi: 10.3847/2041-8213/acdac6 . ISSN   2041-8205.
  14. NANOGrav Collaboration (29 June 2023). "Focus on NANOGrav's 15 yr Data Set and the Gravitational Wave Background". The Astrophysical Journal Letters.