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  • richardmitnick 9:43 pm on March 1, 2018 Permalink | Reply
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    From GBO: “Pulsar Watchers Close In On Galaxy Merger History” 

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    Green Bank Radio Telescope, West Virginia, USA
    Green Bank Radio Telescope, West Virginia, USA

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    Green Bank Observatory

    2018-02-28
    Paul Vosteen

    1
    Astronomers see galaxies merging throughout the universe, some of which should result in binary supermassive black holes. Credit: NASA

    Fifty years after pulsar discovery published, massive new data set moves closer to finding very-low-frequency gravitational waves, researchers say.

    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.

    Gravitational waves are wrinkles in space-time that stretch and squeeze the distances between objects. In 2015, a hundred years after Albert Einstein realized that accelerating massive objects should produce them, these waves were finally detected from black holes with masses roughly 30 times the mass of our sun colliding with each other. However, Einstein’s theory also predicts another kind of wave, one that comes from the mergers of black holes with masses of hundred million times the sun’s.

    Astronomers believe that nearly all galaxies have supermassive black holes at their centers. When two galaxies collide, these black holes will slowly fall toward each other, finally merging long after the initial galaxy collision. In the last stage of this process, as the two black holes spiral closer to each other, strong gravitational waves can be produced.

    While these waves travel at the speed of light, their strength varies quite slowly, on timescales ranging from months to years. This means that gravitational wave observatories on Earth can’t measure them. For that, you need an observatory with detectors light-years apart.

    “We know that galaxy mergers are an important part of galaxy growth and evolution through cosmic time. By detecting gravitational waves from supermassive binary black holes at the cores of merging galaxies, we will be able to probe how galaxies are shaped by those black holes,” said Sarah Burke-Spolaor, assistant professor at West Virginia University.

    2
    Nature publication of the discovery of pulsar B1919+21. Credit: Reproduced by permission from Springer Nature

    Fifty years ago, the February 24, 1968 edition of the journal Nature provided the solution, with the discovery of a new kind of star. This new star was curious, emitting regular radio pulses once every 1.3 seconds. Graduate student Jocelyn Bell (now Dr. Bell Burnell [now really Dame Susan Jocelyn Bell Burnell, one of the many women denied a deserved Nobel]) was the first to spot the signal, seeing it as “a bit of scruff” in her radio surveys. Zooming in on the scruff, Bell saw the regular pulses from the star.

    After first entertaining the possibility that the pulses could be the result of LGM, or “little green men,” the new star was dubbed a pulsar, with the understanding that the pulses represented the rotation rate of the star. Such a rapid rotation rate meant that the star must be small, about the size of a city. Only a few years later, a pulsar in a binary system was found, and the first mass estimate indicated that this tiny object held about one and a half times the mass of our sun.

    “Before this time, no one thought stars so small could actually exist! It wasn’t until a pulsar was found at the center of a supernova remnant in 1968 that astronomers realized that pulsars were neutron stars born in the explosions of massive stars,” said Maura McLaughlin, professor at West Virginia University.

    4
    After detecting unexpected signals at the same location in the sky (top left), graduate student Jocelyn Bell (right) [now Dame Susan Jocelyn Bell Burnell] observed individual pulses from the new source (bottom left) in late 1967. Credit: UK National Science & Media Museum

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    2009 Dame Susan Jocelyn Bell Burnell. Wikipedia

    The fastest pulsars, called millisecond pulsars, spin hundreds of times every second (faster than your kitchen blender!), and are the most stable natural clocks known in the universe. Pulsar astronomers around the globe are monitoring these stellar clocks in order to form a new kind of cosmic gravitational wave detector known as a “Pulsar Timing Array.” By carefully measuring when radio pulses arrive from millisecond pulsars, astronomers can track the tiny changes in the distance from the Earth to the pulsars caused by the stretching and squeezing of spacetime due to a gravitational wave.

    In the US and Canada, a group called NANOGrav (North American Nanohertz Observatory for Gravitational Waves) is searching for these gravitational waves using some of the largest telescopes in the world, including the Green Bank Telescope in West Virginia and the Arecibo Observatory in Puerto Rico.

    NAIC/Arecibo Observatory, Puerto Rico, USA, at 497 m (1,631 ft)

    NANOGrav routinely joins forces with groups in Europe and Australia to improve their sky coverage and sensitivity. Collectively known as the International Pulsar Timing Array, the combined observations from these groups constitute the most sensitive data set in the world for searching for low-frequency gravitational waves.

    6
    International Pulsar Timing Array

    This month, fifty years after the publication of the first pulsar discovery, NANOGrav has submitted a pair of companion papers to The Astrophysical Journal describing eleven years of monthly observations of 45 millisecond pulsars along with the astrophysical implications of their results. For the first time, the data set includes a six-pulsar “high-frequency” sample, with measurements made every week to expand the pulsar timing array’s sensitivity range. NANOGrav is able to set sensitive upper limits that constrain the physical processes at play in galaxy mergers. As their sensitivity improves, NANOGrav is uncovering new sources of background noise that must be accounted for. Most recently, uncertainties in the pull of Jupiter on the sun have been found to affect pulsar timing. As a result, the team is implementing new computational methods to account for this, in effect determining Jupiter’s orbit more precisely than possible except by planetary missions.

    “This is the most sensitive pulsar timing dataset ever created for both gravitational wave analysis and a host of other astrophysical measurements. And with each new release, we will add more pulsars and data, which increase our sensitivity to gravitational waves”, said David Nice, professor at Lafayette College.

    Last year, the journal that announced the discovery of pulsars once again played host to a pulsar first. In November, Nature Astronomy published their first-ever article describing the gravitational wave environment that pulsar timing arrays are working to uncover. By looking at galaxy surveys, the article estimates there are about 100 supermassive black hole binaries that are close enough to affect pulsar timing array measurements. Given their expected future sensitivity, the authors state that pulsar timing arrays should be able to isolate the gravitational waves from a specific individual galaxy within about 10 years.

    “From city-sized pulsars spinning fast in galaxies to large, massive galaxies themselves and their merging central black holes, all in 50 years! That is a large step for humankind, and not one that we could have foreseen. What will the next 50 years bring? Pulsars and gravitational waves will continue to be big news, I’m sure!” said Jocelyn Bell Burnell.

    A century after Einstein first predicted them, gravitational waves were finally detected. Now, 50 years after Jocelyn Bell’s discovery, pulsars have become a new tool for measuring both gravitational waves and the distant black holes that create them. If predictions are correct, the next decade will be an exciting period of discovery for radio astronomers, pulsars, and gravitational waves!

    Links to supporting materials:
    1-page summary of 11-year results: https://nanograv.github.io/11yr_stochastic_analysis/ Submitted to the Astrophysical Journal, Dec 31, 2017

    11-Year Data Release paper: https://arxiv.org/abs/1801.01837 Submitted to The Astrophysical Journal

    Gravitational Wave Search paper: https://arxiv.org/abs/1801.02617 Submitted to The Astrophysical Journal

    See the full article here .

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    Mission Statement

    Green Bank Observatory enables leading edge research at radio wavelengths by offering telescope, facility and advanced instrumentation access to the astronomy community as well as to other basic and applied research communities. With radio astronomy as its foundation, the Green Bank Observatory is a world leader in advancing research, innovation, and education.

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    60 years ago, the trailblazers of American radio astronomy declared this facility their home, establishing the first ever National Radio Astronomy Observatory within the United States and the first ever national laboratory dedicated to open access science. Today their legacy is alive and well.

     
  • richardmitnick 1:37 pm on March 30, 2016 Permalink | Reply
    Tags: , , , , Pulsar timing arrays,   

    From AAS NOVA: “Outlook for Detecting Gravitational Waves with Pulsars” 

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    Amercan Astronomical Society

    Network of pulsars could be used to search for the ripples in space-time.  David Champion NASA JPL
    Network of pulsars could be used to search for the ripples in space-time. David Champion NASA/JPL

    Though the recent discovery of GW150914 is a thrilling success in the field of gravitational-wave astronomy, LIGO is only one tool the scientific community is using to hunt for these elusive signals.

    MIT/Caltech Advanced aLIGO Hanford Washington USA installation
    MIT/Caltech Advanced aLIGO Hanford Washington USA installation

    After 10 years of unsuccessful searching, how likely is it that pulsar-timing-array projects will make their own first detection soon?

    Supermassive Background

    Ground-based laser interferometers like LIGO are ideal for probing ripples in space-time caused by the merger of stellar-mass black holes; these mergers cause chirps in the frequency range of tens to thousands of hertz. But how do we pick up the extremely low-frequency, nanohertz background signal caused by the orbits of pairs of supermassive black holes? For that, we need pulsar timing arrays.

    Pulsar timing arrays are sets of pulsars whose signals are analyzed to look for correlations in the pulse arrival time. As the [spacetime] between us and a pulsar is stretched and then compressed by a passing gravitational wave, the pulsar’s pulses should arrive a little late and then a little early. Comparing these timing residuals in an array of pulsars could theoretically allow for the detection of the gravitational waves causing them.

    Globally, there are currently four pulsar timing array projects actively searching for this signal, with a fifth planned for the future.

    [Current pulsar timing array experiments

    The Parkes Pulsar Timing Array [PPTA] at the Parkes radio-telescope has been collecting data since March 2005.

    CSIRO/Parkes Observatory
    Parkes Phased Array Feed
    Parkes Observatory radio telescope and the Parkes Phased Array Feed

    The European Pulsar Timing Array(EPTA) uses data from the four largest radio telescopes in Europe:
    Lovell Telescope

    Jodrell Bank Lovell Telescope
    Jodrell Bank Lovell Telescope

    Westerbork Synthesis Radio Telescope

    Westerbork Synthesis Radio Telescope
    Westerbork Synthesis Radio Telescope

    Effelsberg Telescope

    MPIFR/Effelsberg Radio Telescope
    MPIFR/Effelsberg Radio Telescope

    Nançay Radio Telescope.

    Nançay decametric radio telescope
    Nançay decametric radio telescope, Nançay, France

    Upon completion the Sardinia Radio Telescope will be added to the EPTA also.
    The North American Nanohertz Observatory for Gravitational Waves uses data collected by the NAIC/Arecibo and [GBO]Green Bank radio telescopes.

    NAIC/Arecibo Observatory
    NAIC/Arecibo Observatory

    NRAO/GBT
    NRAO/GBT ]

    Now a team of scientists led by Stephen Taylor (NASA-JPL/Caltech) has estimated the likelihood that these projects will successfully detect gravitational waves in the future.

    Probability for Success

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    Expected detection probability of the gravitational-wave background as a function of observing time, for five different pulsar timing arrays. Optimistic and conservative assumptions are made for merger rates (blue and red lines, respectively) and environmental conditions (solid and dashed lines, respectively). [Taylor et al. 2016]

    Taylor and collaborators statistically analyzed the detection probability for each of the projects as a function of their observing time, based on the projects’ estimated sensitivities and both conservative and optimistic assumptions about merger rates and environmental influences.

    First the bad news: based on the authors’ estimates, small arrays — which contain only a few pulsars that each have minimal timing noise — will not be likely to detect gravitational waves within the next two decades. These arrays are more useful for setting upper limits on the amplitude of the gravitational-wave background.

    On the other hand, large pulsar timing arrays have far more promising detection probabilities. These include the Parkes Pulsar Timing Array, the European Pulsar Timing Array, and NANOGrav — which each target tens of pulsars, with the intent to add more in the future — as well as the International Pulsar Timing Array, which combines the efforts of all three of these projects. There is an 80% chance that, within the next decade, these projects will successfully detect the gravitational-wave background created by orbiting supermassive black holes.

    Based on this study, the outlook for these large arrays remains optimistic even in non-ideal conditions (such as if supermassive-black-hole merger rates are lower than we thought). So, though we may still have to wait a few years, the possibility of probing an otherwise inaccessible range of frequencies continues to make pulsar timing arrays a promising avenue of study for gravitational waves.

    Citation

    S. R. Taylor et al 2016 ApJ 819 L6. doi:10.3847/2041-8205/819/1/L6

    The science team:
    S. R. Taylor1,2, M. Vallisneri1,2, J. A. Ellis1,2,2, C. M. F. Mingarelli1,2,3, T. J. W. Lazio1,2, and R. van Haasteren1,2,2

    Author affiliations

    1 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

    2 TAPIR Group, MC 350-17, California Institute of Technology, Pasadena, California 91125, USA

    3 Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, D-53121 Bonn, Germany

    See the full article here .

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