IPTA 2014 Meeting in Banff, Alberta Canada

This year’s IPTA meeting will be held in Banff from June 16 through June 27.  Pre-registration has closed, but if you plan to attend please contact the LOC at iptabanff2014@gmail.com as soon as possible.  The meeting format will be consistent with past years and will have a week long student workshop followed by a week of science talks.

New this year will be a Gravitational Wave Detection Workshop held jointly with the GWIC (LIGO/LISA) community on Friday June 27.

Please see ipta.phys.wvu.edu for more information on this meeting and to browse information from past IPTA meetings.  We look forward to seeing you in Banff!

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IPTA J1713 24 Hour Global Campaign

The IPTA and collaborators recently undertook an ambitious, 24-hour-long observation of millisecond pulsar J1713+0747.

The “J1713 24 hour global campaign” was conceived at the 2012 IPTA Science Meeting in Kiama, Australia, as a way to characterize the absolute noise floor for the methods used by the pulsar timing array (PTA) effort. In other words: a perfect pulsar would become a more and more accurate clock over the years depending on how many pulses are gathered in an individual 30min observation. (To be specific, timing precision would be improved by a factor . In other words, four times as many pulses collected in four times as much observing time means twice the timing precision.) J1713 is one of a handful of pulsars observed regularly by the IPTA that’s a serious candidate for the perfect pulsar. With 219 pulses emitted per second, the thirty minute or so observation done weekly over the last eight years yields about 394,000 pulses. Were we to observe it for 24 hours straight, in principle, the error bar on that day’s timing accuracy should go down by a factor of  . Because, however, even J1713 is not really a perfect pulsar, we expect this number to be much lower. How much lower? The answer says a lot about what can be done to understand and minimize timing noise (which is the same as saying, minimize our clock irregularities), and thus, to detect gravitational waves sooner!

The observation used nine of the largest radio telescopes in the world for this campaign: the Parkes telescope in Australia (featured in the movie The Dish), the GMRT (Giant Meterwave Radio Telescope) in India, the Nancay radio telescope in France, the Effelsberg radio telescope in Germany, the WSRT (the Westerbork Synthesis Radio Telescope) and LOFAR (LOw Frequency Array) telescopes in the Netherlands, the Lovell radio telescope in the UK, and the Arecibo Observatory (featured in the movies Contact and Goldeneye) along with the GBT (Green Bank Telescope) in the US. The majority of these facilities are large, single dish telescopes (but not all – take a look in the accompanying pictures) because sensitive single pointings with little or no imaging are the best for accurately timing a pulsar.

Telescopes used in the J1713+0747 Global Observation

Using telescopes located around the world is important, because any single telescope can see pulsar J1713+0747 for much less than twelve hours, depending on the observing site’s latitude. Thus, the telescopes “trade off” between one another – as the pulsar sets from the perspective of, say, the Parkes telescope in Australia, it rises from the perspective of the Lovell telescope in the UK.

Members of the IPTA observed J1713+0747 on June 22nd of this year. Much of the long-lasting telescope operation was conducted remotely from Krabi, Thialand, where the IPTA was beginning its 2013 IPTA Science Meeting. (Having the international collaboration in one place proved to be a significant advantage, as supplemental last-minute ideas came together very quickly.) The 24 hours went successfully. The data accumulated is around 90 terabytes, and was collected at a variety of frequencies.


Figure 1

Figure 2

Figure 3

Figure 4

A few quick comments about the first results: in Figure 1, we show a seven-hour pulse profile from the GBT. Averaged over this amount of time, the signal becomes 6000 times as strong as the noise! In Figure 2, we see many of the telescopes’ data plotted consecutively (all that observed at high-frequencies), in order to show just how long and how stable the pulsar’s behavior is. (When the data points fall on the x-axis, it means there is no deviation from the timing model as it is understood – i.e. everything we know about, such as the Earth’s motion and pulsar’s motion has been subtracted out.) In Figure 3, we see the pulse times-of-arrival from the GBT alone – make sure to scroll left and right! (Each data point is ten seconds worth of arrival times.) Finally, in Figure 4, we see the dynamic spectrum from the GBT, which is a diagram showing the radio frequency spectrum of the pulsar changed over seven hours.

With such a wealth of new data, this pulsar, and thus the entire IPTA project that observes it, will help us to understand better how pulsars work, and how accurately they can be timed in order to detect gravitational waves.

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Introduction to the IPTA Concept

The International Pulsar Timing Array (IPTA) is a consortium of consortia[1] , comprised of the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array (PPTA). The principal goal of the IPTA is to detect gravitational waves using an array of approximately 30 pulsars. This goal is shared by each of the participating consortia individually, but they have all recognized that their goal will be achieved more quickly in collaboration, and by combining their respective resources. Sharing resources will also help to reach other IPTA goals, for example, establishing a pulsar-based reference timescale.

The basic experiment exploits the predictability the pulses from millisecond pulsars (MSPs) and uses them as a system of Galactic clocks. Disturbances in the clocks will be measurable at Earth. A disturbance from a passing gravitational wave will have a particular signature across the ensemble of pulsars, and will be thus detected.

Participating Pulsar Timing Arrays

The experiment is analogous to ground-based interferometric detectors such as LIGO and VIRGO, where the time-of-flight of a laser beam is measured along a particular path and compared to the time-of-flight along an orthogonally oriented path. Instead of the time-of-flight of a laser beam the IPTA is measuring the time- of-flight of an electromagnetic pulse from the pulse. Instead of 4 km arms (as in the case of LIGO) the ‘arms’ of the IPTA are thousands of light-years (the distance between the pulsars and the earth.) Each of the PTAs times approximately 20 millisecond pulsars (MSPs) each month. With significant overlap between the collaborations the total number of MSPs timed by the IPTA (and thus the number of ‘arms’ in the detector) is approximately 30.

These differences between the IPTA and the ground-based interferometers allow them to probe a completely different range in gravitational-wave frequency and thus a different category of sources. Whereas ground-based detectors are sensitive to 10’s-1000’s of Hz, the IPTA is sensitive to 10’s-100’s of microHertz. Their primary source of gravitational waves is supermassive black-hole binaries (billions of solar masses), presumed to exist in plenty in the universe at the centers of galaxies, resulting from previous mergers of those galaxies.

For a listing of the resources of the IPTA please go to “Links” above.

Pulsar timing was tied for top ranking in the “medium size” category for priorities from the Particle Astrophysics and Gravitational Panel of the Astro2010 Decadal Review sponsored by the National Academy. The table is in Table B.1 of the report.

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