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.
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.