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A state-of-the-art pulsar survey to aid gravitational wave detection efforts through pulsar timing

Final Report Summary - PULSAR SURVEY (A state-of-the-art pulsar survey to aid gravitational wave detection efforts through pulsar timing)

Project context and objectives

This project is related to the large-scale effort of gravitational-wave detection. Gravitational waves are the final untested aspect of Einstein's theory of gravity and as such they hold key information for the advancement of physics and astronomy. Possibly the most promising way to detect these very weak ripples in the fabric of space and time is through the highly precise timing of a set of pulsars. These pulsars (commonly referred to as cosmic lighthouses) are neutron stars that emit radio waves from their magnetic poles and rotate up to a thousand times per second, causing the beam of emission to sweep through the universe in a highly regular fashion.

The most useful pulsars for a gravitational-wave search are very old neutron stars that were formed in a binary star system hundreds to thousands of millions of years ago. Normally these pulsars would rotate at rates of a few Hertz, but in binary systems they can accrete matter from their companion star, which causes them to spin up to periods of a few milliseconds (as fast as a common kitchen blender). As such 'millisecond pulsars' are so old and because they have high velocities caused by the supernova explosion that first created them, they have typically moved a long way and consequently are not constrained by the Galactic plane. This is the main reason why these sources are hard to find. Due to the fact that they have no preferred location in the Galaxy, millisecond pulsars are like needles in a haystack. The only way to find them is to search the entire sky, bit by bit.

Work performed

We set out to aid the detection of gravitational waves in a number of ways, but most specifically by setting up a pulsar survey with the Effelsberg radio telescope (located in Bad Münstereiffel, Germany). By searching the Northern sky indiscriminately, this survey is the first large-scale attempt at finding millisecond pulsars that are accessible to European telescopes. To achieve this goal, the telescope was equipped with a new receiver and new digital recording hardware to write the data to disk. Software to monitor ongoing observations was created, as well as a pipeline to process the data and report any possible discoveries. The entire system was tested thoroughly and automated as much as possible, after which the actual survey was commenced. At this point, about 700 hours of telescope time have been invested in the survey, equalling nearly 10 % of the entire Northern sky. Of the data taken, about one-fifth has been analysed. This has led to the discovery of about ten new pulsars, including Effelsberg's first-ever millisecond pulsar discovery.

In order to assess the usefulness of the newly discovered pulsars and to improve the timing of already known pulsars, new timing instrumentation was also installed at the Effelsberg observatory, increasing our timing precision (and therefore our sensitivity to gravitational waves) by several factors. This new hardware has been tested and routine monitoring observations were started last year on old as well as new sources, turning Effelsberg into a cornerstone of the European gravitational-wave detection efforts with pulsars; and making Europe a global leader.

There are two main requirements for gravitational-wave detection through pulsar timing. The first one is to have a sufficient number of pulsars to time; the second one is to time these pulsars to a high enough precision. A prime cause of lacking timing precision is the ionised plasma between us and the pulsar, known as the interstellar medium. This medium introduces delays to propagating signals, and because the medium is neither stable nor homogeneous, these delays vary randomly with time. Until recently, only ad-hoc methods with limited precision were used to correct for these variable delays, leaving substantial corrupting effects in the data.

The second way in which the project increased our sensitivity to gravitational waves was through its investigation of more advanced mitigation schemes for these corrupting effects of the interstellar medium. Specifically, I aided the commissioning of the LOw Frequency ARray (LOFAR), a new telescope with unprecedented sensitivity to very long radio wavelengths. As interstellar plasma strongly effects scale with wavelength, the LOFAR telescope is more sensitive to these effects than other telescopes and can therefore provide highly precise corrective models of interstellar dispersion for use in the gravitational-wave detection efforts.

Archival data has been used to place limits on the strength of the expected gravitational waves (Yardley et al., MNRAS 2011; van Haasteren et al., MNRAS 2011) to evaluate the effects of the interstellar medium in great detail through both simulation and real data analysis (Coles et al., ApJ 2010) so as to test individual pulsar systems and their use for long-term timing efforts and tests of gravity (Freire et al., MNRAS 2011; Lazaridis et al., MNRAS 2011 and Freire et al., MNRAS submitted 2012). Furthermore, collaborations were initiated to investigate the effects of gravitational waves on the timing analysis used to detect them (Ellis et al., MNRAS 2011), to predict the possible precision of pulsar timing efforts in the future (Liu et al., MNRAS 2011) and to improve the techniques used in pulsar timing analyses (Coles et al., MNRAS 2011).

Main results

We have built essential foundations for work that will continue for many years to come. The data being taken for the pulsar survey is expected to be fully completed in five years, at the earliest, and as improvements in computing power become available, the data will serve the astronomical community for at least another decade after that, making this truly a legacy survey. Timing with LOFAR and accurate monitoring of the interstellar medium, meanwhile, opens up an entirely new avenue in high precision timing. My initial investigations indicate that further development of this technique should enable it to become the best practice in the field, providing European gravitational-wave detection efforts with a boost and keeping these efforts on track for an initial detection within the current decade.