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High-time Resolution Astrophysics using Digital Beamforming
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
In the past few years, a possible new population of sources emitting fast andbright transient radio bursts have been discovered. To explore this newtransient phase space, the next generation of radio telescopes, typicallyinterferometers, provide wide observing bandwidths in order to achieve highsensitivity for the detection of weak sources and multi-beaming capabilities toincrease the field of view. In comparison to a traditional single beam radiodish, these telescopes collect vast volumes of data and still rely on thetraditional observing techniques. Localising transient events during theirdiscovery is essential as these events might not be repeatable by nature. I describe the development and construction of a prototype phased array, theManchester University Student Telescope (MUST), capable of multi-beamoperation. I present results of the Yagi antenna beam measurements and theoptimum antenna separations; the investigation of the radio frequencyinterference spectrum around the preferred observing band and setting thesubsequent bandpass filter specifications; simulations of theoptimum configurations of the MUST tiles; and finally I describe implementationof the digital back-end. For the joint task of discovery and simultaneous localisation, the advantagesof single dishes and interferometers are combined in a beamforming approach. Ipresent an investigation into the wide-bandwidth time-domain signalprocessing techniques for time-domain beamforming that can be used in transientand pulsar observations. I discuss the efficient polyphase decomposition forinterpolation digital filters and multiplication-free fractional delay filtersthat can be used to reduce the complexity of the beamformer implementation andavoid high sampling rates. This reduced complexity allows more simultaneousbeams to be formed using time-domain techniques. This analysis was performedfor the MUST array, but is applicable to a wide range of interferometers.I have developed and analysed a new proof-of-concept non-imaging method tolocalise transient sources observed with interferometers or phased array feeds.It utilises the additional spectral and comparative spatial informationobtained from multiple overlapping tied array beams. This allows us to estimatea transient source location with up to arcsecond accuracy in almost real timeand allows the required high time resolution to be preserved. We demonstratethat this method can work for a variety of interferometric configurations,including for LOFAR and MeerKAT, and that the estimated angular position may besufficient to identify a host galaxy, or other related object, withoutreference to other simultaneous or follow-up observations.