2.6 Where to look for 2 An Introduction to Pulsar 2.4 Normal and millisecond pulsars

2.5 Searching for pulsars 

Pulsar searching is, conceptually at least, a rather simple process - the detection of a dispersed, periodic signal hidden in a noisy time series collected using a large radio telescope. The search strategy is to form a large number of time series for different trial DM values. These data are then analysed for periodic signals. We give here a brief description of the basic search techniques. More detailed discussions can be found elsewhere [141Jump To The Next Citation Point In The Article, 178, 130].

A schematic pulsar search is shown in Fig.  11 . The finite bandwidth is split up into a number of channels, typically using a filterbank or a correlator (see e.g. [13]), either of which usually provides a much finer frequency channelisation than the eight channels shown for illustrative purposes in Fig.  11 . The channels are then de-dispersed (see §  3.1.2) to form a single noisy time series. An efficient way to find a periodic signal in these data is to take the Fast Fourier Transform (FFT) and plot the resulting amplitude spectrum. For a narrow pulse the spectrum will show a family of harmonics. To detect weaker signals still, a harmonic summing technique is usually implemented [141]. The best candidates are saved and the whole process is repeated for another trial DM.


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Figure 11: Schematic summarising the essential steps in a ``standard'' pulsar search.

After the data have been processed for a suitable range of DM, a list of pulsar candidates is compiled and the raw time series data are folded modulo each candidate period. In practise the analysis is often hampered by the presence of periodic interference sources which can often look very ``pulsar-like''. Although interference excision schemes (usually based on coincidence analyses of data taken from different points on the sky) work fairly well, interference is an ever-increasing problem in radio astronomy and considerable efforts are required to carry out sensitive searches.

2.6 Where to look for 2 An Introduction to Pulsar 2.4 Normal and millisecond pulsars

image Binary and Millisecond Pulsars at the New Millennium
Duncan R. Lorimer
© Max-Planck-Gesellschaft. ISSN 1433-8351
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