Pulsars - Introduction |
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Initially, pulsars were found as single short bursts coming at the same time intervals. The first pulsar was discovered in 1967 by Jocelyn Bell, a graduate student of Anthony Huish, at the Cambridge University Observatory radio telescope. For this discovery, Huish received the Nobel Prize in 1974. Since then, more 2.5 thousand pulsars have been found. Pulsars are rapidly rotating neutron stars. The axis of rotation of the pulsar does not coincide with its magnetic axis. When the pulsar's magnetic axis crosses the line of sight, we see a pulse (a short flash). Each revolution of the pulsar gives one pulse. The accuracy of the arrival of pulses at small intervals of time is comparable to the accuracy of quartz watches. And at large time intervals (years and decades) the accuracy of the arrival of the pulses of some pulsars may exceed the accuracy of the timestamps of atomic standards. Pulsars can vary greatly in their visible properties. There are pulsars in which radio emission can disappear and then appear. The phase of the signal arrival does not change. Such pulsars are called pulsars with nullings. |
Schematic representation of a pulsar (fig.1)
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Search for pulsars
The essence of any pulsar search program is the search for periodically repeating signals. There are two main ways to search for signals with good repeatability over time.
One of these methods is signal averaging. Its essence is as follows. As a rule, pulsar pulses are not visible in noise tracks. To see the pulsar, you need to increase the signal-to-noise ratio. This can be done by adding pulses in phase. When adding N records with a pulsar period, the pulsar signal increases by N times, and the random noise track increases by the root of N times. As a result, the signal-to-noise ratio increases by the root of N times. Thus, by going over different periods and averaging observations over a long time, you can significantly increase the signal-to-noise ratio and detect a pulsar - if it is there, of course.
Another, and more commonly used, method of searching for periodic pulsar signals is a power spectrum search. The signal is translated into the frequency domain (usually using fast Fourier), and if there were periodic signals in the signal, then you can see peaks in the power spectrum - harmonics. The value corresponding to the inverse frequency of the first harmonic is the period of this harmonic process, i.e. the pulsar period.
If the pulses have poor repeatability in time (for example, as in pulsars with nullings), then another search method is needed. After all, if the fading period of pulsar radiation is longer than the period of its active life, then averaging with the pulsar period may lead not to an improvement in the signal-to-noise ratio, but to its deterioration. A search on the power spectrum will not give anything here either. Therefore, it is more profitable to search for these pulsars by individual pulses. They can be much stronger than the pulses averaged over the entire observation period.