10.1 The nature of Pulsars
Pulsars are compact sources that emit a series of fast radio pulses. They are in fact neutron stars about 20 km in diameter and have a mass of about 1.4 times that of our Sun. This means that a neutron star is so dense that on Earth, one teaspoonful would weigh a billion tons. Because of its small size and high density, a neutron star possesses a surface gravitational field about 2 x 1011 times that of Earth. They can also have magnetic fields a million times stronger than the strongest magnetic fields produced on Earth(12).
Pulsars were first discovered in late 1967 by graduate student Jocelyn Bell Burnell, as radio sources that blink on and off at a constant frequency. Now we observe the brightest ones at almost every wavelength. Pulsars are spinning neutron stars that have jets of particles moving almost at the speed of light streaming out above their magnetic poles.
Figure 10.1 A Pulsar - A spinning Neutron Star
In Figure 10.1 we see a compact stellar remnant spinning about its axis of rotation with an intense magnetic field (created as the star collapsed) at some angle to the spin axis. The intense beams of radiation emerging from the magnetic poles sweep around in space like a light-house beam. Each time the beam crosses an observer's location he sees a short intense radio pulse which has the regular period of the Neutron star's rotation. A typical pulse train is shown in Figure 10.2(13)
Figure 10.2 Pulsar signal
10.2 Observing Pulsars
There are two characteristics of pulsar signals that make observing them different from all the 'noise like' emissions discussed earlier:
The low signal level requires a large aperture antenna and very low noise receiver whilst the pulsed nature of the signal means that the observer cannot use signal integration over a period of many seconds to reduce signal variability. Integration would destroy the pulse structure of the signal. These features make pulsar observations almost the ultimate challenge for the amateur radio astronomer with limited equipment. Some observers using a modest 10 foot(14) (15) diameter dish claim to have observed a number of pulsars with the aid of special post detector software which enhances the pulse structure within background noise. There is still a deal of work going on by professional radio astronomers to understand the emission generation mechanisms in pulsars.
There is a wide variety of pulse rates, emission spectra and source intensities for which a full explanation is currently being sought. For amateur radio astronomers, simply detecting a pulsar would be an achievement of some note. Indeed, one of the members of the Society of Amateur Radio Astronomers (SARA)(16) Jim Van Prooyen(17) has made great efforts to detect pulsars with a 10 foot diameter dish - and larger antennas - by developing special software to recover pulses from a noisy signal. He comments that - "There have been several efforts by amateur radio astronomers to build [pulsar detectors], and for some of us, the detection of pulsars is the Quest for the Holy Grail of amateur radio astronomy. There are a number of notable efforts:
Van Prooyen has published the graph in Figure 10.3 showing the detection of Pulsar B0031-07 which he made using his post detector software capability
Figure 10.3 Amateur detection of Pulsar (Van Prooyen)
10.3 Pulsar emission spectra
A great deal of work has been carried out by professional radio astronomers to determine the nature of the emission spectra from a large variety of pulsars. In general they follow the simple synchrotron shape as given in Figure 2.5, A spectrum of Pulsar B1557-50 is shown in Figure 10.4.
However evidence has been found of a 'turn over point' where the emission is a maximum - falling away on either side as shown in the example in Figure 10.5 for Pulsar B1740-31.(18) This suggests that a useful frequency to observe is in the UHF band (300MHz up to about 1GHz). Note that the power flux density for these Pulsars is in the milli-Jansky range (mJy). A big challenge for amateur observers!
© Dr David Morgan 2011