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Pulsar Detection Observations

A System for Observing Pulsar Emissions


Vela Pulsar

The Vela Pulsar (PSR B0833-45 or PSR J0835-4510) is a radio, optical, X-ray and gamma-emitting pulsar associated with Vela Supernova Remnant, at a distance of about 950 light years. The Vela supernova remnant is the debris of the explosion of a massive star about 10,000 years ago. The pulsar is the collapsed core of this star, rotating about 11 times a second.

It has a period of 89 milliseconds (the shortest known at the time of its discovery) and this remnant from the supernova explosion 11,000 years ago is estimated to be travelling at 1,200 km/s (750 mi/s).

Listen to the 'sound' of Vela as recorded by the NRAO 92m dish at a frequency of 408 MHz - (unfortunately this dish famously collapsed at 9:43 pm on November 15, 1988. ).

A pulsar is a neutron star which spins rapidly. An intense magnetic field surrounding the star forms two radio beams which sweep space as the star rotates. As the beams rotate if one of the beams points in our direction a 'pulse' of radio energy is received here.

Simple geometry shows only one of the two beams can ever point in our direction unless the angle between the rotation and magnetic axes approaches 90°.

So a 'pulsar' - short for 'pulsating star'- is not actually pulsating, but two relatively constant beams sweeping space make it appear so. Remarkably only those pulsars that have one or both of those beams pointing in our direction can be detected as pulsars - meaning the total population of pulsars probably far exceeds the number detected.

Note also that the magnetic field poles of the neutron star need to be offset from the spin axis of the star to effect the sweeping motion of the beams.

The beam width is narrow, resulting in apparent pulse widths of few % of the period of the pulse. A typical figure is 5%. For most pulsars the observed pulse width increases with decreasing frequency due to scattering in the interstellar medium.

Vela Pulsar Characteristics

Name RA DEC P0 (S) W50/W10
(mS)
DM S400
(mJy)
B0833-45 08:35:20.61 -45:10:34.87 0.089328385024 2.1/4.5 67.99 5000

KEY

RA: Right ascension (hours minutes seconds)
Dec: Declination (degrees minutes seconds)
P0: Barycentric period of the pulsar (s)
DM: Dispersion measure (cm-3 pc)
W50: Width of pulse at 50% of peak (ms)
W10:
 Width of pulse at 10% of peak (ms)
S400:
 Mean flux density at 400 MHz (mJy)

Scintillation effects are relatively benign for this pulsar - variations due to diffractive scintillation occur in a much shorter time frame than typical observation times and so such variations average out.  Longer terms diffractive and refractive effects have a low modulation index (variations are not large: generally limited less than 3 dB) making the day-to-day variations relatively small compared with, say, B0329+54.

One notable characteristic is that the radiation from the Vela pulsar is almost completely polarised - predominantly linear, but with some degree of circular polarisation.  This is an important factor as using a linearly-polarised antenna could result in periods of cross-polarisation leading to loss in signal strength.  This consideration drove the adoption of a circularly-polarised antenna system.  Dual orthogonal linear probes could have been used, but that would require a duplication of the receiver chain - a complication not attractive at this stage.