Download a report on pulsars, written for PHAS1901

Jack Carlyle
Dr L Voigt
Pulsars
Table of Contents
Introduction.............................................................................................................................. 1
Discovery..................................................................................................................................2
Observation...............................................................................................................................2
Binary Pulsars...........................................................................................................................3
Pulsar Classes........................................................................................................................... 3
The Significance of Pulsars...................................................................................................... 3
Sources......................................................................................................................................4
Introduction
Pulsars are neutron stars
which appear to emit
electromagnetic radiation
at regular intervals, in
'pulses', from which the
name is derived (though
the stars do not actually
pulsate). The short periods
and regularity have led to
the interpretation that the
pulses are generated by
neutron stars formed in
supernova explosions from
normal stars. Since the
normal stars are rotating,
the neutron stars also
rotate, except they now
have a very high magnetic
field. They also rotate
faster than normal stars
because they are smaller,
so conservation of angular
momentum results in
higher rotational speeds.
The period of rotation is Illustration 1: Schematic view of a pulsar. The sphere in the
middle represents the neutron star, the curves indicate the
Jack Carlyle
Dr L Voigt
almost constant to a very high degree of accuracy, and plasma (ionised gas) is blown out by
the huge magnetic field as it rotates. electrons are accelerated in the magnetic field and emit
synchrotron radiation (ie generated by the acceleration of ultrarelativistic charged particles
moving through a magnetic field), giving rise to a strong beam of electromagnetic radiation
at all wavelengths.
If the earth is in the path swept out by this beam, it will be observed by us as a pulse, in the
same way that light-houses appear to flash from far away.
There are over a thousand pulsars known, and several have been identified in the optical or
x-ray band.
Discovery
All stars appear to “twinkle” in the sky in the optical spectrum due to refraction of light by
the atmosphere, however they also display this phenomenon in the radio band, which arises
from density fluctuations in the solar wind. It was while studying this scintillation in 1967
when Jocelyn Bell, during her phd studies (under professor anthony hewish), discovered
signals in the metre waveband which occurred with incredible regularity and unknowingly
recorded the first observed data for a pulsar. The pulses were so regular that, at first, the
group tried to find a local source of the radiation, and even entertained the idea of extraterrestrial intelligence, but once more pulsing radio sources had been observed in different
parts of the sky, it became clear that they were a natural phenomenon.
Observation
It is expected that the luminosity of a source of synchrotron radiation will have a power-law
−
dependence on frequency, v, given by L  ∝ where ~0.75 . Since v > 1 and β < 1, as
v increases, vβ will increase and v-β will decrease. this means that signal strength of pulses
will be higher when observed at lower frequencies.
Due to the fact that each pulse detected on the earth will correspond to the rotation of the
source pulsar, it is possible to measure the period of rotation by measuring the time between
pulses.
Pulsars do not have a defined characteristic luminosity when observed through an optical
telescope (and many cannot be seen anyway), so absolute magnitudes cannot be used to
determine their distances from us; other methods must be employed, such as measurment of
dispersion. All electromagnetic radiation moves at a constant speed through a vacuum, but is
slowed proportionally to it's wavelength when it travels through a medium. the interstellar
medium is often regarded as a vacuum, but in actual fact is occupied by atoms and free
electrons, albeit at extremely low densities. Therefore lower wavelength pulses will be
detected on earth slightly sooner than higher wavelength pulses (however, this does not
affect the period). This time difference can be measured and used to find the distance of the
Jack Carlyle
Dr L Voigt

1
1
pulsar from the earth using the equation  t =4150n e d f 2 − f 2
1
2

where Δt is the time delay (seconds), ne is the interstellar electron density in cm-3, d is the
distance in parsecs and f1 and f2 are the observation frequencies in MHz.
This equation is derived from the expression for the velocity of propagation of a radio wave
ne
through an ionised medium u =c 1−K
.
f2

Binary Pulsars
Most stars in our galaxy are part of a binary or multiple star system. These usually remain
intact, even in the case of one star becoming a supernova, so a pulsar could exist in a binary
system. these can be identified as their periods have a large cyclic change, owing to the
change in redshift as the pulsar periodically moves towards and away from us.
Pulsar Classes
There are three distinct classes of pulsars, each one corresponding to a source of energy
powering the radiation. All pulsars are neutron stars, but the classes each have very distinct,
observable behaviour.
Rotation powered pulsars are powered by the loss of rotational energy. This is the first group
to have been discovered, and were once known as radio pulsars, but rotation powered
pulsars that emit x-rays have since been found.
Accretion-powered pulsars (also known as x-ray pulsars) are powered by the gravitational
potential energy of the accreted matter, producing x-rays that are observable from earth.
Magnetars are powered by the decay of an extremely strong magnetic field, emitting x-rays
and gamma rays.
The Significance of Pulsars
The study of pulsars has yielded observations which are highly applicable to several areas of
Physics and Astronomy. For example, the observation of pulsars in radio wavelengths have
pushed the limits of our understanding of gravitational waves, confirming predictions made
by general relativity.
It was also via the observation of a distant pulsar which led to the very first detection of an
exarsolar planetary system.
I can therefore say, without hesitation, that I believe pulsars to be one of the most
fascinating, intricate, and indeed beautiful celestial bodies to have been observed in our
universe.
Jack Carlyle
Dr L Voigt
Illustration 2: Composite Optical/X-ray image of the Crab Nebula, showing synchrotron
emission in the surrounding pulsar wind nebula, powered by injection of magnetic fields
and particles from the central pulsar.
Sources
Zeilik & Gregory, Introductory Astronomy & Astrophysics
Duncan R. Lorimer, Binary and Millisecond Pulsars at the New Millennium
D. R. Lorimer & M. Kramer, Handbook of Pulsar Astronomy
Jack Carlyle
Ingrid H. Stairs, Testing General Relativity with Pulsar Timing
ULO materials
910 words (not including contents & sources)
Dr L Voigt