By
Douglas L. Simmons
Copyright © 1979, 2005
PROLOG:
I have, since early youth, taken a great interest in all things astronomical.
During the later years of the 1970's I was a member (for two of those years, President) of The Calumet Astronomical Society, of Northwest Indiana.
The following is a presentation I drafted in 1979 which, based upon the knowledge and theories of that era detail the formation and structure of pulsars; a variety of the class of objects known as neutron stars.
While the manuscript itself is outdated, these fourty some years later, and the images were drawn by hand, photographed onto 35 mm slides and, only recently, scanned into digital images, the information is still relevant and accurate. Presuming the reader has some prior knowledge of matters astronomical, it is most certainly still useful as an introduction to pulsars and perhaps still informative to those with some prior knowledge.
It is presented with the hopes of educating and leading the reader on to further studies of their own.
Should I find that this article elicits the volume of attention to justify taking time from other activities, I will attempt of draft and post other article of a similar vein.
Until then, please do enjoy this short exploration of the enigmatic Pulsar.
Pulsars were discovered in 1967 by a group of Cambridge Astronomers, led by Antony Hewish. This initial discovery led to the development of entirely new techniques for studying the magnetic spectrum of rapidly varying phenomena, from low radio to gamma ray frequencies.
Identification of pulsars with neutron stars, which were orginally postulated by Walter Baade and Fritz Zwicky in 1934, gave the first observational evidence for the actual existence of neutron stars.
It is truly amazing just how much has been learned about pulsars in the (at the date of this writing) 12 short years since their discovery.
b. Models
All present models for pulsars are based on the belief that they are rotating neutron stars, coupled with theories of the mechanisms which power them.
The number of observed facts during 1968 through 1969 gave scientists a chance to try out some new and varied theories.
Most pulsars have a stable period of better than one part in 10,000,000 over intervals of a few months. The fact that they are so stable shows that a great amount of mass, hence large inertia, is involved in the timing processes. That the pulses are emitted by a very compact body is shown by their short periods.
White dwarfs or neutron stars were obvious first candidates for study as the originators of pulsar signals.
In this light, three distinct mechanisms were seen as possibilities for producing the signals.
- - radial pulsation's, such as is found in cephids.
- orbital motion
- rotation
Radial pulsation's received first attention, although the times involved were too fast for white dwarfs and too slow for neutron stars. When the Vela and Crab pulsars were discovered, radial pulsation's were completely ruled out because of the extremely short periods of these pulsars. Less than one second.
Orbital motion was ruled out, because even if two neutron stars were so close as to be touching the orbits would not be fast enough to produce the signals and the tidal forces involved would tear the two bodies asunder. In addition, for an orbital model the periods would be getting shorter while it can be observed that in actuality, for all observed objects, they are increasing.
Left with only the rotation model to consider, white dwarfs were eliminated as they are too large to withstand the stresses involved in such fast rotation. In order to rotate at the speeds required to match the observed signals without having a surface velocity exceeding the speed of light the pulsar must have a radius of less than 1,700 kilometers. Neutron stars seemed to fit the bill very well.
c. Basic Observational Properties.
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Image #1
As of 1977, one hundred and forty nine pulsars were known (Many more have been discovered since that time). All emit broad band radio noise in the form of a periodic sequence of pulses.
While the observed pulse intensities vary over a wide range-sometines entire pulses are missing-the basic timing of the pulses is periodic.
If the instrumental time constant is reduced to about one micro second (one thousandth of a second) a more complex structure is revealed and some pulsars are shown to have sub pulses. If the time constant is further reduced a much smaller micro structure is shown.
The time period of the pulsar, rotation, is described as longitude so the pulse period equals 360 degrees. The latitude of the emission varies little and basically we are looking at the same area of the neutron star in each pulse we see.
Except for x-ray sources in binary systems, which will be discussed later, all known pulsars were discovered at radio frequencies. There have been some reports of detection of long period gamma ray pulsars but (at the time of writing) these haven't been confirmed.
Only the Crab and Vela pulsars (PSR 0531 21 and PSR 0833-45) are known to emit outside radio frequencies.
- (From this point on the reader will assume that any observational data, or conclusions drawn from that data date to the time of writing.
- --Thank you.)
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Image #2
The observed periods of pulsars lie between 0.03 and 4.0 seconds. Most of them have periods of less than one second.
There is an apparently significant dip in distribution centered on periods close to one second. This suggests two classes of pulsars, short period and long period types.
The Crab pulsar has the shortest period known, 0.0331 seconds, and pulsar PSR 0525 21 has the longest at 3.7454 seconds.
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Image #3
Image #3 shows a plot of pulsar positions in galactic coordinates. The observed concentration of pulsars along the galactic plane is a real effect, and shows that observed pulsars are in our galaxy.
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Image #4
Here (image #4) we see a plot of positions on the galactic plane. The number of pulsars decreases with distance from the center of the galaxy, and probably has something to do with the concentration of stars and types of stars located in different parts of the galaxy.
Accurate distances for pulsars are hard to obtain, and some measurements are, no doubt, in error, most of the estimates are close to the actual distances of the pulsars. These range from 100 to 20,000 parsec.
Successful searches for pulsars have been conducted by ten different groups. But there are still many pulsars which have not been found due to low luminosity's, too great a distance form earth, or the beamed emission does not sweep the solar system. Many more have probably been missed due to the limits of the observational methods used in the searches.
The pulsars located so far, as a group, belong to the galactic disc and few are found above or below the plane of the galaxy. The more distant pulsars always lie within a few degrees of the plane.
Pulse profiles are very complex and each one is completely unique.
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Image #5
The shape of the integrated pulse profile is somewhat frequency dependent but they are stable at any given frequency. (400 MHz is the frequency used for purposes of classification due to the fact that most observations have been made at this frequency)
There are two groups of pulsars; Type S (simple) and Type C (complex). Pulsars which have a drifting sub pulse are called Type D or Type SD and CD when greater classification is needed.
Most of the energy from the signal is confined to a small portion of the period, although some pulsars have additional components of significant energy content called inter pulses. These inter pulses are situated approximately half way between the main pulses. Most of the time the inter pulse contains only a small percentage of the amount of energy emitted by the main pulse. The inter pulse could be explained by the polar model of the emission mechanism, this model will be discussed.
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Image #6
b. Energies
Pulse emission energies can vary from pulse to pulse and more slowly over longer periods of time and at times emission can cease completely but in all cases the period remains stable.
Energies can also vary at different frequencies. At lower frequencies emission cut off is sometimes due to the interstellar medium, this energy will then show up as a continuous signal with the same spectrum as the pulsar.
c. Polarization.
Pulsar signals are frequently polarized. The type of polarization can tell mayn things about the pulsar, such as emission mechanism, make up of the nebulosity around the pulsar, and the magnetic field of the pulsar.
Signal polarization can also describe the interstellar medium the signal passes through on it's way to earth.
d. Stability
Generally integrated pulse profiles remain stable over long periods of time which is why they are an important part of emission studies.
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Image #7
Some pulsars under go what is called mode changing, the integrated pulse profile will change to another mode which is stable as long as it remains dominant. The pulsar will always revert back to the original mode in a few hours.
To long term change in pulse profiles of intrinsic polarization has ever been detected in any pulsar.
Individual pulses can vary greatly in intensity, shape and polarization from one period to the next. Often these changes are random in nature but sometimes there are periodic changes. Sub Pulses in the main pulse can also change greatly from pulse to pulse.
Drifting sub pulses change longitude or time position within the pulse profile in a constant direction either toward the leading edge of toward the trailing edge. Sub Pulses drifting toward the leading edge are often found in type SD and those drifting toward the trailing edge are found in type CD pulsars.
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Image #8
The Crab nebulae is one of the strongest sources of radio and x-ray emission in the sky. At optical frequencies apparent magnitude is only about 8.4 and even though it has a low surface brightness the nebulae can easily be seen with a small telescope. The nebular emission is almost entirely generated by the synchrotron process, and about 12% of the emission from the nebulae is at radio frequencies.
Most of the mass in the nebulae is contained in the outer filaments, this equals about one solar mass. The total mass of the nebulae is not greater than 2 solar masses.
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Image #9
This chart (image #9) shows the optical polarization of the nebulae.
The nebulae is expanding and most of the energy released by the pulsar is spent in maintaining this expansion against losses due to the effects of the interstellar medium.
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Image #10
b. Crab Pulsar
There are two pulsars near the Crab nebulae and one of them is within the nebulae itself. This is the remnant of the super nova observed by the Chinese over 900 years ago.
This pulsar is the energy supply for the nebulae.
Optical pulses were detected in one of a pair of 16th magnitude stars near the center of the nebulae in 1969. This was managed by means of television and electronic image enhancement of a telescopic image of the star. It is still only one of two pulsars detected optically (up to 1979 more may have been found since), the other being the Vela pulsar.
For the amateur to detect the stars in the nebulae would require photography of an telescope with an aperture above 12.5 inches for visual observation.
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Image #11
In images #11 and #12 we see pulse profiles, at different frequencies for the Crab and Vela pulsars. You can see that the pulse profile for the Crab emissions much the same at all frequencies.
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Image #12
With the Vela pulsar it is shown that there is a marked difference in the radio pulse, optical and gamma ray pulses.
Most of the information now known about pulsars is based on observations made in frequencies ranging form 10 MHz to 10 GHz. There is observational evidence that flux densities decrease rapidly above this range.
Objects radiating at x-ray frequencies, while related to pulsars, do not radiate significantly art radio frequencies.
X-ray pulsars have periods ranging from 0.7 to 835 seconds (14 minutes) and are all believed to be members of binary systems.
Only one radio pulsar is known to be a member of a binary system, this is PSR 1913 16. This pulsar has the second shortest period known-0.059 seconds-and a very short orbital period of 7.75 hours.
Pulsation's at optical and gamma ray frequencies have been detected for the Vela pulsar but none at x-ray. In addition, the position of this pulsar is uncertain due to difficulties of pinpointing the radio source. If the star believed to be the source of the optical pulses is indeed the pulsar then only part of it's light comes from pulsed emission because the magnitude of the star is greater than the magnitude of the optical pulses detected.
Several radio pulsars have been observed in the gamma ray regions but searches for more than this have proven fruitless.
b. Binayr X-ray Sources.
A number of binary x-ray sources have been detected and the information given here is based on seven of these which were studied very closely.
All binary x-ray sources are believed to be made of a collapsed object, a neutron star of a black hole, and a main sequence or post main sequence star that is normal except for the tidal distortion and asymmetrical heating caused by the x-ray source.
Optical variations in the visible companion of about 0.1 magnitude are probably due to tidal distortion and x-ray heating of one face of the star.
X-ray eclipses are observed in five of the seven systems studied here which indicates that their orbit has little or no inclination to our line of sight.
The mass of both objects can be determined from the optical component if its spectrum is not overly distorted due to gas streaming from the star to the compact companion.
In four of the objects studied the x-ray source is seen to be modulated and three of these have periods comparable to those of radio pulsars.
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Image #13
These (image #13) are the integrated profiles for three of the group studied: Hercules X-1, Centarus X-3 and SMC X-1.
SMC X-1 is actually in the Small Magellanic Cloud as is shown by its distance estimate of 65,000 parsecs.
Integrated profiles for these three objects is seen to change shape considerably over days or weeks, unlike pulsars which remain the same all the time.
X-radiation is probably generated in these systems by accretion of mass form the normal star which is absorbed into the compact companion. Visible components range between 2 and 30 solar masses, while the x-ray components fall between 1 and 3 solar masses.
Only a neutron star is compact enough to generate emission energies observed. However, further study of Cyg X-1 could show that a black hole is needed to fill the energy requirements of the x-ray source (it is now believed that Cyg X-1 is a black hole).
These objects do not emit in the radio range or if they do it is blocked by the gasses falling into the compact body. Evidence favoring the accretion model is taken from spectroscopic data and by observed speeding up of pulse periods which can only be explained by this model.
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Image #14
The polar emission model seems to work well for these objects and agrees with the model used for radio pulsars.
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Image #15
This (image #15) chart shows two models used to explain the emission process the accretion and stellar wind models.
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Image #16
This (image #16) chart shows the velocity curve and variations in intensity of different emission frequencies as two components of an x-ray system travel in their orbits.
c. Possibility Of Binary Pulsars
Approximately half the stars in the Milky Way are binaries and it is noteworthy that radio pulsars are, in general, solitary objects. Binary pulsars would be useful for a number of observational purposes, such as, an absolute measure of mass and testing of gravitational theories.
Pulse stability's are on the order of 0.001 second on a time scale of several years. Differences in amplitude of pulses caused by gravitation of bodies as small as the earth could tell of planetary companions to pulsars
It has been determined by this method that no pulsar, other than PSR 1913 16 can have orbiting companions approaching solar mass.
d. Binary Pulsar 1319 16
The only binary radio pulsar known was discovered in July of 1974. Of immediate interest was its short period, second only to that of the Crab pulsar. Cyclic variations in the pulsars period led to the discovery of the companion.
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Image #17
The velocity curve shown is consistent with an orbital period of 7.75 hours and a semi major axis of orbit of 1.0 solar radius, or about 400,000 miles. The pulsar contains about 0.13 solar masses of matter, and is believed to lie at a distance of about 5,000 parsecs from the sun.
To this date (1979) it is only detectable at radio frequencies and the companion has not been directly observed. Lack of eclipses and other information indicates that the companion is also a compact body and may, in fact, also be a pulsar.
A few years study of changes in the orbital period will give enough information to make comparison tests of different gravitational theories and with better testing instruments we will be able to make direct measures of gravity emissions from the system. This is possible because both bodies are effectively point sources of gravity wave emission.
e. Evolution Of Close Binary Systems
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Image #18
Stage 1 - The evolution of the hypothetical close binary system shown begins with two upper main sequence stars of 20 solar masses and 8 solar masses. The 20 solar mass star evolves off the main sequence and begins to deposit material on its companion.
Stage 2 - During this stage, which lasts 30,000 years, nearly 15 solar masses of material is transferred to the companion. At the end of this stage we have a helium star of 5 solar masses and a main sequence star of 15 solar masses.
Stage 3 - By this stage, conservation of angular momentum has lengthened the orbital period to about 11 days. After « million years the helium star explodes into a super nova, ejecting 3 solar masses of material and leaving a compact remnant of 2 solar masses. Probably a neutron star.
Stage 4 - The orbital period has increased to 13 days. The 23 solar mass star evolves into a blue super giant in 4,000,000 years.
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Image #19
Stage 5 - With a strong stellar wind coming from the blue giant, accretion makes the compact body a powerful x-ray source. This continues for 40,000 years.
Stage 6 - The blue giant expands to fill roche lobe and excessive accretion blocks x-ray emission. Compact body can accept only a small part of the mass transferred and about 17 solar masses is lost to the system in this stage. After 200,000 years we see a compact star of 2 solar masses and a helium star of 6 solar masses. The orbital period is now about 0.2 days.
Stage 7 - When the second helium star goes super nova it will leave a system of two compact stars of companions will become unbound from their orbits and there will be two runaway compact stars.
Stage 8 - From stage 4 on, the system has a pulsar in it but emission is blocked by plasmas in the system, so we should not expect to observe pulsars with main sequence companions (to date we haven't).
At stage 8, the system is cleared of all matter except two compact bodies so one or both of them can be observed as pulsars, whether they remain together or not.
It is predicted that there are 4,000 unresolved compact binaries with massive companions of more than 15 solar masses in the Milky Way. We can observe only about 1/5 of the existing pulsars due to beaming effects. Data suggests also that a close binary system of 8 solar masses or less will survive a supernova explosion of one of the members only about 3% of the time. Therefore, pulsars should be isolated and posses large runaway velocities.
The only time a pulsar will have a nebulae around it is if it were a solitary object when it went supernova. Should two pulsars form a binary system they will sweep the space around them clean and if they become unbound from their orbits they will leave nebulae behind due to their high radial velocity.
The most remarkable characteristic of pulsars is the stability of their basic pulsation periods which are almost as good as atomic frequency standards. No other stellar object can display such timing phoneme.
There are two types of changes in pulse times, steady increases in the period and unpredictable irregularities.
A braking index is used to determine the age of the pulsar. Knowing how much the pulse if increasing now will tell what the period was in the past. The braking index changes with time, in other words, the slower a pulse period is the slower it increases in length and the faster the period the more rapidly it increases in length. The faster the period the more rapidly it increases due to reaction with nebulae and magnetic fields around the pulsar.
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Image #20
Sometimes, pulsar periods will show a sudden time decrease, however, the pulsar rapidly reverts to the original braking index.
These sudden decreases in period are known as glitches.
Using the two component model, pulse emissions are seen as being due to outer charged layers of neutron star and change in the rotation period of the outer layers will take time to transfer to the inner neutronium layers which are semi liquid in state and loosely coupled to the outer layers.
Pulse glitches are believed to be caused by star quakes in the crust of the pulsar which causes settling, reducing the diameter of the pulsar and so increasing the period, or rotation rate of the star.
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Image #21
Measure of signal absorption by neutral hydrogen atoms gives information on HI cloud structure (neutral hydrogen=no electrical charge) used to determine electron density in interstellar space if pulsar distance is known from an independent source. Since pulsars are effectively a point source, broadening of the signal beam can also infer information about the interstellar medium.
Dispersion of the signal can give information about the HII (electrically charged hydrogen clouds) in the galaxy.
c. Faraday Rotation And The Galactic Magnetic Field
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Image #21B
Circular polarization of pulsar signal can give information on galactic magnetic field structure. Scattering of the signal gives information on gas and dust clouds between earth and the pulsar. Information is also taken from ghost signals received from a reflected source, such as a gaseous nebulae between the pulsar and earth.
The rotating neutron star model for pulsars is the most widely accepted model for the reasons already given. Pulsars have strong magnetic fields, possibly the strongest in the universe. This, combined with a rapidly rotating object produces immense electric fields which accelerate charged particles to colossal energies.
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Image #22
In this chart (image #22) you see a schematic of a neutron star.
The star consists of an outer crust which is much the same as the interior of a white dwarf star and consists of iron nuclei surrounded by a sea of degenerate electrons, compressed to a tremendous density. The neutron stars crust is solid.
Below this is the inner crust which is made of neutron rich nuclei in a sea of electrons. This is a super conducting semi fluid. Between the inner crust and the core are normal neutrons.
The core consists of super fluid neutrons at fantastic densities. This fluid too is a superconductor.
Behavior of electrons remains normal throughout the neutron star.
10. EMISSION MECHANISMS
The polar emission mechanism is now the most widely accepted model.
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Image #23
This model postulates emission from the regions of the magnetic poles. This would give tow beams from each pulsar, one from the north magnetic pole and one from the south.
Only one of these beams crosses the earth so we are seeing along the magnetic axis of the pulsar, either straight on or at a slight angle.
The signal is actually not a pulse but a continuous beam, we see it as a pulsed emission because the beam only sweeps the earth during a small portion of the pulsars rotation period.
Radio emissions must be coherent (maser emissions) and highly focused to produce the emission intensities we observe as thermal emission is not strong enough for these energies.
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Image #24
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Image #25
When the pulsar is within a nebulae then gases near the star are excited by emissions from the pulsar and emit strongly by absorbing these energies and then re-emitting them at lower frequencies. This is seen as the compact source emitted in the crab nebulae.
If the magnetic axis were aligned close enough to the equator then we could see both pulses emitted by the pulsar, when inter pulses are received we may be looking at the very edge of the pulse from the magnetic pole not aligned with us.
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Image #26
Due to processes involved in the emission mechanism around periods of about one second, gamma ray energies at the emission region are not sufficient to support pulsed emission. The gamma rays play an important part in the forming of electron and positron pairs which are necessary for the emission process to function.
This belief is borne out by the lack of pulsars with periods near one second.
Decay of the magnetic fields in the pulsar makes them appear older than they really are and further decay leads to cessation of pulsar emission.
Alignment of the magnetic axis with the rotational axis could also cue the pulsar to turn off and die.
So.
You can see that pulsars are far from a simple blinking light in the sky. They are the tools of tomorrow. Tools which can tell us as much about the universe as radio astronomy has told us of pulsars.
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I have drawn my knowledge of astrophysics and astronomy from many sources, none by direct study (unless you wish to count time spent in some ones corn field with my 4 1/4 inch rich field Newtonian - stargazing).
Although available in many other publications the information used in this presentation owes much to the book: Pulsars by R.N. Manchester and J.H. Taylor. Published by W. H. Freeman and Company, San Francisco
Thank You for keeping my thoughts alive.
- --the author
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- Thank you for keeping my thoughts alive.
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