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Pulsars, History and Amateur Detection
Partial Information from Pulsars, Mechanisms and Detection CD by Jeffrey M. Lichtman and James Van Prooyen
(Other Information � See Acknowledgments)
The Life of a Star � Pulsar Formation
Throughout a stars life in the main sequence, stars fight a dramatic battle against the forces of gravity- Gravity tries to collapse the star by pulling its outer layers towards its center. But the star fights back by releasing nuclear energy, which is fueled by a rich supply of hydrogen. Eventually, usually after billions of years, stars deplete their fuel supply and must give up the fight. Some aging stars die quietly- others suffer violent deaths. The method depends on a star mass. Stars about the same size as our Sun become white dwarfs, which shine for a very long time from leftover heat. Stars that have about 10 times the Sun's mass blow apart and often form neutron stars. Scientists believe that the Crab Nebula came from such a star.
A Stars Collapse
Once a large star exhausts its fuel supply, gravity takes over and the star is collapsed without opposition. Usually a star will find other sources of fuel like helium, carbon, oxygen and nitrogen, but these offer a short reprieve. Eventually the densities at the center of the star get so high that the star cannot collapse much more at all. Instead all the pressure from the collapse is "stored," ready for release. Finally the conditions become so extreme at the center of the star that all the "stored" pressure from the years of collapse is released in a single brilliant burst: a nova or a supernova, depending on the mass of the star. This explosion throws off the outer layers of the star and compresses its core even more. It was a supernova that created the Crab Nebula. During the explosion, the star gives off more energy than a galaxy of 1 00 billion stars. The outer layers being ejected create an expanding shell of dust and gas that become a supernova remnant.
Below is a radio picture of Cassiopeia A, supernova remnant. This remnant is considered the strongest galactic radio object, other than the sun.
(Neutron Star: A collapsed star of extremely high density. Generally these objects have slightly more mass than the Sun, but are only about 10 km in radius. A neutron star has intense gravity, and may also have an intense magnetic field and fast rotational component.
Supernova: A super massive star may undergo a cataclysmic explosion at some point after it has exhausted its internal hydrogen used for fuel. The famous crab nebula is a beautiful example of the still-expanding gases surrounding a supernova explosion that occurred in the year 1054.)
The Birth of Neutron Stars
Besides the interstellar debris, supernova explosions often leave behind a cinder, the stars dense collapsed neutron core, which was created by the compression of electrons and protons. Called a neutron star, the object is about 1 0 miles wide, has a mass greater than our Sun, and a density of about a billion tons per teaspoonful. Because of its small size and high density, the neutron star possesses a gravitational field 300,000 times stronger than the Earth's. Its rotation also increases dramatically during the collapse. Most celestial objects rotate, but neutron stars rotate very rapidly. The neutron star in the Crab Nebula rotates 30 times per second or 3.4 million miles per hour. It is the only kind of star that can rotate rapidly without breaking apart.
Pulsar Mechanics
Some neutron stars -- such as the Crab -- emit radio waves, light, and other forms of radiation that appear to pulse on and off like a lighthouse beacon. Called pulsars, they don't really turn radio waves on and off -- it just appears that way to observers on Earth because the star is spinning. Astronomers pick up the radio waves only when the pulsars beam sweeps across the Earth.
Pulsars possess a powerful magnetic field that traps and accelerates charged particles, and shoots them through space as radio waves. Their rapid rotation makes them powerful electric generators, capable of accelerating charged particles to energies of millions of volts. The Crab, the youngest and most energetic pulsar, produces enough energy to power the nebula and make it expand. The real difference between a neutron star and a pulsar is that a pulsar has a magnetic field that is misaligned with the rotation axis -- being tilted at an angle of about 30 degrees to the rotation poles.
A pulses energy output lights up and expands the nebula around it. This action robs energy from the pulsar's rotation, so that it spins slower over time. This "spin-down" rate is a tiny percentage per year, so that it will take about 10,000 years for the pulsar to slow to half its current rotation speed. As time progresses, the Crab's pulses will become less intense, and its X-ray emissions eventually will end. The nebula itself will disappear after only a few thousands years. Eventually only the radio pulsar, beaming every few seconds, will remain.
First discovered in 1967, scientists jokingly dubbed pulsars LGM for "Little Green Men," because the radio signals were so regular it seemed to be a sign of intelligent life. Scientists can predict the arrival times of pulses a year ahead with an accuracy of better than a millisecond. They have cataloged more than 300 of them. But only two, the Crab and Vela, emit detectable visible pulses. The Crab emits radiation throughout the entire spectrum, including gamma and X-rays.
The Crab Nebula is one of the most famous radio objects. It is an optically viewable supernova remnant, and also a very strong radio source, The radio power of this object puzzled scientists, because it was recognized as being a supernova expansion discovered by Chinese astronomers as a 'guest star', in 1054 AD By this time its heat would have radiated away into outer space, making it a very weak thermal radiator. Because it is a very strong radio source, astrophysicists had to find another mechanism for its radiated energy. In the supernova event, a pulsar was created in the Crab which sends out pulses at a rate greater than 33 per second, It is now believed that the pulsar continually pumps up' the energy of the leftover debris. The Crab pulsar is one of several that have been identified as central figures of supernovae debris, this particular pulsar has also been proven to pulse optically in synchronization with its radio bursts.
Crab Nebula (Hubble Telescope
It is important to note that although the Crab Nebula is a strong radio source that is easily detectable with amateur equipment, detection of the pulsar is much more difficult. Observation of pulsars requires very large antenna aperture which is associated with very small beam width. This tends to separate the pulsar energy emission from surrounding sky noise. Additional signal processing equipment is also needed to fetch the pulses from the usual thermal noise produced by radio receivers.
Pulsar Detection @ 406 MHz
(James Van Prooyen, Software Engineer, Grand Rapids Radio Observatory, GRRO
grro@dnx.net
Over the past five years, Jim has developed a few different versions of software for the purpose of detecting Pulsars. In Jim�s words - As a software engineer by trade and with my interest in Radio Astronomy, the two areas seemed to be perfect for this research.
My first task was to find the best system for doing this type of research. As many of us are on one budget or another, this was my first concern.
In my research, I had read about the detection of Pulsars, the history of Jocelyn Bell and Dr. Anthony Hewish. In addition, I had read of the efforts of an amateur radio astronomer named Robert M. Sickels (Fort Pierce, FL, deceased 1993) and his work at 612 MHz. In further readings, In addition, I found notes on the frequency range of 406 � 410 MHz as well as some microwave regions as well.
While attending a radio astronomy conference, at the National Radio Astronomy Observatory, Green Bank, WV, I was introduced to the 406 MHz area by an engineer named Carl Lyster. Carl is a design engineer for Radio Astronomy Supplies (RAS) and, is responsible for all system developed and sold by RAS.
After returning from the conference, I researched different off the shelf receivers/scanners being used for radio astronomy. In doing my research on equipment, I was interested in the best receiver I could find and at a reasonable price. As we all know, we work on a budget. Most of the receivers were quite highly priced and were not made primarily for doing radio astronomy. After careful consideration, I decided to use the RAS 406.7 MHz Radio telescope (which was field proven and showed excellent system sensitivity),
Hardware
I started the process of adapting the current software made for the RAS system. I found that while the system software was more than adequate for doing radio astronomy continuum observations, I would also need to design a software package for my special project, Pulsars.
Let�s take a look at my system �
Software Overview (Early On)
All versions of the program continue to use the 406.7 MHz system and it associated software. Other Radio Telescope receivers/computers may be used as long as the output file format is compatible with the input file format of the Pulsar Detector. See the section titled �File Formats� form more information.
A minor modification of RAS data collection program is needed to run this program. See the section titled �RA Modifications� for more information.
Work in several different areas is being done to improve this system.
Processing
The pulsar detector works in the following manner. The signal is sampled using time base that is some integral fraction of the period of the pulsar. Each sample taken during the duration of a single pulse period is assigned to a bin. The sample can then be folded back so that samples from corresponding bins are added together. Each time we add a new series of bin values for the ongoing totals, we "renormalize" by subtracting an amount from all of the bins so that the weakest bin value is brought back to zero. Random noise in each bin tends to average out and bins, which have a slightly higher average, will gradually "grow" above the others. A picture then emerges which shows how the average strength of the pulsar signal varies over its period.
A filter based on the pulsar period is then used for final processing of the data. data is sent to a file, the file name is selected from the catalog based on its index in the catalog.
Processing File
The pulsar detection system is made up of several different files
RA_PSR2.BAS Source code file for the pulsar detector
RA_PSR2.EXE Executable program
RA_PSR2.CAT The catalog of 1300 know pulsars
RA_PSR2.CMD The command file, this has information on the
data files to be processed.
RA_PSR2.SET Setup data
Output Files
The output file has arrays of data for plotting in Excel. Currently there are 4 different data arrays saved for plotting, they are as follows:
Array Name Use
Raw Totaled Bin's Raw data
Time Corrected Bin's Used to check processing
Processed Bin's The bin after summing and normalization
Filtered Bin's The final product of the processing
several are included in this report
Example data file which are plotted in this report are:
PSR76.DAT
PSR269.DAT
PSR1117.DAT
PSR1133.DAT
PSR1161.DAT
Please note that the internal catalog name is used, many older operating systems will not support the naming standard currently in use for naming pulsars.
Latest Hardware/Software Update
- New 16 bit high speed A/D (Fall of 2005)
- Higher speed sampling, up to 1000 time per second. This allows us to
plot the full wave form of most pulsars.
- Survey of 45 pulsar done to test the system (receiver, and software)
- New directory subsystem that allows continues operation of the data
collection system. The data collection system names file based on date and time of day.
Future Developments in Progress
- A 4 node Beowulf cluster of computers has been added to do processing
in real time.
- Interface between data collection system and Beowulf cluster in is
almost complete with initial testing starting in March of this year.
- New features added to do search for standard Pulsars and RRAT's.
Please note:
Beowulf is a design for high-performance parallel computing clusters on inexpensive personal computer hardware. Originally developed by Donald Becker at NASA, Beowulf systems are now deployed worldwide, chiefly in support of scientific computing.
See the following web page for more information on Beowulf computers: en.wikipedia.org/wiki/Beowulf_ (computing)
A Bit of Pulsar History by Robert M. Sickels
Premier amateur radio astronomer, Robert M. Sickels was the first to do so, in the mid 1980s, at his home and lab in Fort Lauderdale, Florida. The operating frequency was 408 MHz and 612 MHz, using a 12 foot parabolic antenna.
Actual Pulsar Observation (Robert M. Sickels)
The pulses emanating from the pulsar are usually embedded in the noise floor. In order to hear the pulses, one must have additional hardware or software to make the pulses audible.
Bob Sickels used a number of different circuits to do this. Each one showed some degree of being able to extract the pulses. One of these was the Audio Pulse Enhancer, designed by Sickels.
Something New?
In a recent piece in the Journal of Science, NATURE (Nature 439, 817-820 (16 February 2006) | doi:10.1038/nature04440, titled Transient radio bursts from rotating neutron stars� http://taco.nature.com/nature/journal/v439/n7078/full/nature04440.html#abs
� As directly quoted� -the discovery of a previously unknown type of source, varying on timescales of minutes to hours. Eleven objects characterized by single, dispersed bursts having durations between 2 and 30 ms. The average time intervals between bursts range from 4 min to 3 h with radio emission typically detectable for <1 s per day.
Correspondence to: M. A. McLaughlin1 Correspondence and requests for materials should be addressed to M.A.McL. (Email: Maura.McLaughlin@manchester.ac.uk).
AXP�s
Another fairly new object is the Anomalous X-ray Pulsar (AXP). These objects emit powerful Gamma and X-Rays as received by the ESA's gamma-ray observatory. Less than ten are known at this time.
Pulsar Catalog
Below is a partial catalog of known pulsars, it gives their sky positions pulse period and relative intensity. The two north hemisphere pulsars most are likely to be detected at:
0329 + 54 and 0531 +21. Pulse period is used to derive the number of pulses per second. i.e. 1/pp for 0329 +54 = 1.399 pulses per second, which also represents approximately 14 pulses each 10 seconds in duration.
Ascension Declination Period in Seconds 408 MHz - K Degrees
01 00 +65 1,6792 58
01 36 457 1.2837 51
01 38 +59 1.2229 55
01 54 +61 2.3517 60
03 29 +54 7145 56
03 40 +53 1.9346 52
03 55 +54 .l564 49
04 02 +61 .5949 45
04 48 +46 .6385 56
05 31 +21 .0331 (Crab) 103
05 40 423 .2460 46
06 11 +22 .3349 52
08 l8 -41 .5454 53
08 33 -45 .0892 (Vela) 190
09 50 +08 .2531 22819
13 25 -43 .5327 112
15 24 -39 2.4176 67
15 52 -23 .5326 43
15 56 -44 .2571 127
16 00 -27 .7783 49
16 12 +07 1.2068 46
16 12 -29 2.4776 57
16 20 -42 .3646 142
16 41 -45 .4551 392
16 48 -42 9734 295
16 49 -23 1.7037 61
17 00 -32 1.2118 121
17 00 -18 .4777 56
17 02 -18 .2990 61
17 06 -16 .6531 50
17 17 -29 :6204 172
17 18 -02 .4777 56
17 18 -32 .4772 209
NOTE: The above information has been taken from the Radio Astronomy Handbook,
Robert M. Sickels.
Useful Websites
http://www.nrl.navy.mil/home.htmlhttp://www.nrl.navy.mil/home.htmlhttp://www.nrl.navy.mil/nrl/direct/code.7000.htmlhttp://www.nrl.navy.mil/nrl/direct/code.7000.htmlhttp://rsd-www.nrl.navy.mil/http://rsd-www.nrl.navy.mil/http://rsd-www.nrl.navy.mil/7210/http://rsd-www.nrl.navy.mil/7210/http://www.nrl.navy.mil/cgi-bin/chas/whois.plhttp://www.nrl.navy.mil/cgi-bin/chas/whois.pl
http://astronomy.swin.edu.au/pulsar/
Acknowledgements
I would like to thank the following people for there help, expertise and with supplying information for this publication.
Dr. Michael Kramer - Jodrell Bank, UK,
Dr. Marshall, CSIRO
Bryan Gaensler � NRL,
Mr. James Van Prooyen � Software Design Engineer
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