Celebrating the 50th anniversary of pulsar thus neutron star discovery
In the winter of 1967 Cambridge radio astronomers discovered a new type of radio source of such an artificial seeming nature that for a few weeks some members of the group had to seriously consider whether they had discovered an extraterrestrial intelligence. Although their investigations lead them to a natural explanation (they had discovered pulsars), they had discussed the implications if it was indeed an artificial source: how to verify such a conclusion and how to announce it, and whether such a discovery might be dangerous. In this they presaged many of the components of the SETI Detection Protocols and the proposed Reply Protocols which have been used to guide the responses of groups dealing with the detection of an extraterrestrial intelligence. These Protocols were only established some twenty five years later in the 1990s and 2000s...
In July 1967 a new low-frequency radio telescope started working at the Lord’s Bridge station of the Mullard Radio Astronomy Observatory (MRAO) of the University of Cambridge. Antony Hewish had led the design and construction of this novel telescope, a collection of wooden poles with wires strung between them, built to discover more of the newly found quasars and measure their sizes, by watching them flicker as the Interplanetary Medium passed in front of them. Covering two hectares, this was the largest telescope then working at this long (4-m) wavelength.(Submitted on 4 Feb 2013)
Our method of utilising scintillation for the quantitative measurement of angular sizes demanded repeated observations so that every source could be studied at many different solar elongations. In fact we surveyed the entire range of accessible sky at intervals of one week. To maintain a continuous assessment of the survey we arranged to plot the positions of scintillating radio sources on a sky-chart, as each record was analysed, and to add points as the observations were repeated at weekly intervals. In this way genuine sources could be distinguished from electrical interference since the latter would be unlikely to recur with the same celestial coordinates. It is greatly to Jocelyn Bell’s credit that she was able to keep up with the flow of paper from the four recorders.
One day around the middle of August 1967 Jocelyn showed me a record indicating fluctuating signals that could have been a faint source undergoing scintillation when observed in the antisolar direction. This was unusual since strong scintillation rarely occurs in this direction and we first thought that the signals might be electrical interference. So we continued the routine survey. By the end of September the source had been detected on several occasions, although it was not always present, and I suspected that we had located a flare star, perhaps similar to the M-type dwarfs under investigation by Lovell. But the source also exhibited apparent shifts of right ascension of up to 90 seconds which was evidence against a celestial origin. We installed a highspeed recorder to study the nature of the fluctuating signals but met with no success as the source intensity faded below our detection limit. During October this recorder was required for pre-arranged observations of another source, 3C 273, to check certain aspects of scintillation theory, and it was not until November 28th that we obtained the first evidence that our mysterious source was emitting regular pulses of radiation at intervals of just greater than one second. I could not believe that any natural source would radiate in this fashion and I immediately consulted astronomical colleagues at other observatories to enquire whether they had any equipment in operation which might possibly generate electrical interference at a sidereal time near 19h 19m .
In early December the source increased in intensity and the pulses were clearly visible above the noise. Knowing that the signals were pulsed enabled me to ascertain their electrical phase and I reanalysed the routine survey records. This showed that the right ascension was constant. The apparent variations had been caused by the changing intensity of the source. Still sceptical, I arranged a device to display accurate time marks at one second intervals broadcast from the MSF Rugby time service and on December 11th began daily timing measurements. To my astonishment the readings fell in a regular pattern, to within the observational uncertainty of 0.1s, showing that the pulsed source kept time to better than 1 part in 10^6 . Meanwhile my colleagues Pilkington, and Scott and Collins, found by quite independent methods that the signal exhibited a rapidly sweeping frequency of about -5 MHz s-1 . This showed that the duration of each pulse, at one particular radio frequency, was approximately 16 ms.
Having found no satisfactory terrestrial explanation for the pulses we now began to believe that they could only be generated by some source far beyond the solar system, and the short duration of each pulse suggested that the radiator could not be larger than a small planet. We had to face the possibility that the signals were, indeed, generated on a planet circling some distant star, and that they were artificial. I knew that timing measurements, if continued for a few weeks, would reveal any orbital motion of the source as a Doppler shift, and I felt compelled to maintain a curtain of silence until this result was known with some certainty. Without doubt, those weeks in December 1967 were the most exciting in my life.
It turned out that the Doppler shift was precisely that due to the motion of the Earth alone, and we began to seek explanations involving dwarf stars, or the hypothetical neutron stars. My friends in the library at the optical observatory were surprised to see a radio astronomer taking so keen an interest in books on stellar evolution. I finally decided that the gravitational oscillation of an entire star provided a possible mechanism for explaining the periodic emission of radio pulses, and that the fundamental frequency obtainable from white dwarf stars was too low. I suggested that a higher order mode was needed in the case of a white dwarf, or that a neutron star of the lowest allowed density, vibrating in the fundamental mode, might give the required periodicity. We also estimated the distance of the source on the assumption that the frequency sweep was caused by pulse dispersion in the interstellar plasma, and obtained a value of 65 parsec, a typical stellar distance.
While I was preparing a coherent account of this rather hectic research, in January 1968, Jocelyn Bell was scrutinising all our sky-survey recordings with her typical persistence and diligence and she produced a list of possible additional pulsar positions. These were observed again for evidence of pulsed radiation and before submitting our paper for publication, on February 8th, we were confident that three additional pulsars existed although their parameters were then only crudely known. I well remember the morning when Jocelyn came into my room with a recording of a possible pulsar that she had made during the previous night at a right ascension 09 h 50 m . When we spread the chart over the floor and placed a metre rule against it a periodicity of 0.25s was just discernible. This was confirmed later when the receiver was adjusted to a narrower bandwidth, and the rapidity of this pulsar made explanations involving white dwarf stars increasingly difficult.
The months that followed the announcement of our discovery were busy ones for observers and theoreticians alike, as radio telescopes all over the world turned towards the first pulsars and information flooded in at a phenomenal rate. It was Gold (8) who first suggested that the rotation of neutron stars provided the simplest and most flexible mechanism to explain the pulsar clock, and his prediction that the pulse period should increase with time soon received dramatic confirmation with the discovery of the pulsar in the Crab Nebula (9, 10). Further impressive support for the neutron star hypothesis was the detection of pulsed light from the star which had previously been identified as the remnant of the original explosion. This, according to theories of stellar evolution, is precisely where a young neutron star should be created. Gold also showed that the loss of rotational energy, calculated from the increase of period for a neutron star model, was exactly that needed to power the observed synchrotron light from the nebula.
PULSARS AND HIGH DENSITY PHYSICS Nobel Lecture, December 12, 1974 by ANTONY HEWISH
I had sole responsibility for operating the telescope and analyzing the data, with supervision from Tony Hewish. We operated it with four beams simultaneously, and scanned all the sky between declinations +50' and -10' once every four days. The output appeared on four 3-track pen recorders, and between them they produced 96 feet of chart paper every day. The charts were analyzed by hand by me. We decided initially not to computerize the output because until we were familiar with the behavior of our telescope and receivers we thought it better to inspect the data visually, and because a human can recognize signals of different character whereas it is difficult to program a computer to do so.
After the first few hundred feet of chart analysis I could recognize the scintillating sources, and I could recognize interference. (Radio telescopes are very sensitive instruments, and it takes little radio interference from nearby on earth to swamp the cosmic signals; unfortunately, this is a feature of all radio astronomy.) Six or eight weeks after starting the survey I became aware that on occasions there was a bit of "scruff' on the records, which did not look exactly like a scintillating source, and yet did not look exactly like man-made interference either. Furthermore I realized that this scruff had been seen before on the same part of the records - from the same patch of sky (right ascension 1919).
The source was transiting during the night - a time when interplanetary scintillation should be at a minimum, and one idea we had was that it was a point source. Whatever it was, we decided that it deserved closer inspection, and that this would involve making faster chart recordings as it transited...
A few days after that at the end of November '67 I got it on the fast recording. As the chart flowed under the pen I could see that the signal was a series of pulses, and my suspicion that they were equally spaced was confirmed as soon as I got the chart off the recorder. They were 11/3 seconds apart. I contacted Tony Hewish who was teaching in an undergraduate laboratory in Cambridge, and his first reaction was that they must be manmade. This was a very sensible response in the circumstances, but due to a truly remarkable depth of ignorance I did not see why they could not be from a star. However he was interested enough to come out to the observatory at transit-time the next day and fortunately (because pulsarsrarely perform to order)the pulses appeared again. This is where our problems really started. Tony checked back through the recordings and established that this thing, whatever it was, kept accurately to sidereal time. But pulses 11/3 seconds apart seemed suspiciously manmade. Besides 11/3 seconds was far too fast a pulsation rate for anything as large as a star. It could not be anything earth-bound because it kept sidereal time (unless it was other astronomers). We considered and eliminated radar reflected off the moon into our telescope, satellites in peculiar orbits, and anomalous effects caused by a large, corrugated metal building just to the south of the 41/2 acre telescope.
Then Scott and Collins observed the pulsations with another telescope with its own receivers, which eliminated instrumental effects. John Pilkington measured the dispersion of the signal which established that the source was well outside the solar system but inside the galaxy. So were these pulsations man-made, but made by man from another civilization? If this were the case then the pulses should show Doppler shifts as the little green men on their planet orbited their sun. Tony Hewish started accurate measurements of the pulse period to investigate this; all they showed was that the earth was in orbital motion about the sun.
Meanwhile I was continuing with routine chart analysis, which was falling even further behind because of all the special pulsar observations. Just before Christmas I went to see Tony Hewish about something and walked into a high-level conference about how to present these results. We did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem - if one thinks one may have detected life elsewhere in the universe how does one announce the results responsibly? Who does one tell first? We did not solve the problem that afternoon, and I went home that evening very cross here was I trying to get a Ph.D. out of a new technique, and some silly lot of little green men had to choose my aerial and my frequency to communicate with us. However, fortified by some supper I returned to the lab that evening to do some more chart analysis. Shortly before the lab closed for the night I was analyzing a recording of a completely different part of the sky, and in amongst a strong, heavily modulated signal from Cassiopea A at lower culmination (at 1133) 1 thought I saw some scruff. I rapidly checked through previous recordings of that part of the sky, and on occasions there was scruff there. I had to get out of the lab before it locked for the night, knowing that the scruff would transit in the early hours of the morning.
So a few hours later I went out to the observatory. It was very cold, and something in our telescope-receiver system suffered drastic loss of gain in cold weather. Of course this was how it was! But by flicking switches, swearing at it, breathing on it I got it to work properly for 5 minutes - the right 5 minutes on the right beam setting. This scruff too then showed itself to be a series of pulses, this time 1.2 seconds apart. I left the recording on Tony's desk and went off, much happier, for Christmas. It was very unlikely that two lots of little green men would both choose the same, improbable frequency, and the same time, to try signalling to the same planet Earth.
Over Christmas Tony Hewish kindly kept the survey running for me, put fresh paper in the chart recorders, ink in the ink wells, and piled the charts, unanalyzed, on my desk. When I returned after the holiday I could not immediately find him, so settled down to do some chart analysis. Soon, on the one piece of chart, an hour or so apart in right ascension I saw two more lots of scruff, 0834 and 0950. It was another fortnight or so before 1133 was confirmed, and soon after that the third and fourth, 0834 and 0950 were also. Meanwhile I had checked back through all my previous records (amounting to several miles) to see if there were any other bits of scruff that I had missed...
At the end of January the paper announcing the first pulsar was submitted to Nature. This was based on a total of only 3 hours' observation of the source, which was little enough. I feel that comments that we kept the discovery secret too long are wide of the mark. At about the same time I stopped making observations and handed over to the next generation of research students, so that I could concentrate on chart analysis, studying the scintillations and writing up my thesis.
A few days before the paper was published Tony Hewish gave a seminar in Cambridge to announce the results. Every astronomer in Cambridge, so it seemed, came to that seminar, and their interest and excitement gave me a first appreciation of the revolution we had started. Professor Hoyle was there and I remember his comments at the end. He started by saying that this was the first he had heard of these stars, and therefore he had not thought about it a lot, but that he thought these must be supernova remnants rather than white dwarfs. Considering the hydrodynamics and neutrino opacity calculations he must have done in his head, that is a remarkable observation!
... It has been suggested that I should have had a part in the Nobel Prize awarded to Tony Hewish for the discovery of pulsars. There are several comments that I would like to make on this: First, demarcation disputes between supervisor and student are always difficult, probably impossible to resolve. Secondly, it is the supervisor who has the final responsibility for the success or failure of the project. We hear of cases where a supervisor blames his student for a failure, but we know that it is largely the fault of the supervisor. It seems only fair to me that he should benefit from the successes, too. Thirdly, I believe it would demean Nobel Prizes if they were awarded to research students, except in very exceptional cases, and I do not believe this is one of them. Finally, I am not myself upset about it - after all, I am in good company, am I not?
By S. Jocelyn Bell Burnell8th Texas Symposium on Relativistic Astrophysics13-17 Dec 1976. Boston, Mass.