Friday, August 17, 2007

Charles Schisler, radio astronomer of 1967, and his Messier Objects type list

Dick Manchester requested about 10 minutes during the Friday Afternoon discussion to be given over to Mr. Charles Schisler. Dick had spent some time talking with Charles about observations Charles had made beginning in the summer of 1967. This summary is based on Charles' presentation on Friday August 17, my discussions with Charles on Thursday August 16, and conversation with Dick Manchester.

Charles was a radar man in the military at the age of 41, stationed on the coast of Alaska, from June 1967-June 1968, at Clear Air Force Base. There, the US military was using active radar at 420 MHz, to scan the horizon between 240 and 360 azimuth (East of North, looking out over the Pacific), using three antennae which covered 40 degree swaths (240-280, 280-320 and 320-360 degrees azimuth). They also had sensitivity at two different altitudes in these azimuthal areas (+5 and +10 degrees). Each antenna would directionally sweep across its 40 degree area in 5 seconds. The purpose of these antennae was to watch the horizon for ICBMs, which should rise in altitude should they come over the horizon. This was active radar: a radio pulse would be sent out at 420 MHz, and if a target were found, return a bounced signal, giving its distance (from the time lag) and direction. The radar system would find signals from the transient pulses -- it contained a high pass filter, so that only the highly variable radio pulses bouncing back from targets would be observed, and not the time average hiss of a persistent radio source. In this way, the equipment like that of Anthony Hewish and Jocelyn Bell Burnell, who were looking for the scintillation of radio signals from quasars. The system could not, therefore, detect persistent radio signals -- only highly variable ones. Signals were presented immediately on a screen, and were not (apparently) archived on paper.

See the Clear Air Force Station website for some information about the instrument.

The timing capabilities of the radar are not entirely clear to me at this writing. We heard from Charles that the radar had a range of 3000 miles; this would mean that it could resolve a bounced radio signal, which travels out and back in 20 milliseconds or shorter, filtering signals on a much longer timescale. It's possible the timing capability estimated this way is off by a factor of 2 (to 40 milliseconds), but not 10.

Charles had been a navigator on bombers (B-47s), where celestial navigation was part of his training. So, the day he detected a radio source which decreased in altitude, going East, he knew not to dismiss it as interference. He was pleased that he was assigned to the same azimuthal area the next day, and knew to search for the same signal at about the same time, and when the signal re-appeared, descending in altitude, but only 23 hours and 56 min later, he knew immediately that he had observed a celestial source. It appeared again on a third day, confirming the detection.

This was of no interest to his commander, so Charles looked into this in his spare time, driving some hours to the nearest library, where he was surprised to find a catalog of celestial radio sources -- "General Catalog of Discrete Radio sources" (Howard and Maran ApJ Supp. 10, 1 1965 [ADS]). He had not known that celestial radio sources had been documented. In this way, Charles determined that he had observed a radio source Taurus A -- the Crab nebula.

During the year, he noticed several celestial radio sources in this way, and he carefully noted the location of each one. Some were in the radio catalog he found at the library, some were not. Had the work not been classified, it may have been able to provide a very sensitive radio survey of a wide swath of the sky. I imagine Charles' list should have been useful to the radar men. It's a modern day Messier Objects list, where the original Messier Objects list were extended optical sources which did not move, and so were not comets and could be safely ignored by the avid comet hunters of the day. Here, Charles' list is of variable radio sources which were tied to the celestial sphere, and so were not intern-continental ballistic missles on their path to destroy the West Coast.

A question, which is of historical interest, arises: were these detections the first historical detections of coherent radio pulsations from a pulsar? If so, it is possible they predate the August 6, 1967 detection, described to us by Jocelyn Bell Burnell this week, and which began their odyssey of figuring out what these signals were. The question has no scientific import today, having no interpretive implication of physical interest. Neither would a positive (or negative) answer diminish the primacy of discovery by Bell-Burnell and Hewish. However, it is of historical interest to know, when was the first observational evidence capable of revealing the existence of neutron stars received?

I asked Charles if he had, in the radio observations he had made, ever seen the radio signals give a train of pulses -- the tell-tale sign of a radio pulsar source. He responded that, no, he had not seen any trains of pulses. The screens he had were not set up for that sort of thing. Even so, one could put the question this way (which I did, with Dick Manchester): if an astronomer who knew of the existence of radio pulsars had seen the information returned by the radar system, could they have concluded they were observing coherent radio pulsations?

The answer, it appears, is "no, but". The design of the system was to detect the fast return pulses of the active radar bursts. Thus, even a radio astronomer who was fully knowledgable of the possibility of coherent pulsations, would have concluded that one was seeing fast radio variability from a celestial source, but would not have had the information necessary to conclude one was observing coherent pulsations -- the signal which indicates a radio pulsar, and that Jocelyn Bell Burnell first identified in her data on August 6, 1967. But: it would certainly have prompted observations with the recording capability to resolve the pulsations.

1 comment:

René Breton said...

I must say that I found Mr. Schisler's talk very interesting. We don't get to read such pieces of history in textbooks.

Once again, it shows how subtle is the nature of discoveries: Mr. Schisler could have let this little "annoying" signal go and not bother about it but instead he carefully recorded details of what he observed.

As Bob pointed out, shedding light on this story is mainly of historical interest (here, I don't want to diminish the work that has been done, it's totally worth it). However, it could have been even "better". Let's imagine that Mr. Schisler's observations would have detected some transient neutron star bursts or supernovae. Just like the Chineese records of SN 1054, it could have had a bigger scientific impact, even several years later.

I wonder how many such discoveries will remain unknown forever...