Friday, August 17, 2007

Pulsar timing and LIGO

Like gravitational waves? Got timing? Willing to get some?

As you'll hear in my talk, LIGO could double the number of pulsars it's looking for with more timing. Michael Kramer and Andrew Lyne are co-authors on a couple of our papers, and there's room for more. We'll discuss this at the October meeting, and we've got an open letter here you can read and pass around.

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.

Neutron star/LIGO meeting

Hi all,

We're holding our second annual "Neutron star experts/LIGO Scientific Collaboration" meeting later this year. Please come join us in Hannover, Germany on Oct 20, 2007. Details can be found at :

Our first such meeting was held Nov 06 at MIT and proved to be both fun and informative. People from the neutron star community gathered and discussed with LSC folks issues relating to the detection of gravitational waves from spinning compact objects, and how the two groups can collaborate together to do better and more informative science. Details at the webpage linked above.

Tuesday, August 14, 2007

10 Interesting Pulsar Questions

Earlier today, Franco Pacini suggested accumulating 10 Interesting Pulsar Questions. His goal: that we not have another meeting until at least one is answered. However, posing such questions is interesting in itself. Pose your list in the comments -- your best pulsar-related questions (needn't be 10).

On another topic: there are 46 people approved to post to this blog, we are getting hundreds of hits a day, and yet there is only one person (so far) who has posted new items.

Recommend Places to Eat

It's always a struggle to figure out where to eat locally. But, with 185 attendees and one day of eating, that's sufficient to make recommendations. So make them! Add comments below.

Monday, August 13, 2007

Discussion: Monday Morning Session

These are some comments, paraphrased from the speaker, during the 20-minute discussion following the morning session. For the complete -- and correct -- version of this discussion, I recommend going to the webcast page and watching it yourself. You can add your own comments and amplications as attached coment.

Vicky Kaspi: What sorts of observations would be most useful for testing your models?

A. Spitkovsky: The kinds of observations we heard about this morning (drifting subpulses) are certainly useful. GLAST will tell us something.

Y. Lyubarsky: Agree with Anatoly.

J. Weisberg: Observers should keep an open mind about their work, as the appearance / disappearance of pulsars turns out to be a very interesting phenomenon...

Jon Arons: It's clear these objects produce a pair-plasma, and that must also produce gamma -rays. For GLAST, with 30x improved sensitivities, if it does not detect gamma-ray emission, there will need to be a lot of erudite theorist squirming to explain that. Also, what is the carousel? The basis of the magnetospheric model which now exists can be used to produce a modulating wave model through that magnetosphere -- explorations in this direction may be quite useful.

Peter: Since it seems that the torque on the magnetic field axis towards alignment or towards orthogonality must act on a short timescale, then why are not most pulsars either aligned or orthogonal and is this explained in your models?

A. Spitkovsky: I have not looked at the torques in my models at a quantitative level. However, if the torques were really large then almost every pulsar would be precessing and that is not seen.

J. Arons: Timescale of alignment depends on internal structure.

D. Lai: I think the theoretical situation is not promising. Example, Anatoly's simuations stil produce a braking index of three, and to get a different braking index, some other physics must be introduced. Perhaps you can comment on this.

A. Spitkovsky: Nature need not be "nice", and so this would lead to not a "nice" model. This requires introducing more complicated physics; it is an unclean problem, yes.

P. Weltevrede: Is there only one drifting sub-pulse?

J. Rankin: Perhaps not all the bright components in the center of pulses are not cores.

D. Lai: Following on J. Arons' comment. How would you differentiate between carousel drifting subpulses, and a wave-type configuration Jon refers too?

--- D. Fox, R. Rutledge

Complete Proceedings Webcasted

The proceedings are going to be webcasted. Today you'll want the link to Monday's session.