Fermi gamma-ray source-type determination

Xandro BA
Xandro BA
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Topic 197417

In general i got an idea how Fermi does detections and how directionality is determined of gamma-detections. What i didn't get, and what relates to this project, is how it's determined whether the source is a gamma-ray pulsar and not another kind of gamma-ray source. Are the wu's detections in general and through processing them it's determined it's a pulsar or something else? Or is at an earlier stage looked at certain signs, if you wish, in the data (pre-Einstein) to make that determination?

Mike Hewson
Mike Hewson
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Fermi gamma-ray source-type determination

Excellent thoughts, the crux of the problem indeed. It comes down to accumulated detections, plus by what criteria one defines 'a pulsar'.

Unlike the radio pulsar searches, our data points are discrete* ie. single photon arrivals in time series ( from a given direction ). Why are they recorded in lumps ? Several reasons :

- photons do that. This is quantum mechanics. The interaction of same with a detector is yes/no, no half detections/arrivals. Of course some may pass through and not interact with the detector bulk at all.

- inverse square dilution of photon flux with distance from source. What might be a fire hose at source is now dispersed to single drops here and there.

So particularly if you have a pulsing source then not all pulses will result in a detection. For radio frequency photons the arrivals, per pulse, are in huge bunches. The photon numbers are actually discrete ( we have never seen fractional photons ) but the numbers are so vast, their individual energies so small, and our detectors so 'clumsy' that we can't tell the difference b/w a flux of compared to or .

Now to pulsing. What does that mean ? A ( very ) regular repetition. You can apply the Fourier stuff just as well to discrete sources as continuous ones**.

But there is an even deeper issue. Perhaps it would be best to use the phrases 'pulsar as observed' as distinct from 'pulsar as hypothecated'. The first is a set of data records ( including detector characteristics, calibrations, information about the context of acquisition etc ... ), the second is a statement about a presumed mechanism(s) ie. what is actually happening way out there to produce the stream of particles that we detect here. Clearly one can have many hypotheses per given data record.

Now if only one of us could pop over to some nebula or three, check it out, and be back by next weekend! :-)

Cheers, Mike.

* Well, we make the radio pulsar records discrete during collection ie. digitise for processing. But the feed horn/waveguide at the radio dish focal region is an 'analogue' device.

( edit ) And I guess there is a third category of 'pulsar candidate' which is, strictly speaking, what we define by our work here at E@H. By what process the label changes/firms-up I don't know.

Err .... Ben ??

** I have too glibly contracted a vast amount of effort in that sentence, see here and here for glimpses of the greater horror, noting carefully this statement :

Quote:
..... In roughly one in a million revolutions of the pulsar, a single photon reaches the detector ...

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Xandro BA
Xandro BA
Joined: 23 Jul 13
Posts: 49
Credit: 4,522,731
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Thanks for taking the time to

Thanks for taking the time to explain and for the links. Interesting!

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