[evlatests] Zeeman problem?

[Michael Rupen]

Hi Barry --

   boy, would I love this to be the explanation (regardless of how silly
we'll all feel :)  It should be easy to confirm:

1- This would have been true with the old correlator as well as the new,
   but I thought we were told that the old correlator did not see this
   sort of problem.  Emmanuel, can you confirm that was on much weaker
   lines?

2- This argument suggests that the self-noise varies from channel to channel
   directly proportional to the signal in that channel.
   - this should be seen in all Stokes parameters, so the noise in
     self-cal'd Stokes I should be similar.  Hard to check with recent A
     cfg data but should be obvious in older stuff, and in a new D cfg image.
   - Stokes V spectrum should show noise prop.to the line in that channel.
   - Higher-resolution observations should show different noise levels
     depending on the shape of the spectral line.
   This might be tough to see but perhaps do-able.

   I believe we looked at some very strong lines in L band.  Did those show
   the increased noise?

3- Self-noise should integrate down with time; my impression was that
   this did not.  Emmanuel, can you confirm?  It might be interesting to
   try stacking the data from several observations in this regard.

Cheers,

                  Michael

> We've been thinking for a while about looking to see whether the
> Zeeman problem occurs with the phased array.  So I thought I'd
> try a thought experiment to see how a perfect instrument would
> behave.  To simplify further I made the thought experiment a
> filter bank, rather than a correlation spectrometer, which
> obviously must give the same result.  Looking at the output of
> this receiver, the system receiver noise is, in SEFD terms, about
> 12 Jy, the antenna SEFD divided by the number of antennas.
>
> So, in the center frequency channel, the maser line _really_
> dominates the system, by a factor of 25 or so.  That being the
> case, the RMS on the channel will be the maser flux divided by
> the square root of BT.  For B = 2 kHz, T = 25m (roughtly the
> July 12 setup), the expected rms on the central channel is
> 300 Jy / sqrt(2000 * 1500) = 170 mJy.
>
> In the end channels, off the line, the power is about 25 times lower,
> so the rms will be also, about 7 mJy.
>
> It seems to me that this is not incompatible with what Emanuel
> sees in his reduction of the Zeeman observation.  The peak excursion
> on his Zeeman pattern is 290 mJy, about 1.7 sigma. something not
> unexpected.  That is, what we see is about (within a factor of two,
> anyway) what we would expect with a perfectly functioning correlator.
>
> We may have a Zeeman problem, but I no longer regard the evidence
> for it as very convincing.
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