[evlatests] Zeeman problem?

[Rick Perley]

    Right.  The key question to me is why the spectra shown by Emmanuel 
from the VLA data did not seem to show the higher
noise in the line, as Barry's (quite correct, in my opinion) argument 
suggests should be present.   Separating a noise origin from a gain 
instability origin should be relatively simple ... (says here ...)

   

Michael Rupen wrote:
> 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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>>     
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