[evlatests] Zeeman problem?

[Barry Clark]

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.