[evlatests] Zeeman problem?

[Barry Clark]

Basically, the rule is that in calculating signal-to-noise in a line
it stops going down after about 15 Jy.  It's no faster or easier
to look at a 300 Jy line than at a 15 Jy line, and it just buys you
trouble in the form of counter-intuitive looking spectra and other
problems.

Steven T. Myers wrote:
> 
> Interesting. The noise should go down with integration time, are there 
> longer observations that show this?  E.g. 8 hours should be a factor 4 
> lower fluctuations if due to noise...
> 
> On Tue, 20 Sep 2011, Barry Clark wrote:
> 
>> 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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> |:| Steven T. Myers                      |:|  Tenured Astronomer       |:|
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