[daip] issues with RM task?

[Eric Greisen]

Sayali Avachat wrote:
> Hello, 
> 
> I am a PhD student of Florida Institute of Technology. I am working on 
> data analysis of galaxy M87. 
> I have obtained RM maps for one of the time epochs in my data. But I am 
> not sure if the RM is giving me the right thing. I tried to plot 
> position angle Versus wavelength squared to see the lambda^2 signature 
> of Faraday rotation in my data, making a least squares fit. The slopes 
> of position angle Vs lambda^2 are way higher than the maximum value of 
> rotation measure in my RM map. Also I do not see any negative RM values 
> in my map (which it should give me since there is negative rotation 
> measure in M87 jet, as per earlier work on it). Its range is 0-1300 
> rad/m^2, while slope of PA Vs lambda^2 is around 2000-4000 rad/m^2.
> I am not sure if I am entering the inputs in RM properly. First I tried 
> setting to default APARM values and then tried successively to increase 
> the APARM(4) 5000, 10000, 15000 etc.
> The RM maps looks different each time (obviously) but the range still 
> stays same, 0-1300 rad/m^2.  Even setting the initial guess for RM value 
> by APARM(3) does not help getting the reasonable  result.
> What is that I am missing when plotting RM map? Is there any other 
> parameter that I need to set to get what I want? Or is there any other 
> task to obtain RM map?

I am sorry to be slow to answer your query and it would probably be best
if I could actually get your image of position angles to play with. 
Fitting RMs to only a few frequencies is a tricky business and is now 
being replaced with "rotation measure synthesis" which is also turning 
out to be a tricky business.  Our task FARS does that but it really only 
works well with 100's of frequencies.  COMB does a simple algorithm with 
only 2 frequencies (and a rather poor help file description)

I still have the images from August :
   8475840 Aug  7 13:31 M87_03_XUKQ-1.FITS
   8631360 Aug  7 13:31 M87_03_XUKQ-2.FITS
   8544960 Aug  7 13:31 M87_03_XUKQ-3.FITS
   8490240 Aug  7 13:31 M87_03_XUKQ-4.FITS
  25251840 Aug  7 13:31M87_03_XUKQ.PATRAN.FITS
Are there more recent ones I should look at?

Eric Greisen