[daip] [!12656]: AIPS - Offset source moving in RA during observation

[Andy Biggs]

Andy Biggs updated #12656
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Offset source moving in RA during observation
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           Ticket ID: 12656
                 URL: https://help.nrao.edu/staff/index.php?/Tickets/Ticket/View/12656
                Name: Andy Biggs
       Email address: abiggs at eso.org
             Creator: User
          Department: AIPS Data Reduction
       Staff (Owner): -- Unassigned --
                Type: Issue
              Status: Open
            Priority: Default
                 SLA: NRAO E2E
      Template group: Default
             Created: 07 September 2018 04:11 PM
             Updated: 07 September 2018 04:11 PM
           Reply due: 11 September 2018 04:11 PM (4d 0h 0m)
      Resolution due: 03 June 2021 12:00 AM (999d 7h 49m)

Hi,I've been reducing some archival VLBA data from 2000 of a gravitational lens. The project code is GX006 and this was a longtrack observation that regularly swapped between standard continuum setups at 8.4 and 15 GHz. Despite the 'G' in the code, this is VLBA only. One image (A) is very bright (~400 mJy) and is a AGN core with a short jet. The other image (B) is ~12 times fainter and 1.6" away. I perform standard data reduction in AIPS with fringe fitting peformed initially using a point-source model at the phase centre. I then map and self-cal using IMAGR/CALIB.The resulting map of A looks pretty good, but that of B looks much worse. The noise is higher, there are significant residuals around the source. As a test, I subtracted the clean components of A using UVSUB and phase calibrated the data using a map of B alone. The phase solutions showed essentially a linear slope with time and a subsequent map of B looked fantastic. As a further test, I wrote the UVSUB'd data to disk and fitted a point source to B in difmap. This shows (see attachment) that B drifts during the observation, predominantly in RA and this will be what the B-only phase self-cal is correcting for. The drift is pretty big and I doubt very much that it has an astrophysical origin. I see the same effect at X and U Bands.I'm hoping that someone can help me understand the origin of this problem and perhaps correct for it. The only thing that I've come up with so far is whether this could be anything to do with the earth orientation parameters. I was looking at the VLBA memo relating to the large errors found in 2005 and I was interested to see that the effect of these is to move a source in RA i.e. similar to what I see. Now, these data were not observed during a time in which the EOPs were known to be greatly in error and neither am I phase referencing, but this is the only possibility I've been able to think of so far. If there are errors in the EOPs, and generally speaking there is always some level of error, these will be taken out through self-calibration for A, but might it be that B cannot be properly corrected because it's not at the phase centre?Alternatively, the separation between the two lensed images lies along PA=144 deg i.e. approximately north-south. Therefore, the shift in B could almost be viewed as a small rotation.Thanks in advance for your help.Andy

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