Green Bank NGC5044 HI data calibration issues

In previous discussion last month while I am in Boston, we already concluded there is no HI detection on NGC5044. This post focus on a particular bad on-off pair , scan number 37 and 38 in the second session of our observation. (37 is on scan; while 38 is off scan)

All other on-off pairs provide good calibration solution as shown below: (showing only 1.4-1.42GHz for demonstration purpose.) With antenna temp around 0.3K

Typical calibrated spectrum of second session. this is showing the scan 35/36 pair

However, On/off pair scan 37/38 is particularly bad. with antenna temp ~ 8K with slightly different spectral shape

The change in shape can also be found in wider frequency range as shown here : 1.35-1.45 Ghz

This is a typical calibrated spectrum

and this is the scan37/38:

A closer inspection on individual integration shows a decrease in counts from scan 37 to 38

(showing 1.41-1.425 GHz here, and only in YY polarization)

The last normal integration from scan 19-37 is scan37, integration number 3. The Counts at 1.41GHz is 9.5e9 counts

which reduced to 9e9 counts in the next integration ( integration number 4 of scan 37)

and 8e9 counts in integration 5

It remains at this level until integration 16, which then reduce again. integration 17 reduce to 7.5e9 and 6.5e9 in integration 18-24

In scan 38, similar thing occurs, in integration3 and onwards, this level drop to 5.3e9

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In the other polarization XX:

Drop occurs in scan38 integration 0 : where it dropped from 8e9 to 6.5 e9

and subsequently to 5.2e9 counts in integration 14 of scan38

videos for the scans

For ON scans polarisation Y

For ON scans polarisation X

For OFF scans polarisation Y

For OFF scans polarisation X