What is in the NondetectionLimit packet?

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#1

Wondering what to do with the NondetectionLimit in the LSST alert schema. Before arranging the storage of a few billion of these, I’d like to know what they mean!

There are only 4 attributes in the schema:

  • ccdVisitId
  • midPointTai (== MJD)
  • filterName
  • diaNoise

I am guessing that the last of that list is the payload. Does the diaNoise have units of nanoJansky, like the source flux?

For analysis of light curves, we would quote a 5 sigma upper limit, since the a diaSource is created on the condition that psfFlux >=5 sigma. Is diaNoise the 1-sigma noise measurement? Therefore the flux upper limit is 5-sigma = 5*diaNoise?

I am also guessing that the NondetectionLimit packet only appears within an alert, so that a diaObjectId and its sky position can be inferred, is that right?

#2

Hi @roy,

We haven’t actually implemented the diaNoise field in pipelines yet so I can’t give you definitive answers right now. I expect that indeed, the noise estimates will have the same units as the source flux and will be a one-sigma estimate.

(The reason we provide a noise estimate rather than the 5 sigma limiting magnitude directly is that the latter is not appropriate for estimating limits applicable for variable sources.)

DM-36281 is the ticket to watch.

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