The purpose of this post is to hopefully clarify, with references, the various LSST data products that are relevant to this discussion, outline the cases where User Generated data products might be necessary, and identify which components of the LSST pipelines might be used to create them. The following paragraphs have been written with the novice LSST user in mind.
To reiterate, the question is how to recreate the calibrated light curve of the central engine of a low-luminosity AGN that is embedded in a host galaxy. In other words, which LSST data products deliver the flux of the AGN (quiescent + variable components) as a function of time, with no contamination from the extended host galaxy.
First, to summarize the most relevant parameters in the difference imaging analysis (DIA) catalogs DIASource and DIAObject, which are measured from the single visit and difference images, and explain why they are inadequate for this goal. From Table 1 in Section 3.3.1 of the Data Products Definitions Document (DPDD; ls.st/lse-163):
DIASource.psFlux = calibrated flux for a point source model; measures flux difference between the template and the visit image (this would be the variable component of the AGN only), and
DIASource.totFlux = calibrated flux for point source model measured on the visit image centered at the centroid measured on the difference image (this would combine the underlying host and the quiescent and variable components of the AGN).
These two data products cannot yeild the quiescent + variable component of the AGN only. To get this would require that the DIA pipeline deblend the quiescent point source and extended host components in the template image using the point source model from the detected variable component, and store this information with the DIASource. This type of processing is not included in the DIA pipeline.
Second, to summarize the relevant parameters in the Data Release Object and Source catalogs (Tables 4 and 5 of the DPDD), which are measured from the single visit and the deep coadd images (note that the deep coadd images are not the same as the template images). As described in S.4.1 of the DPDD, “The master list of Objects in the Data Release will be generated by associating and deblending the list of single-epoch DIASource detections and the lists of sources detected on coadds”. This means that the AGN detected in a difference image will also be in the Object catalog.
Furthermore, “…to enable studies of variability, the fluxes of all Objects will be measured on individual visits (using both direct and difference images), with their shape parameters and deblending resolutions kept constant. This process is known as forced photometry” (DPDD, S.4.1). Forced photometry is described in S.4.2.4 of the DPDD as: “the measurement of flux in individual visits, given a fixed position, shape, and the deblending parameters of an object. It enables the study of time variability of an object’s flux, irrespective of whether the flux in any given individual visit is above or below the single-visit detection threshold.” For an AGN light curve, we should look to the parameters in the ForcedSource catalog.
As Gordon identified in his Community post on this thread on Feb 26,
ForcedSource.psDiffFlux = point source model flux on difference image
should be equivalent to the DIASource.psFlux of the AGN, so long as two conditions are met: (1) the same template image was used and (2) the centroid determined from the coadd and used for forced photometry is the same as the centroid for the difference image. The first condition should be true for a given data release, but the second condition may not be true for AGN components with low signal-to-noise ratio. Although ForcedSource.psfDiffFlux conceptually measures the same quantity as DIASource.psFlux, it is potentially much better for non-moving Objects because it uses detections and positions obtained from all epochs, not just the measurement epoch.
However, as discussed in the first few paragraphs, these measurements from the difference image are insufficient for the goal of obtaining the AGN flux. Instead, we turn to:
ForcedSource.psFlux = point source model flux on direct image.
Since the point source model is the deblended child, this is the flux of the AGN component only, without the host galaxy contribution, in a single-visit image. Therefore, this is the one to use to create the light curve of the AGN – but there are caveats for our particular situation of a low-luminosity AGN.
As Jim Bosch notes in his Community post on this thread from Feb 21, “it’s up to how well we can deblend that pair, and that will depend a lot on the relative fluxes of the components and how extended the host galaxy is”. It is also important to note that the deblending is done - and the point source model defined - based on the coadd image, which might not provide the optimal results for low-luminosity AGN. Depending on the science goals, the conclusion is that the ForcedSource data products alone might not be sufficient for studies of embedded AGN below some luminosity ratio threshold. All or some low-luminosity AGN may require a User-Generated pipeline to, e.g., stack all images from epochs when the AGN was brightest, redefine the point source model, rerun the deblender, and make new measurements of ForcedSource.psFlux.