How Are Resolved Alerts below 1" Separation Treated?

Science Alerts & Brokers
#1

As far as I understand, DIA-Sources that occur within 1" of each other will be associated to the same DIA-Object.

When we have strongly lensed transient, we may have multiple images that will be resolved and detected but have a separation below 1". Let’s for simplicity’s sake assume that the strong lensing system is a double, so we only have two images. Let’s denote them Img1 and Img2. I assume that no DIAObject is associated with the position of the strongly lensed images before hand. In that case then I have the following questions:

  1. Given an initial detection of a transient at Epoch 0, is it correctly understood that the centroid, with which DIAObject membership of later detections is decided, is fixed to the centroid determined at Epoch 0? Or is the centroid of the DIAObject allowed to change over time as more DIASources are associated with the object?

  2. Assume that Img1 is detected at Epoch 1. Then, at a later visit Epoch 2, both Img1 and Img2 are detected and resolved. As far as I understand, then the DIASource of Img2 will be associated to the DIAObject of Img1. Will the DIAObject then contain two different DIASources at Epoch 2, or will the two DIASources be combined into a singular detection?

  3. Assume that both Img1 and Img2 are detected at Epoch 1 and constitute two different DIASources. At Epoch 2, both Img1 and Img2 are detected again. In this case of simultaneous detection of resolved DIASources, how is the creation of and association with a DIAObject handled?

  4. I saw mention of an end-of-night process to tidy up duplicate DIAObjects / other association issues in this thread. How would edge cases like the ones I’ve described be handled in such a process? Are they being considered?

Tagging @anaismoller / @anais.moller, @BrunoSanchez who I’ve pestered with these questions a bit as well. :smiley:

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#2

The broad answer to your questions is that we do not currently deblend detection on the image difference, so it is not possible to create multiple DiaSources from the same visit that are closer than about 4.5". That radius is approximate, and is based on the (varying) footprint size of the DiaSources. The association radius is 1" as you state, and any DiaSource detected within that radius of a DiaObject will be associated with that DiaObject. We do have plans to implement deblending for image difference detections in the future, which would allow DiaSources to be created that are closer together.

Given an initial detection of a transient at Epoch 0, is it correctly understood that the centroid, with which DIAObject membership of later detections is decided, is fixed to the centroid determined at Epoch 0? Or is the centroid of the DIAObject allowed to change over time as more DIASources are associated with the object?

That is currently correct. The DiaObject location is fixed at the epoch of initial detection. We plan to update the DiaObject location in the future as more DiaSources are associated, but that is not yet implemented.

#3

Thanks for the answer, Ian. I’m specifically interested in the case where the two sources are not blended, but have low separation. Would two such sources within 4.5" of each other simply be assumed to be blended together regardless? Also, where does this 4.5" number come from? As far as I remember, the expected PSF is on the order of ~1", no?