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Why can an object move off the crosshairs in WISE images? (DIsk Detective)

Mar 03, 2022

Hello! Finally a new post about citizen science! I will be covering this form the perspective of the Disk Detective project, so you may want to pay it a quick visit before reading so you can make sense of the following post. Also check the additional links included in the post for additional information.

I will cover a case that was shared by another user of the Disk Detective project, when I was asked my opinion of why the object was moving off the crosshairs, in this case specifically in the WISE 3 and 4 bands. That can happens for a number of reasons, but since we are limiting in this case the question to the WISE (or unWISE, speaking of the coadd used for the DD site) we are not going to speak of situations like high proper motion stars.

WISE images were taken for the different bands quite close to each other in terms of date, so you are not going to see movement there unless the object is flying across the Milky Way and in our backyard (in astronomical terms of course, you can visit Backyard Worlds website for a project that deals with this type of situations).

So, if we discarding actual movement… what can cause the impression of movement in a set of WISE images? Faint objects, objects to faint to be visible in optical bands but what can glow brightly in IR, Radio or X-Ray bands.

Ok, but how I can determinate that easily? Well, that is not always possible but to our luck now thanks to surveys like Gaia (DR2 and EDR3) we have data about a lot of faint objects that a few years back were impossible to track. Of course, there still a lot of lurkers around that are not going to be present even in Gaia. There is were moderns survey’s that deals with other parts of the electromagnetic spectrum comes in, more on this below.

Let´s see the images of this particular subject, number 70403227 for the Disk Detective site. As you can see, everything looks quite normal until the end, were the object “moves” in unWISE 3 and 4.

If you open the VizieR result page for this object, and search in Gaia EDR3, there are two objects presents with the second one being quite faint compared to our DD subject. Could be that the culprit?

We need some additional tools and IRSA Finder Chart gives us just that, that even if not exactly precise, is enough to get a ballpark of the distance from the center of our coordinates to the point we want. Let´s notice that in IRSA shows us the original WISE images, while in the DD site we see the unWISE version.

So, taking that in consideration looks like the second object in Gaia EDR3 is too far away, and additionally is not point to the right position if we use its coordinates.

In that case, we have anything closer to the value given by the ruler earlier?

Yes! In this case, we have a clear winner that is inside the suggest distance thanks to this survey. It’s not exactly centered in WISE 4 but it’s there. So in this case, at least for the purpose of the Disk Detective project, we cannot trust in these WISE images because this faint object that is not being picked up by optical surveys is brighter in the WISE 3 and WISE 4 bands, overpowering and pretty much erasing from the pictures the signal from the star in the center of our coordinates.

In conclusion, the object never moved or shifted from the crosshair, but a faint second object is glowing more than the central star is those particular bands, while being extremely faint in any other wavelengths except for radio. And regarding of this subject being useful for the research done in Disk Detective, sadly we have to discard it since we cannot trust in the IR excess of the WISE 3 and 4 images, that are fundamental for our approach there.

Additionally checking the rest of the results in the VizieR page, that bit of an offset in respect of the allWISE coordinates sounds for me like an indication of that second object messing up measurements of the central star.

 I hope that this particular case analysis helps other citizen scientist in similar situations!

 Hope you like it, invite me a coffee and see you in the next one!

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