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Add images with HST data from the different datasets, showing 1999 and 2000 data separately.
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Eric Jensen
Eric Jensen
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EDIT:

I just realized that it would be possible to test the idea above by downloading the individual filter images from the HST data and looking at them directly. Fortunately, the Hubble Legacy Archive makes this pretty easy to do, and even provides pre-combined color images for datasets where multiple filters were taken. So we can directly download images made from data only before and only after the outburst of that object and see what they look like.

This search gives all WFPC/2 images of NGC 3314, and we can see that there are some from 1999 and some from 2000. Those are the pre- and post-outburst data, respectively, that were combined in the original press release images shown above. If you look in the column that says "Level", the Level 4 data products are color composite images. The two options there are composites of the 1999 data and of the 2000 data. Let's look.

Here are the 1999 (pre-outburst) images, which are only in red and blue filters. Presumably the color composite fills the green channel with some combination of the red and blue:

NGC 3314 1999 HST data

And here are the 2000 (post-outburst) data, combining images in red, green, and blue filters:

NGC 3314 2000 HST data

Below I've zoomed in and taken a screenshot, trying to get the images centered as close to the same as possible. I've positioned them so the object in question is just to the left of the red crosshairs. Top is 1999, bottom is 2000.

1999: NGC 3314 1999 HST data zoomed in

2000: NGC 3314 2000 HST data zoomed in

You can see that the newly-brightened object is basically white, not green. The individual, isolated red, green, or blue pixels are noise. In any image, you will have some pixels that are extra-bright, either from noise in the detector, or from a cosmic ray hit that was not removed entirely. Since they are random noise, they aren't in the same place in every image. So if you make a color composite image, then those randomly-bright pixels will show up as the color of whatever filter they appeared in, since they are bright only in that image and dark in the images taken in the other filters.

EDIT:

I just realized that it would be possible to test the idea above by downloading the individual filter images from the HST data and looking at them directly. Fortunately, the Hubble Legacy Archive makes this pretty easy to do, and even provides pre-combined color images for datasets where multiple filters were taken. So we can directly download images made from data only before and only after the outburst of that object and see what they look like.

This search gives all WFPC/2 images of NGC 3314, and we can see that there are some from 1999 and some from 2000. Those are the pre- and post-outburst data, respectively, that were combined in the original press release images shown above. If you look in the column that says "Level", the Level 4 data products are color composite images. The two options there are composites of the 1999 data and of the 2000 data. Let's look.

Here are the 1999 (pre-outburst) images, which are only in red and blue filters. Presumably the color composite fills the green channel with some combination of the red and blue:

NGC 3314 1999 HST data

And here are the 2000 (post-outburst) data, combining images in red, green, and blue filters:

NGC 3314 2000 HST data

Below I've zoomed in and taken a screenshot, trying to get the images centered as close to the same as possible. I've positioned them so the object in question is just to the left of the red crosshairs. Top is 1999, bottom is 2000.

1999: NGC 3314 1999 HST data zoomed in

2000: NGC 3314 2000 HST data zoomed in

You can see that the newly-brightened object is basically white, not green. The individual, isolated red, green, or blue pixels are noise. In any image, you will have some pixels that are extra-bright, either from noise in the detector, or from a cosmic ray hit that was not removed entirely. Since they are random noise, they aren't in the same place in every image. So if you make a color composite image, then those randomly-bright pixels will show up as the color of whatever filter they appeared in, since they are bright only in that image and dark in the images taken in the other filters.

Bounty Ended with 200 reputation awarded by uhoh
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Eric Jensen
Eric Jensen
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tl;dr No additional data of interest, but I explain how I searched, and I can explain the green color.

It seems that there have not been any additional observations of that object, which perhaps isn’t too surprising given how faint it is - 21st magnitude is possible only with big telescopes.

Simbad lists that object under the name of “EQ J103712-274051” but doesn’t list any other names, nor any references besides the IAU Circular already mentioned. That’s a purely coordinate-based designation, with the “EQ” meaning it is based on equatorial coordinates. Simbad says that that designation is internal to Simbad for objects of unknown nature and is to be avoided in the literature.

Looking at ADS, the only citation for the IAU Circular is a 2019 paper by Bonanos et al., which is a catalog of HST variable sources. That seems promising on its face, but digging into the paper it turns out that the sources they flag are different ones, and the citation is in a footnote that says, “This object is unrelated to the transient in NGC 3314 reported in the IAU Circular 7388 by Keel & Frattare (2000).”

I also looked for further observations in Vizier- that can be a good way to find observations of a given source, even if it wasn’t called out specifically in a publication. While that search yields lots of objects within 5”, especially in HST observations, none of them seems to quite match up in coordinates and magnitudes with what Keel & Frattare report in the IAU Circular.

Regarding the green color that drew their attention to the object in the first place, here’s my interpretation of what they are saying there. First, note that HST filters are (usually) named with “F” for filter (as opposed to “G” for grism), a number that gives the filter effective wavelength in nm, and then “N”, “M”, or “W” for “narrow”, “medium”, or “wide”, indicating the width of the filter bandpass. So the F450W filter is similar to Johnson B, for example, and the F555W and F675W filters are green and red, respectively.

From 1999, when the object wasn’t bright, they had blue and red images. From 2000, after it brightened, they had blue, green, and red images. (The 1999 “red” data, F814W, are actually near-infrared, but I’m guessing they used those in the red channel for the Heritage image.) So in combining to make an RGB (color) image, they had two each of red and blue images (one before and one after brightening), but only one green image (after brightening), which they would have needed to weight doubly in the composite to get a reasonable color balance. That’s why that object ended up looking green - they weighted the “bright” green data twice as much as the “bright” red and blue data.