Did I Image 2I/Borisov this Morning?

Bottom line up front, at this point I have no idea if I photographed 2I/Borisov, (the comet, hereafter) or not.

Here’s what I have to go on so far. The first image depicts a Stellarium rendering of the star field in which the comet was located when I imaged it this morning. Its computed location is indicated with a red X.

Stellarium frame rendering of Comet 2I/Borisov’s star field as of 20191207 0514EST. Annotations by Jim Johnson, December 7, 2019.

Next is an actual telescopic image from this morning. It is annotated to guide the reader through this discussion.

Annotated image of Comet 2I/Borisov taken 20191207 0514EST by Jim Johnson.

The frame scale and orientation are almost exactly the same for both the Stellarium rendering, and this morning’s photograph. Also in both images are green lines that highlight groupings of stars I used to determine that I had at least photographed the star field that should contain the comet.

In the actual photograph, I have placed a red circle in approximately the same relative location that the Stellarium rendering expects the comet to be, but I cannot see anything in this location.

I have also circled in blue two objects in the photograph that are not in the Stellarium rendering, and that are brighter than I expect the comet to be. I believe that these two objects are not stars, because I have done a lot of detailed comparisons between Stellarium renderings and actual photographs of the same star field. I have never found Stellarium to miss a star.

But I cannot say that they are the comet. That there are two unidentified objects instead of one in the photograph is suspicious.

Their brightness is suspicious too. The comet is projected to be 15.6 magnitude. Keeping in mind that a larger number equates to a dimmer object this would be a very dim object, even if it were a star. But comets are diffuse objects, not pinpoint objects like a star. This means that a comet’s reflected light is spread out over a wider area than a star, thus making it appear dimmer than a star of the same magnitude.

In the annotated photograph, I marked the magnitudes of three reference stars in orange. All three of these stars are of a lower (brighter) magnitude than the comet is projected to be, but even so, two of these three stars appear dimmer than the two circled objects. The third (11.2 magnitude), suggests that both of the unexplained objects in the blue circle are roughly 11th magnitude.

As I confessed at the beginning of this article, I do not know if I captured Borisov or not, but I have some guesses. One thing that could account for the separation between the blue circle (actual location of something), and the red circle for the computed location is that cometary data are updated periodically, and perhaps a future update will provide a more accurate computed location that places them within the blue circle. Another possible scenario that I would assess as highly unlikely, is that the comet broke into two pieces, and all of the freshly exposed ices caused the comet to flare up in brightness. Still not probable is that the flare up pushed the comet off of its original path, thus creating the discrepancy between the blue and red circles.

I will keep an eye out for news and cometary data updates for the next few days and report any new findings.

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