DISCOVERY OF AN UNKNOWN SUPERNOVA REMNANT IN CEPHEUS

Not far from the well-known supernova remnant CTA1 (SNR G119.5+10.2) in the constellation Cepheus, we succeeded in identifying the previously unknown SNR candidate G115.5+9.1 "Scylla", with the special feature of its very high latitude. G115.5+9.1 is therefore also one of the northernmost known supernova remnants. Its extremely faint luminosity, without a clear X-ray or radio emission shell, and its very fine filaments meant that G115.5+9.1 remained undiscovered for so long. On some surveys, the SNR can certainly be seen, but it can be mistaken for part of larger HII structures of Cepheus.

Location of the Scylla SNR in the constellation Cepheus © Northern Sky Narrowband Survey, Stefan Ziegenbalg

Initial deep test images with narrow-band H-alpha filters revealed that the fine filaments are visible segments of an emission shell with a diameter of approx. 1 degree in the night sky.
Further tests with OIII and SII filters also showed clear signals, albeit much weaker than H-alpha.

A check of various surveys, especially in the radio, X-ray and gamma bands, revealed no conspicuous emissions, which is not unusual for such a faint SNR candidate.

We collected all the data in a report and sent it to Professor Robert Fesen from Dartmouth College for his expertise. Professor Fesen was able to take some test images of the northern and southwestern regions of the suspected new SNR at the MDM Observatory in November 2024.

© Robert Fesen

The images taken along the suspected envelope in the northern and southwestern sections show many thin filaments that Professor Fesen believes are consistent with ISM shocks. So we had another likely SNR discovery on our list, albeit an extremely faint candidate.

But it's an interesting discovery because it's so far from the Galactic plane and in the middle of a relatively quiet ISM region. Also, G115.5+9.1 is quite large with about 1 degree in diameter (the visible part). If possible segments in the east and northeast are also included, then G115.5+9.1 may even have an extension of up to 1.5 degrees.

SCYLLA AND CHARYBDIS

Some people may be familiar with the expression “being caught between Scylla and Charybdis”. It means being in a hopeless predicament in which you have to choose between two equally dangerous alternatives.

The expression comes from Greek mythology, more precisely from Homer's “Odyssey”. Scylla and Charybdis were two sea monsters that lived in the Strait of Messina and posed a dangerous threat to sailors. Scylla was a six-headed monster, while Charybdis was a whirlpool that devoured ships.

The choice of name “Scylla” for the newly discovered supernova remnant is inspired by its unconventional appearance. The red filaments of H-alpha gas are reminiscent of the tentacles of the mythological sea monster. Like the ancient creature that drew sailors into the depths, the supernova remnant with its filigree structures exudes a certain menace and fascination.

Supernovae are powerful cosmic events that signify both destruction and transformation. The naming “Scylla” honors this duality and combines the scientific discovery with the rich symbolism of Greek mythology.

WHERE IS CHARYBDIS?

The Scylla SNR is not the only previously unknown object in the image, it also contains two planetary nebulae, one of which will be presented here in particular.

Firstly, the already known PN “M 2-55” (PNG 116.2+08.5) can be found in the image at about 5 o'clock below the Scylla SNR.
On the other hand, the previously unknown PN candidate “Sai2” (PNG 113.8+08.4), which was discovered by our team member Yann Sainty in the course of data collection for the Scylla project, can also be found in the image. Sai2 is hidden at about 2 o'clock on the right edge of the image. The PN candidate has the shape of a classic bubble with roughly equal amounts of OIII and H-alpha. The team decided on the name “Charybdis” to correspond to the protagonist "Scylla" in the image.

PN candidate Sai2 (PNG 113.8+08.4) “Charybdis” and the PN “M 2-55” (PNG 116.2+08.5).

While “Scylla” attracts attention with its striking red filaments, “Charybdis” remains rather inconspicuous at the edge of the picture. This constellation reflects the mythological tale in which Charybdis, as a whirlpool, represented a hidden danger that pulled ships into the depths unnoticed.
With a diameter of 1.6 arc min, Charybdis is not one of the largest and most conspicuous objects in the cosmos, but this does not detract from its simple elegance and beauty.

PN candidate Sai2 (PNG 113.8+08.4) "Charybdis"

A ROCKY ROAD TO THE FINISHED PICTURE

The challenges of data collection:
Capturing the SNR turned out to be a complex process that presented several challenges. To begin with, there was uncertainty about the exact extent of the nebula, which required a lot of test imaging for precise mapping. The subsequent planning of the images with four different telescope setups required careful coordination to capture both the SNR itself and its surroundings with maximum visual presence and impact.

The actual data collection took place over 29 nights. The low brightness of the SNR and the high concentration of dust in the region made the acquisition considerably more difficult. In order to obtain a usable signal, the exposures had to be limited to nights with very low moon brightness, as otherwise the scattered light from the moon would have overlaid or even destroyed the faint signal. This restriction slowed down the process considerably, but was essential to ensure image quality.

The following data processing posed a further challenge. The combination of the images from four different telescopes, each with different field rotations, required precise technical knowledge. Even slight deviations in the field rotation could affect the fine structures of the SNR filaments. The preservation of these details therefore required a great deal of effort and careful processing steps.

Marcel Drechsler on the complexity of image processing:
Anyone who knows me knows that, in addition to research, my passion is image processing of astronomical data. My profession as a freelance graphic designer and my long experience in the art scene help me to create images with a certain visual impact without neglecting the scientific background. I always strive for a balance between aesthetics and scientific accuracy. The tools available to the image editor are image composition, image orientation, color palette and color mixing, and forms of contrast modification. With these options, the image editor has the possibility at hand that the viewer quickly skims the image or pauses with interest because he notices something that he has perhaps never seen before in this form, or something that catches his attention.

I also faced this challenge with Scylla. A previously unknown supernova remnant that is largely hidden behind cosmic dust. The SNR is only visible at a few spots. Another problem was that the filaments are incredibly thin and delicate. In contrast to the mighty swathes of dust in the Cepheus constellation, the SNR filaments, some of which are only one or two pixels thick, would hardly have been noticeable, which - admittedly - frustrated me a lot at first. In the course of working on the image data, I therefore decided to slightly widen the filaments of Scylla and add a subtle glow to make the H-alpha filaments in particular appear more prominent in the image.

The decisive modification to the image was an image rotation of 90 degrees counterclockwise. This placed the SNR in a strictly vertical position slightly above the center of the image. By pure coincidence, an elongated dust formation and a very straight part of Scylla form a perfect triangle that opens upwards to the SNR - what a lucky moment when I realized this coincidence. The image was intentionally aligned along the central axis to emphasize this cosmic geometry lesson.
After some initial skepticism, the image of Scylla is now one of my favorite images of our team and I am very happy to have invested so many hours of work.

ACKNOWLEDGMENTS

The team would like to thank Professor Fesen from Dartmouth College for his expertise and ongoing support. We greatly appreciate that he trusts us as amateurs and gives his valuable time for our discoveries. We hope for many more exciting projects and hopefully a long lasting collaboration!

We would like to express our sincere thanks to Professor Zouhair Benkhaldoun, the director of the Oukaimeden Observatory, for his support in setting up our observatory within his institution. His willingness to give us this opportunity and his active help were crucial to the successful start of our project. We are confident that this collaboration will allow us to make valuable contributions to scientific research.