Hoinga – Supernova Remnant G249.5+24.5: A New Horizon in Amateur Astrophotography

Team New Horizons is thrilled to unveil 'Hoinga – Supernova Remnant G249.5+24.5' – a comprehensive colour visible light image of this little-known yet spectacular supernova remnant.

Our journey began with a chance encounter during a routine sky survey and evolved into an ambitious six-month project. Spanning 600 hours of observations, bolstered by encouragement from the original discovering scientists, and requiring days of intricate processing, our efforts have culminated in this unprecedented view of Hoinga.

This image offers a detailed view of the remnant's intricate structure and expansive nature. We hope it provides a new perspective on Hoinga and highlights the ongoing value of dedicated amateur observations in astronomy.

A Chance Encounter: Illuminating a Hidden Cosmic Giant

In December 2023, during a routine sky survey, New Horizons Team Director Tim Shaeffer directed Ken Hall to use his RASA11 to image a part of the sky in the constellation Hydra. Tim had identified a "faint possible blob" in a survey as part of our ongoing search for new celestial objects.

Initial imaging revealed unexpected SNR-like filamentary structures, prompting further investigation.


The 'faint blob' and ensuing identification of SNR filamentary like structure in our first survey image

Follow-up sessions using different filters confirmed we had stumbled upon something intriguing and incredibly large. The structure practically filled a 5-degree frame, ruling out molecular clouds and suggesting we were looking at an enormous, largely unexplored supernova remnant.

Our excitement grew as we realised this wasn't a new discovery, but rather the recently identified 'Hoinga – Supernova Remnant G249.5+24.5,' first detected in late 2020. With no published colour images of Hoinga in visible light, we embarked on an ambitious project to create one, unaware that it would evolve into a six-month endeavour inspired by the original discoverers.

A Hoinga: A Vast Stellar Echo Concealed in the Cosmos

Hoinga (SNR G249.5+24.5) is one of the largest supernova remnants ever discovered in X-rays, with a diameter of about 4.4 degrees - nearly 50% larger than the well-known Veil Nebula and covering an area 76 times that of the full moon in our night sky.

In late 2020, Prof. Dr. Werner Becker of the Max Planck Institute for Extraterrestrial Physics discovered Hoinga using the eROSITA X-ray telescope aboard the Spektrum-Roentgen-Gamma (SRG) space observatory during its first all-sky survey. The name "Hoinga" comes from the medieval name of Prof. Becker's hometown.

What makes Hoinga unique is its location far from the galactic plane, where most supernova remnants are typically found. Its discovery was confirmed by Associate Professor Natasha Hurley-Walker using archival radio survey data at the International Centre for Radio Astronomy Research in Australia.

Capturing the Elusive: A Six-Month Journey to Image Hoinga

Hoinga's unique characteristics - its large size, diffuse emission, soft X-rays, high latitude, age, and distance all combine to make it an exceptionally challenging and faint target for astrophotography.

As a southern hemisphere object with optimal viewing conditions from January to May, Ken dedicated his remote observatory's two telescopes to the task of capturing sufficient data for a high-quality image. By March 2024, we had amassed and pre-processed nearly 300 hours of usable data using both the Redcat51/2600MM and RASA11/QHY600 telescope combinations.

Steeve then undertook the complex task of creating the final image, combining OIII, Ha, and RGB datasets along with a 4-panel Ha mosaic from the RASA. The initial result was exciting, and we were eager to share it with the astrophotography community and the original discoverers.


Our initial colour image shared which we shared with the original discoverer's

While preparing the image description, we realised that Hoinga's visible extent might be even greater than anticipated. Overlaying our image on the original discoverers' composite X-Ray and Radio image revealed potential visible light structures extending beyond our captured field of view.


Animation of our initial colour image superimposed on the Composite X-ray and radio image from the Hoinga MPE press release (with permission) [3]

This realisation led to another three months of data collection and processing to complete our expanded image. The process was challenging but fuelled by encouraging feedback from the original discoverers...

Bridging Amateur and Professional Astronomy: Validation from Hoinga's Discoverers

Sharing our initial colour image of Hoinga with its original discoverers led to unexpected and exciting collaborations. Prof. Dr. Werner Becker of the Max Planck Institute for Extraterrestrial Physics responded enthusiastically:

"Sometimes, some emails differ from the daily emails in the inbox folder. Yours is undoubtedly one of those. Many, many thanks for your message and fantastic image, which comes as a surprise! Fantastic work, and it's great that you made these observations."

Prof. Becker suggested combining our data with eROSITA observations could potentially identify new insights into Hoinga's emissions. He also shared information on our previous Huntsman Nebula discovery and proposed a collaborative short report paper.

Prof. Natasha Hurley-Walker from the International Center for Radio Astronomy Research also praised our image and shared an import paper titled "Far-UV and Optical Emissions from Three Very Large Supernova Remnants Located at Unusually High Galactic Latitudes."[2] 

This paper, which notably includes contributions from renowned amateur astrophotography discoverers Marcel Drechsler and Xavier Strottner confirms Hoinga's status as a bona fide supernova remnant and contains the first optical images of Hoinga. It reveals numerous sharp, filamentary structures in both far-UV and Hα emissions, characteristic of shock fronts in supernova remnants.

Revealing Hoinga: Navigating the Complexities of Astrophotography Processing

By June 2024, we had accumulated a wealth of data with about 80% of our carefully filtered observations deemed suitable for the final image. Steeve then set out to bring these diverse, high-quality data sets together into a coherent image, describing the process:

“Processing the image posed significant challenges, primarily due to the need to combine datasets from two very different telescopes while maintaining the highest resolution and detail. The pre-processing stage was the most time-consuming aspect, especially considering the original combined resolution was about 16,000 pixels in length, which considerably slowed down the process.

Another major challenge was managing the vast dynamic range present in this image. The field contains both very bright small galaxies and ultra-faint filament details that even 80 hours of exposure on a RASA telescope struggled to capture. To address this, I developed an HDR processing workflow using multiple stretch factors for different parts of the field, preventing the cores of galaxies from being clipped and revealing their intricate inner details, as well as the faintest parts of the supernova remnant.

Noise reduction also required a meticulous approach. For the extremely faint regions of the SNR, even after using DeepSNR, I had to remove various small-scale layers with MMT as the signal became noisier, while applying minimal noise reduction to the brighter parts of the image. This process was particularly extreme for this image, necessitating the combination of many different layers to achieve a result that was as seamless as possible while staying true to the raw data and maintaining an even noise profile across the image.

The H-alpha and OIII emissions were, of course, continuum subtracted. For the colour data of the galaxies, I relied solely on the RGB data, supplemented with non-continuum subtracted H-alpha as galaxy luminance layer. The extensive integration time for these filters allowed for a good amount of small scale detail, each of which had to be handled individually, and there were hundreds of them."


Inverse starless image clearly identifying the scale of Hoinga's complete shell 

Additionally, the southern part of the SNR is much brighter than the northern part. The northern region is so faint that it required a completely different processing and stretching approach compared to the south. Careful blending was necessary, only incorporating components that were clearly SNR signals. It was easy to introduce non-existent details, and it took considerable effort to faithfully showcase the entire structure of the object, allowing for a true understanding of its morphology and appreciation of the dynamic range while ensuring every detail in the data was visible.

Illuminating Hoinga: Reflections and Inspirations

Team New Horizons' exploration of Hoinga has taken us on an unexpected journey. What began as curiosity about a 'faint possible blob' evolved into a six-month project, producing the first detailed colour visible light image of this spectacular supernova remnant while fostering meaningful collaborations with the scientific community.

This project exemplifies the power of curiosity-driven exploration and persistence in astrophotography. As we unveil Hoinga's intricate structure, we're reminded that the cosmos still holds many secrets waiting to be uncovered. We hope our work inspires fellow amateur astronomers to approach familiar skies with renewed wonder.

References:
[1] Becker, W., Hurley-Walker, N., Weinberger, C., et al. (2021). Hoinga: a supernova remnant discovered in the SRG/eROSITA All-Sky Survey eRASS1. Astronomy & Astrophysics, 648, A30. https://www.aanda.org/articles/aa/full_html/2021/04/aa40156-20/aa40156-20.html
[2] Fesen, R. A., Drechsler, M., Weil, K. E., et al. (2021). Far-UV and Optical Emissions from Three Very Large Supernova Remnants Located at Unusually High Galactic Latitudes. The Astrophysical Journal, 920(2), 90.  https://ui.adsabs.harvard.edu/abs/2021ApJ...920...90F/abstract
[3] Max Planck Institute for Extraterrestrial Physics. (2021, March 3). Hoinga – the largest supernova remnant ever discovered with X-rays. Retrieved from https://www.mpe.mpg.de/7572057/news20210303