APOD: 2026 June 11 - The Mermaid Nebula Supernova Remnant

This image is a collaboration between my friend Sy Ming and myself. Data was captured entirely by Sy Ming across 4 nights under very dark Bortle-1 skies from Kukerin, Western Australia during an astronomy trip, and processed by myself. The choice of target was picked by him too. The goal for this project was to image the elusive G296.5+10.0 SNR in its entirety, revealing the full extent of its structure. The main image displayed shows a crop of the 'Betta Fish / Mermaid' portion, together with the bright Rho Centauri blue-white binary star dominating the field (see revision B for the widefield).

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Introduction

G296.5+10.0 (ESO 217-25) is a large, ancient supernova remnant in the constellation Centaurus. It was first detected as a soft X-ray source during the HEAO A-1 sky survey (Tuohy et al., 1979) and subsequently identified at radio wavelengths. A landmark radio continuum study by Roger et al. (1988) revealed its remarkable bilateral shell structure, later confirmed through detailed quantitative analysis by Storey et al. (1992) as one of the archetypal 'barrel-shaped' supernova remnants with two oppositely-facing emission crescents (see SN 1006 as another prominent example). The Northern crescent is evocatively referred to as the Mermaid or Betta Fish Nebula.

2026-04-18-19-20-21-Mermaid-FSQ106-P2-AS-WSM-16-10-web-sm.jpg The above wider FOV shows the full extent of G296.5+10.0 SNR

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Data Capture

In order to image this target, a fast, widefield instrument was required, but it also needed to be airline portable. The natural choice was the Takahashi FSQ106EDX4, which when paired with a Sony IMX571 sensor provides a FOV of about 2.5°. Since an OSC camera was used, an Antlia ALP-T dual band filter was chosen; however, an incorrect filter was used by mistake (the high-speed version), leading to a ~40% decrease in signal.. an unfortunate lesson!

Astronomy ‘camp’ - being based in Singapore means flying out of the country for deep-sky imaging! The imaging rig used is the right-most set up. Photograph by L. Yeo.

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Three nights were spent imaging in dual narrowband, with the final night reserved for OSC to capture star colours and dark dust detail. Unfortunately, clouds moved in and limited OSC capture to less than an hour. A mono setup would likely have yielded better signal and less signal 'contamination' associated with OSC dual-narrowband.

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Physical Properties

Lying well above the Galactic plane at a Galactic latitude of +10°, this SNR benefits from relatively low interstellar extinction, yet remains very faint in optical wavelengths. Recent multi-wavelength analysis (Eppens, Combi et al., 2024) employing XMM–Newton, Fermi-LAT, and ATCA data places the distance at approximately ~4,600 light-years and its age at roughly 14,000 years.

The bilateral morphology (the source of the Betta Fish / Mermaid nicknames) is thought to arise from the interaction of the supernova blast wave with either a non-uniform surrounding interstellar medium or, more intriguingly, the magnetized wind of the massive progenitor star. Spectropolarimetric observations by Harvey-Smith et al. (2010) revealed a highly ordered Faraday rotation structure across the remnant, providing compelling evidence that the progenitor was likely a red supergiant whose magnetized stellar wind sculpted the circumstellar environment prior to the supernova.

The SNR spans approximately 90 arcminutes across its long axis, larger than three full Moons, making it one of the largest apparent-size SNRs on the sky. Yet its extremely low surface brightness makes it a formidable imaging challenge. Given the excellent conditions in Western Australia, it felt appropriate to make use of this opportunity to image this faint object.

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The Neutron Star: 1E 1207.4-5209

At the geometric centre of G296.5+10.0 lies one of the most studied neutron stars in X-ray astronomy: 1E 1207.4-5209. Classified as a Central Compact Object (CCO), this radio-quiet neutron star was first identified as an Einstein Observatory X-ray source and later extensively studied with ASCA, ROSAT, Chandra, and XMM-Newton. It exhibits 424ms X-ray pulsations and uniquely, its spectrum displays absorption features at ~0.7 and ~1.4 keV, interpreted as either cyclotron resonances or atomic transitions in an ultra-strong magnetic field.

Perhaps most remarkably, 1E 1207.4-5209 has been proposed as an 'anti-magnetar', a neutron star born with an anomalously weak dipolar magnetic field (~10¹⁰ G vs 10¹²–10¹³ G for typical young neutron stars). This makes the G296.5+10.0 system a unique astrophysical playground for studying both SNR evolution and exotic neutron star physics.

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A Challenging Target

G296.5+10.0 remains one of the most rarely imaged supernova remnants among amateur astrophotographers. Its optical faintness demands long narrowband integration times, dark southern skies, and careful processing. The brightest portions of the shell represent only one crescent of the full bilateral structure, and there is a great deal of additional extremely faint emission that is only just visible in this image that an even longer integration could certainly help reveal.

We hope you enjoy this glimpse of one of the sky's most elusive supernova remnants.

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Here were some images I referenced when processing this image:

This IOTD widefield by Claudio showcased impressive Ha signal but the Oiii detail showed signs of selective masking/painting which I wanted to avoid https://www.astrobin.com/7m33v4/0/
This APOD by Neil showcased the Ha edges particularly well and I loved how hauntingly the nebula was displayed https://apod.nasa.gov/apod/ap240918.html
This IOTD by Marco was likely one of the first few deep images of this target https://www.astrobin.com/kpl1kc/
This TP by DSRO was coloured naturally with a realistic depiction of how Oiii would look like (instead of the typical blues favoured by modern processors) https://www.astrobin.com/ehthve/

G296.5+10.0 Mermaid (Betta Fish) SNR