Teleios (G305.4-2.2) -- the mystery of a perfectly shaped new Galactic supernova remnant (2505.04041v1)

Abstract: We present the serendipitous radio-continuum discovery of a likely Galactic supernova remnant (SNR) G305.4-2.2. This object displays a remarkable circular symmetry in shape, making it one of the most circular Galactic SNRs known. Nicknamed Teleios due to its symmetry, it was detected in the new Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) radio-continuum images with an angular size of 1320"x1260" and PA = 0 deg. While there is a hint of possible H$\alpha$ and gamma-ray emission, Teleios is exclusively seen at radio-continuum frequencies. Interestingly, Teleios is not only almost perfectly symmetric, but it also has one of the lowest surface brightnesses discovered among Galactic SNRs and a steep spectral index of $\alpha=-0.6\pm 0.3$. Our estimates from HI studies and the Sigma-D relation place Teleios as a type Ia SNR at a distance of either ~2.2 kpc of ~7.7 kpc. This indicates two possible scenarios, either a young (under 1000 yr) or an older SNR (over 10000 yr). With a corresponding diameter of 14/48 pc, our evolutionary studies place Teleios at the either early or late Sedov phase, depending on the distance estimate. However, our modelling also predicts X-ray emission, which we do not see in the present generation of eROSITA images. We also explored a type Iax explosion scenario that points to a much closer distance of <1 kpc and Teleios size of only ~3.3 pc, which would be similar to the only known type Iax remnant SN1181. Unfortunately, all examined scenarios have their challenges, and no definitive supernova (SN) origin type can be established at this stage. Teleios's symmetrical shape suggests expansion into a rarefied and isotropic ambient medium. The low radio surface brightness and the lack of pronounced polarisation can be explained by a high level of ambient rotation measure (RM), with the largest RM being observed at centre.

Overview of the Discovery of Teleios, a New Galactic Supernova Remnant

The paper presents the serendipitous discovery of a new Galactic supernova remnant (SNR), referred to as G305.4--2.2 or Teleios, characterized by its outstanding circular symmetry. The exploration of Teleios, identified in the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) radio--continuum images, enriches the astronomical understanding of SNRs. The object's angular size is reported as 1320$^{\prime\prime}$ by 1260$^{\prime\prime}$ with a position angle of 0°, revealing a near-perfect symmetry rarely observed in similar celestial bodies.

Key Characteristics

• Circular Symmetry: Teleios is among the most circular SNRs known.
• Low Surface Brightness: It possesses one of the lowest surface brightnesses of any detected Galactic SNR, calculated at $\sim$5.1$\times$10$^{-23}$~W~m$^{-2}$~Hz$^{-1}$~sr$^{-1}$.
• Spectral Index: The steep spectral index of $\alpha\,=\,-0.6\pm0.3$ indicates either a young age (under 1000 years) or a somewhat older state (over 10000 years).
• Possible Distances: The investigation suggests a distance of either $\sim$2.2 kpc (near-side) or $\sim$7.7 kpc (far-side), leading to corresponding physical diameters of 14 or 48 pc.

Implications and Theoretical Considerations

Teleios provides a challenging subject for classifying SNRs due to its symmetry and low brightness. A significant portion of the paper is dedicated to exploring various scenarios regarding the potential origins of Teleios, evaluating whether it could stem from a core-collapse supernova (CC SN), a type Ia, or a less conventional type Iax SN. Each scenario presents unique aspects and limitations, and current observations have not definitively identified Teleios's SN origin type.

Core-Collapse SN Scenario

• Proximity to Star Forming Regions: Although Teleios is below the Galactic Plane, massive stars capable of CC SN explosions may exist at these locations.

Type Ia SN Scenario

• Symmetry and Location: If Teleios originated from a type Ia SN, its symmetric shape aligns with expectations of such explosions.
• Absence of X-rays: Unexpectedly, there is no detectable X-ray emission, challenging assumptions about its evolution as a typical type Ia SNR.

Type Iax SN Scenario

• Comparison to Known Instances: Teleios is compared to SN1181, the only known type Iax SNR, but discrepancies in size affect this interpretation.

Future Steps

Immediate next steps involve acquiring more high-resolution multi-frequency data, which could elucidate the object's expansion velocity and other critical parameters. There is room for additional research to improve understanding, as Teleios's orientation may significantly impact observed symmetry and brightness through alignment with magnetic field lines. The nature of Teleios remains intriguing, highlighting the importance of continued observation and modelling in understanding its true composition and origin.