ASKAP$-$EMU Discovery of"Raspberry": a new Galactic SNR Candidate G308.73+1.38

We report the ASKAP discovery of a new Galactic supernova remnant (SNR) candidate G308.73+1.38, which we name Raspberry. This new SNR candidate has an angular size of 20.7 arcmin $\times$ 16.7 arcmin, and we measure a total integrated flux of 407$\pm$50 mJy. We estimate Raspberry's most likely diameter of 10$-$30 pc which would place it at a distance of 3$-$5 kpc, on the near side of the Milky Way's Scutum$-$Centaurus Arm. We also find a Stokes$-$V point source close to the centre of Raspberry with a $\sim$5$\sigma$ significance. This point source may be the remaining compact source, a neutron star, or possibly a pulsar, formed during the initial supernova event.

1. INTRODUCTION Supernova remnants (SNRs) are the resultant expanding structures that remain after the death of massive stars in a supernova explosion.It is generally accepted that the population of known Galactic SNRs (≈300, Green 2022) vastly underrepresents the expected population (Ball et al. 2023) and the discovery of new Galactic SNRs is vital in filling this knowledge gap.

DATA
As a part of the large-scale ASKAP−EMU project, the SNR area of the sky was observed in December 2023 with a complete set of 36 ASKAP antennas at the central frequency of 943.4 MHz and bandwidth of 288 MHz.All data are available through the CSIRO ASKAP Science Data Archive (CASDA 1 ).The observation containing this object is the tile EMU 1342−60 corresponding to ASKAP scheduling block SB54095.The data was processed using the ASKAPsoft pipelines, including multi-frequency synthesis imaging, multi-scale clean, self-calibration and convolution to a common beam size (Guzman et al. 2019).The resulting 943 MHz EMU Stokes−I image has a root mean squared (rms) sensitivity of σ = 33 µJy beam −1 , while Stokes−V image rms is σ = 20 µJy beam −1 .The synthesised beam for both images is 15 ′′ ×15 ′′ .

RESULTS AND DISCUSSION
We propose that Raspberry is a candidate for a new Galactic shell-type SNR based primarily on its radio morphology (Figure 1).The extended shell at radio frequencies seen in Raspberry resembles typical shell-type SNRs.Additionally, we searched the Wide-Field Infrared Survey Explorer (WISE, Wright et al. 2010) all-sky maps and found no infrared emission that spatially corresponds with the observed radio shell.This lack of infrared emission indicates that the radio shell is purely non-thermal, formed by synchrotron emission from ultra-relativistic particles energised by the expanding shock front, typical of a shell-type SNR (Filipović & Tothill 2021).
The typical average value for an SNR spectral index is α = −0.5, obtained from theoretical models (Bell 1978) and observations (Bozzetto et al. 2017).Assuming Raspberry's spectral index of α = −0.5 and integrated flux density of S I = 407±50 mJy, we calculate a scaled flux of S 1GHz = 389 mJy giving surface brightness of Σ 1GHz = 1.4×10 −20 W m −2 Hz −1 .The Σ−D calibration model from Pavlović et al. (2018) works well for Galactic shell-type SNRs similar to Raspberry, and we compare our measured values to estimate the diameter.Comparing with Pavlović et al. (2018, their Fig. 3), we estimate a diameter of D = 10−30 pc.This diameter would place Raspberry at a distance of 3−5 kpc, which is in the Milky Way's near side of the Scutum-Centaurus Arm, based on its Galactic longitude.
We detect a possible circularly polarised point source near the centre of Raspberry, observable only in the Stokes−V image (Figure 1, inset).The coordinates of this point source are RA(J2000) = 13 h 39 m 26.s 8 and Dec(J2000) = −60 • 54 ′ 38.′′ 5, giving the source an offset of 111 ′′ from Raspberry's geometric centre.The point source has a Stokes−V flux density of 100 µJy.Given that the local rms in the Stokes−V image is 20 µJy beam −1 , we estimate that the point source has a significance of ∼5σ.However, the source is not detected in the Stokes−I image.This point source could potentially be Raspberry's compact source, a neutron star, or possibly a pulsar, produced in the initial supernova (SN) event.No optical or infrared counterpart can be identified in existing surveys.

CONCLUSION
We present the serendipitous discovery of a new Galactic SNR candidate Raspberry (G308.73+1.38).The SNR identification is primarily based on the object's radio-continuum morphology and the absence of an infrared counterpart.We observe a Stokes−V point source close to Raspberry's geometrical centre that has the potential to be a central compact source of the SNR.
Further multi-frequency observations are required to confirm Raspberry's identity as an SNR.Particularly, additional radio-continuum bands would allow us to confirm the non-thermal origin of the radio emission (via spectral index); full Stokes parameters would allow us to study polarisation which is a strong characteristic of the shell SNRs; X-ray and high-energy observations would allow us to further determine the multi-frequency spectrum of this object; and a dedicated pulsar search would allow us to determine the true nature of the potential point source at the heart of Raspberry.

Figure 1 .
Figure 1.RGB composite image where the total intensity map of Raspberry, observed by ASKAP at ν = 944 MHz, is in red and blue while WISE 12 µm infrared image is in green.To present the structure of Raspberry, we used different colourmaps and adjusted contrast levels.A linear scale is applied to all images.The inset is the ASKAP Stokes−V zoomed-in image showing the possible progenitor source.