Astronomers from the Yunnan University in China and other institutions have carried out photometric and spectroscopic observations of a recently detected supernova designated SN 2024aecx. Results of the observational campaign, published May 26 on the arXiv preprint server, shed more light on the properties of SN 2024aecx, suggesting that it is a Type IIb supernova.

Supernovae (SNe) are powerful and luminous stellar explosions occurring when a star is transformed by nuclear fusion or gravitational collapse. In general, they are divided into two main groups based on their atomic spectra: Type I and Type II. Type I SNe lack hydrogen in their spectra, while those of Type II showcase spectral lines of hydrogen.

SN 2024aecx was identified on December 16, 2024 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in NGC 3521—a late-type spiral galaxy at a distance of some 37 million light years. It is one of the earliest discovered (within a day) supernovae after the explosion. It had a discovery magnitude of approximately 14.68.

Follow-up observations of SN 2024aecx conducted one day after its detection have classified it as a Type IIb supernova. However, further investigation, performed on December 19, 2024, indicated that it was a Type Ic SN.

In order to resolve this discrepancy, a team of astronomers led by Yunnan University’s Xingzhu Zou has conducted low-resolution spectroscopic and optical imaging of SN 2024aecx.

The observations found that early spectra of SN 2024aecx show a weak signature of hydrogen lines that could only be seen up to about one month after the explosion. The light curves of this supernova in all bands show a distinct feature of two peaks.

According to the paper, the first peak is likely due to shock cooling emission. It turned out that the duration of the shock cooling phase is longer in bluer bands, but the rise time to the secondary peak (after the explosion) is shorter in those bands.

The researchers underlined that the early phase light curve evolution of SN 2024aecx showcases similarity with the typical Type IIb SNe, but its decay rate in different bands is significantly faster in the post-peak phase.

The observations found that SN 2024aecx has a peak absolute magnitude (second peak) of approximately -17.94 mag. This makes SN 2024aecx one of the brightest events in the SNe IIb sample. However, it does not follow the general trend that brighter SNEs exhibit longer rise times.

Results of the study suggest that the explosion of SN 2024aecx had an energy at a level of 160 quindecillion erg and the ejecta mass was about 0.7 solar masses.

When it comes to the progenitor of SN 2024aecx, the authors of the paper assume that it had a radius within a range of 169–200 solar radii and its envelope mass was between 0.03 and 0.24 solar masses.

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