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arXiv · 2610.27807

What Shapes the Ultraviolet Spectra of Type Ia Supernovae?

Snehasish Bhattacharjee·Yen-Chen Pan·Anirban Dutta·Shubham Gupta·Chia-Yu Cheng·Charles D. Kilpatrick·Matthew R. Siebert
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Abstract

Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) exhibit significant diversity, but their physical origin remains poorly understood. We investigate UV spectra of SNe Ia observed with the Swift UltraViolet and Optical Telescope. Our expanded pre-maximum sample probes dependencies on decline rate, ejecta properties, and host environment. We recover the correlation between UV flux and decline rate near maximum in the near-UV (NUV), and find correlations at pre-maximum phases in both the NUV and mid-UV (MUV). This trend is strongly wavelength dependent, with slow decliners showing higher NUV but lower MUV flux than fast decliners. Synthetic TARDIS spectra varying the 56Ni abundance show that enhanced 56Ni suppresses the MUV while increasing the NUV, broadly reproducing the trend. We find no significant correlation between UV flux and ejecta velocity, while Si II 6355 pEW shows trends similar to those with decline rate. We also find no significant correlations with host-galaxy properties, although host environment may contribute to the remaining UV diversity among SNe with similar decline rates. Early-excess SNe show enhanced flux mainly in the NUV rather than a broad UV excess, suggesting that strong circumstellar interaction is not necessarily required to explain the observed excess emission. Peculiar subclasses show distinct UV properties, with brighter and hotter subclasses showing higher UV flux, although the current sample is small and does not cover all SN Ia subtypes. Our results suggest that the UV diversity of SNe Ia is shaped by multiple mechanisms that contribute differently across the UV wavelength range.

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Snehasish Bhattacharjee, Yen-Chen Pan, Anirban Dutta, Shubham Gupta, Chia-Yu Cheng, Charles D. Kilpatrick, Matthew R. Siebert. 2026-10-20. What Shapes the Ultraviolet Spectra of Type Ia Supernovae?. https://arxiv.org/abs/2610.27807

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