The Ultraviolet Spectra of 2003fg-like Type Ia Supernovae

Snehasish Bhattacharjee^1*, Yen Chen Pan¹

¹Astronomy, National Central University, Zhongli, Taiwan

* Presenter:Snehasish Bhattacharjee, email:snehasish@astro.ncu.edu.tw

2003fg-like Type Ia supernovae (03fg-like SNe~Ia) are rare subtype of SNe~Ia, photometrically characterized by broader optical light curves and bluer ultraviolet (UV) colors compared to normal SNe~Ia. In this work, we study four 03fg-like SNe~Ia using \textit{Swift} UltraViolet and Optical Telescope (UVOT) grism observations to understand their unique UV properties and progenitor scenario(s). We report 03fg-like SNe~Ia to have similar UV features and elemental compositions as normal SNe~Ia, but with higher UV flux relative to optical. Previous studies have suggested that the UV flux levels of normal SNe~Ia could be influenced by their progenitor properties, such as metallicity, with metal-poor progenitors producing higher UV flux levels. While 03fg-like SNe were previously reported to occur in low-mass and metal-poor host environments, our analysis indicates that their UV excess cannot be explained by their host-galaxy parameters. Instead, we demonstrate that the addition of a hot blackbody component, likely arising from the interaction with the circumstellar material (CSM), to the normal SN~Ia spectrum, can reproduce their distinctive UV excess. This supports the hypothesis that 03fg-like SNe~Ia could explode in a CSM-rich environment.

Reference: S. Bhattacharjee et al., The ultraviolet spectra of 2003fg-like Type Ia supernovae, Monthly Notices of the Royal Astronomical Society, Volume 542, Issue 4, October 2025, Pages 2752–2767

Keywords: supernova, UV spectroscopy, circumstellar medium (CSM)