Program: GN-2021A-Q-108

Despite their importance as the cosmic distance indicator, the origin of type Ia supernovae (SNe Ia) is still obscured. Recent studies indicated that SNe Ia discovered within a few days of their explosions, especially for those with brightness excess at such early time play an irreplaceable role in addressing the long-standing progenitor and explosion mechanism issues. The large aperture of the Subaru telescope and the wide field of the Hyper Suprime-Cam (HSC) make Subaru/HSC the most powerful survey facility of studying SNe Ia soon after their explosions. In semester 2021A, we plan to carry out the third observing run of the MUlti-band Subaru Survey for Early-phase SNe Ia (MUSSES) that is optimized to catch the first light of SNe Ia at redshift below 0.15. At least three ultra-early SNe Ia with luminosities over ~500 times fainter than their peaks will be discovered and particularly, the deepest photometry of early-phase SNe Ia by far can be promisingly achieved during the observation. The ultra-deep photometry and early-time spectroscopy with Subaru/HSC and simultaneous Gemini/GMOS observations not only can rule out or directly identify a small non-degenerate companion of the SN Ia progenitor system but also can test the double white-dwarf merger scenario by constraining the nearby CSM distribution for the first time.