Extended Data Fig. 1: The early-time comparisons of SNe 2018evt, and 1991T.
From: Newly formed dust within the circumstellar environment of SN Ia-CSM 2018evt
The spectrum of SN 2018evt (black curve) closely resembles the spectrum of SN 1991T at day -9 (red curve) in panel (a)95,96, which exhibits the strong Fe III λ 4404, λ 5129 absorptions and Hα emission, visible Si III λ 4564 absorption, Ni III blends around 4750Å, 5350Å, and weak S II W and Si II λ 6355 as marked (for example,98). Panel (b) compares the early-time photometry of SN 2018evt (stars), and SN 1991T (solid circles)97. A power law f ∝ \({\left(\tau +{{{{\rm{t}}}}}_{{{{\rm{r}}}}}\right)}^{{{{\rm{n}}}}}\) is applied to fit the early V - and c − b and photometry, where τ = (t-\({{{{{\rm{t}}}}}_{B}}^{\max }\))/(s(1+z))23,\({{{{{\rm{t}}}}}_{B}}^{\max }\)= 58352, s = 1.0 for stretch value and z = 0.02523 for the redshift of SN 2018evt. The fitting yields a rise time tr = 18.76 ± 0.24 days, and a power-law index n = 1.57 ± 0.07 (grey curve). The estimated tr is consistent with the V -b and rise time tr(V) = 20.00 ± 0.68 days of SN 1991T/1999aa-like events23. The interpolations of BVRI light curves of SN 1991T are shown in dashed curves. The inset in panel (b) compares the B -V color curves between SNe 2018evt and 1991T, indicating a color difference < ~ 0.1 mag at similar phases. The corresponding Milkyway extinction is 0.05 mag88 for SN 2018evt. The error bars shown represent 1 − σ uncertainties of magnitudes, and colors.
