SN Ia 2025bvm in the Seyfert galaxy NGC 4156: Early detection and follow-up observations

Aims. We investigated the photometric and spectroscopic evolution of SN Ia 2025bvm to perform a detailed classification and provide an independent distance estimate for the host galaxy NGC 4156. Methods. We present UBVRI photometry for a period between days −16 and 80 relative to the B-band maximum. Six optical spectra were taken between days −3 and 28. We used different fitting methods to derive the basic photometric parameters. We determined the expansion velocities from the blueshift of the Si II absorption line and the equivalent widths of the interstellar Na I lines. Results. SN 2015bvm exhibits a photometric evolution typical for Type Ia supernovae; the Δm ₁₅ parameter and the rise time are both close to their mean values. The rising part of the light curves shows no signs of excess flux. The color curves indicate a significant color excess of E(B − V)_tot = 0.22 ± 0.04 mag. The interstellar Na I lines in the host galaxy are stronger than expected for this value of dust extinction. SN 2025bvm is notable for its high expansion velocity at maximum light and the presence of a plateau in the velocity temporal evolution. We estimate a distance modulus of μ = 34.84 ± 0.10 mag, consistent with the value derived from the host galaxy redshift.