SN 2025rbs is a Type 1A supernova in NGC 7331 (distance 44m LY) that was reported on 14^th July 2025. The plot shows two spectra I obtained in subsequent weeks (4, 10 August) when it had faded a little from its brightest about Vmag 12.1, around 29^th July. The spectra are both normalised at the same point around 6600 Angstroms but separated vertically to allow ease of comparison.

The spectra show characteristic features of type Ia [1]. There is a deep absorption trough around 6150A associated with an Si II feature having rest wavelength 6355A. It is substantially blue shifted due to its association with the explosion wavefront moving at around 10,000k/s. This feature is persistent in time over the two snapshots here.

In contrast, there are two absorption features of S II, either side of 5400A which are already reduced in the earlier of the two spectra, and which have completely disappeared in the later one. They are replaced here by a strong emission feature associated with Fe II. Finally, strong emission lines at the blue end of the spectrum, present earlier on, have disappeared in the later spectrum.

[1] Filippenko, Alexei V. 1997. ‘Optical spectra of supernovae’. Annual Review of Astronomy and Astrophysics 35 (Volume 35, 1997): 309–55.

Available at https://ned.ipac.caltech.edu/level5/March03/Filippenko/Filippenko_contents.html