AstroNote 2025-193

Spectroscopy of the fast-rising blue optical transient AT2025ngt (ZTF25aaurzyo, Camilo et al., AstroNote 2025-190) was obtained using the Low Resolution Imaging Spectrometer (LRIS) on the Keck I 10m telescope, located on the summit of Maunakea, Hawaii.  We obtained two exposures of 600s each starting on 2025-06-21 at 10:39 (UT), approximately 9.02 observer-frame days after the first detection.  The observational setup employed the 600/4000 blue grism and 400/8500 red grating, providing continuous coverage from 3140-10250 Angstroms.

The reduced spectrum shows a blue continuum extending aross the entire spectral range.  Several broad, relatively weak absorption features are evident in the blue region, resembling those seen in SN2018gep (e.g., Ho et al. 2019, ApJ 887, 169) and other fast-rising blue SNe Ic-BL (e.g., iPTF16asu, SN 2006aj).  These lines have been previously atrributed to CII, CIII, and OII, although the lines are weaker and velocities somewhat lower in AT2025ngt at a similar phase post-explosion (see attachment for comparison).  On this basis we provisionally classify AT2025ngt as a peculiar Type Ic-BL supernova.

Swift observations have been requested, and further follow-up is ongoing.