AstroNote 2024-280

We observed the location of the fast transient and LFBOT candidate ZTF24abjjpbo / AT 2024wpp (Ho et al. 2024, AstroNote 2024-272; see also Srinivasaragavan et al., AstroNote 2024-276 and Margutti et al., AstroNote 2024-278) with the Keck Cosmic Web Imager (KCWI) integral field unit spectrograph on the Keck II 10m telescope.  Observations were carried out beginning at 1:30 UT on 2024 Oct 02 in photometric conditions with ~1" seeing, using the medium slicer (16"x20" FOV) and low resolution gratings (0.53 and 0.92 "/pixel) with the blue and red sides spanning 350-1000 nm.

Spectra of the transient itself, and of the diffuse, extended galaxy centered approximately 3.1" southeast of the transient (seen in Pan-STARRS and Legacy Survey imaging of the field) were extracted and reduced using the KCWI data reduction pipeline.

The spectrum of the transient is blue and essentially featureless.  In particular, we do not identify any emission or absorption lines of H, He I, or He II at z=0 that would indicate a origin associated with the outburst of a Galactic foreground variable.

The spectrum of the host galaxy shows numerous features in both emission and absorption at a common redshift of z=0.0868.  In particular, we identify H-alpha, H-beta, H-gamma, NII, OIII, and OII narrow-line nebular emission, and broad stellar absorption features of various Balmer lines and Ca H+K.  The line ratios are characteristic of star-forming galaxies.

Associating this with the redshift of the transient itself and based on recent photometric follow-up with the Liverpool Telescope, we estimate a peak optical absolute magnitude (rest-frame g-band) of approximately M_g = -21.5 mag, similar to (but slightly exceeding) that of AT2018cow.  The projected offset between the transient and the galaxy center is approximately 5 kpc.