GCN Circular 17733
D. A. Perley (Caltech) reports: I acquired observations of the optical afterglow candidate of GRB 150423A (Pagani et al., GCN 17726; Varela et al., GCN 17732) using the Low Resolution Imaging Spectrometer (LRIS) on the Keck I 10m telescope. Two epochs of V- and i-band imaging were acquired, the first beginning at UT 07:13 (approximately 0.75 hours after the trigger, as soon as the target cleared the telescope Nasmyth limit), the second beginning at UT 13:12 (approximately 6.75 hours after the trigger). The sources mentioned by Varela et al. are well-detected in both epochs. Source 2 remains constant between the frames, while Source 1 declines by approximately 0.9 magnitude in both V- and i-band, confirming that it is the optical afterglow. Shortly after the second imaging epoch, I also acquired 1600 seconds of low-resolution spectroscopy with the slit oriented to cover both S1 and S2. A faint trace is detected for both sources. Source 2 shows a clear emission line in the blue arm and at least one marginal emission line in the red arm, indicating a low to moderate redshift. Source 1 (the afterglow) shows no clear features in emission or absorption, but the signal is very weak. Further analysis will reported at a later time pending wavelength calibration of the spectra.