GCN Circular 17733

D. A. Perley (Caltech) reports:

I acquired observations of the optical afterglow candidate of GRB 
150423A (Pagani et al., GCN 17726
Not found
 
 
; Varela et al., GCN 17732
Not found
 
 
) 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.