TITLE: GCN CIRCULAR NUMBER: 6444 SUBJECT: GRB 070521: Subaru observations and possible host detection DATE: 07/05/21 23:46:59 GMT FROM: Nobuyuki Kawai at Tokyo Tech T. Hattori, K. Aoki (Subaru Telescope, NAOJ), and N. Kawai (Tokyo Tech) report on behalf of the Subaru GRB team: "We observed the field of GRB 070521 (Guidorzi et al., GCN 6431) with FOCAS on the Subaru Telescope starting at 07:31(UT) (40 min after the trigger) with total exposures of 1920 sec (120s x 16) in z'-band and 360 sec (120s x 3) in i'-band. We detected a faint source inside the XRT error circle at RA=16:10:38.55, DEC=30:15:27.7 with a positional uncertainty of 0.5" and an approximate magnitude of z' ~ 23.5. We obtained 3600 sec long-slit spectra of this faint source covering a wavelength range from 6000 to 10000 Ang. We detected [O III] 5007, 4959 emission lines at z=0.553. If this object is the host galaxy of GRB 070521, its location at a relatively low redshift makes it a good candidate for searching the supernova component. Note, however, that the absence of a bright afterglow may indicate that the GRB source is embedded in a region with high extinction, or the host identification is spurious. The image may be viewed at http://www.hp.phys.titech.ac.jp/nkawai/grb/070521/. We thank the Subaru Observatory for the support of this observation."