GRB 100418A: a Long GRB without a Bright Supernova in a High-Metallicity Host Galaxy

We present results of a search for a supernova (SN) component associated with GRB 100418A at a redshift of 0.624. The field of GRB 100418A was observed with FOCAS on the Subaru 8.2m telescope under a photometric condition (seeing 0.'' 3-0.'' 4) on 2010 May 14 (UT). The date corresponds to 25.6 days after the burst trigger (15.8 days in the restframe). We made imaging observations in the V, R-c, and I-c bands, and two hours of spectrophotometric observations. We obtained a resolved host galaxy image that is elongated 1.'' 6 (= 11 kpc) from north to south. No point source was detected on the host galaxy. The time variation of the Re-band magnitude shows that the afterglow of GRB 100418A has faded to R-c greater than or similar to 24 without any SN-like rebrightening, when we compare our measurement to reports in GCN circulars. We could not identify any SN feature, such as broad emission-lines or bumps in our spectrum. Assuming the SN is fainter than the 3 a noise spectrum of our observation, we estimated the upper limit on the SN absolute magnitude to be M-Ic,M-obs > -17.2 in the observer frame I-c-band. This magnitude is comparable to the faintest type Ic SNe. We also estimated the host galaxy properties from the spectrum. The host galaxy of ORB 100418A is relatively massive (log M-star/M-circle dot = 9.54) compared to typical long ORB host galaxies, and has 12 + log (O/H) = 8.75.