GRB 100418a: A long GRB without a bright supernova in a high-metallicity host galaxy

Abstract

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 8.2 m 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; Rc, and Ic 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 Rc-band magnitude shows that the afterglow of GRB 100418A has faded to Rc ≥ 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 σ noise spectrum of our observation, we estimated the upper limit on the SN absolute magnitude to be MIc,obs > -17.2 in the observer frame Ic-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 GRB 100418A is relatively massive (log M*=Mocir = 9.54) compared to typical long GRB host galaxies, and has 12 + log (O=H) = 8.75. © 2012. Astronomical Society of Japan.