Optical follow-up observation of Fast Radio Burst 151230

The origin of fast radio bursts (FRBs), bright millisecond radio transients, is still somewhat of a mystery. Several theoretical models expect that the FRB accompanies an optical afterglow (e.g., Totani et al., 2013, PASJ, 65, L12; Kashiyama 2013, ApJ, 776, L39). In order to investigate the origin of FRBs, we perform gri-band follow-up observations FRB 151230 (estimated z less than or similar to 0.8) with Subaru/Hyper Suprime-Cam at 8, 11, and 14 days after discovery. The follow-up observation reaches a 50% completeness magnitude 26.5 mag for point sources, which is the deepest optical follow-up of FRBs to-date. We find 13 counterpart candidates with variabilities during the observation. We investigate their properties with multi-color and multi-wavelength observations and archival catalogs. Two candidates are excluded by the non-detection of FRB 151230 in the other radio feed horns that operated simultaneously to the detection, as well as the inconsistency between the photometric redshift and that derived from the dispersion measure of FRB 151230. Eight further candidates are consistent with optical variability seen in active galactic nuclei (AGNs). Two more candidates are well fitted with transient templates (Type IIn supernovae), and the final candidate is poorly fitted with all of our transient templates and is located off-center of an extended source. It can only be reproduced with rapid transients with a faint peak and rapid decline, and the probability of chance coincidence is similar to 3.6%. We also find that none of our candidates are consistent with Type la supernovae, which rules out the association of Type la supernovae to FRB 151230 at z