S20A0042abst
S20A0042
We propose to observe the COSMOS UltraDeep (UD) field with HSC in order to investigate the unexplored time domain of the variable Universe - the long and faint. The COSMOS UD field has been regularly observed in the last three years with the i and z bands with the median limiting magnitudes of 26.0 mag and 25.5 mag, respectively. By adding the 4th year time-domain data as deep as the last three years, we investigate faint transients lasting for more than 1000 days. No transient surveys have monitored the same field as deep, wide, and long as we propose. We expect to discover high-redshift pair-instability and superluminous SNe and intrinsically long-lasting SNe. Long-term AGN activities can also be investigated. Unknown long-timescale transients may be discovered by investigating the unexplored time domain exceeding 1000 days. Accumulating the deep and long time domain data now is very important to prepare for JWST, whose Cycle 1 proposal deadline is on 1 May 2020, and LSST. We also share the acquired data with the community so that the COSMOS UD depth can be made even deeper, which benefits many other science cases.
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