NICER observation list: Target ID 251801

AGN Reverberation mapping in is a well established technique in the optical/UV, where delays between the H-beta line and the continuum are used to probe the optical broad line region (BLR) and estimate the black hole mass. The techniques was applied recently in X-rays, obtaining an upper limit to the delay of the narrow Fe-K line in NGC 4151 relative to the continuum of ~6 days. This is a new powerful tool to probe the circumnuclear environment. We propose monitoring this source to constrain the delay instead of the upper limit. The unique flexibility and sensitivity of NICER will allow unprecedented view of the line emitting region, and of the location of the cold gas and its relation to the ionized BLR, guiding development of accretion and BLR formation models.