Early NIR echo and the time variable mass loss history of Type IIn SN 2024kgi

We present a comprehensive analysis of the long-term photometric and spectroscopic monitoring campaign of the Type IIn Supernova (SN) 2024kgi. The SN reaches at peak an r-band absolute magnitude of-19.81 plus or minus 0.06 mag and a bolometric luminosity of about 2 times 10 43 erg s-1. We observe a break in the lightcurve at day about 312, after which the luminosity decline changes from t-1.0 to t-4.5, likely due to the shock sweeping the dense CSM. We introduce a semi-analytical lightcurve modeling approach for CSM-interaction in transients that accounts for variable diffusion time. We hence estimate a CSM mass of 2.34+11.8-1.6 M and with a density profile ρ csm proportional to r-2.74, indicating progressively increasing mass loss toward the SN explosion. The ejecta signatures emerge as broad H, He, and Ca II triplet lines, at day about 80, much earlier than expected from a spherically symmetric CSM, suggesting asymmetry in the CSM. After the lightcurve break, the H lines exhibit a wavelength-dependent deficit in the red-wing flux, indicating new dust formation in the ejecta and/or in the post-shock gas. We observe an NIR excess from day about 40 onwards. We attribute the early NIR excess before the lightcurve break solely to the NIR echo from pre-existing dust, as there are no other indications for new dust formation. The late-time NIR excess likely has contributions from both the preexisting and newly formed dust. The steady increase in mass loss, slow CSM velocity, and asymmetric CSM favor a binary interaction-induced mass loss.