We present the results of near-infrared ( NIR ) multi-epoch observations of the optical transient in the nearby galaxy NGC300 ( NGC300-OT ) at 398 and 582 days after the discovery with the Infrared Camera ( IRC ) onboard AKARI .
NIR spectra ( 2 – 5 { \mu m } ) of NGC300-OT were obtained for the first time .
They show no prominent emission nor absorption features , but are dominated by continuum thermal emission from the dust around NGC300-OT .
NIR images were taken in the 2.4 , 3.2 , and 4.1 { \mu m } bands .
The spectral energy distributions ( SED ) of NGC300-OT indicate the dust temperature of 810 \pm 14 { K } at 398 days and 670 \pm 12 { K } at 582 days .
We attribute the observed NIR emission to the thermal emission from dust grains formed in the ejecta of NGC300-OT .
The multi-epoch observations enable us to estimate the dust optical depth as \gtrsim 12 at 398 days and \gtrsim 6 at 582 days at 2.4 { \mu m } , by assuming an isothermal dust cloud .
The observed NIR emission must be optically thick , unless the amount of dust grains increases with time .
Little extinction at visible wavelengths reported in earlier observations suggests that the dust cloud around NGC300-OT should be distributed inhomogeneously so as to not screen the radiation from the ejecta gas and the central star .
The present results suggest the dust grains are not formed in spherically symmetric geometry , but rather in a torus , a bipolar outflow , or clumpy cloudlets .