Hot graphite dust in the inner regime of NGC 4151

We model the near-infrared (NIR) spectral energy distribution (SED) of NGC 4151 with a 3D radiative transfer skirt code, using which torus only (TO) and ring and torus (RAT) scenarios are studied. In the RAT models, a graphite ring-like structure (clumpy or smooth) is incorporated between the torus and the accretion disc. We vary the inclination angle (i), inner radius (of the torus and the ring, Rin,t and Rin,r, respectively), torus half-opening angle (σ), optical depth ($τ9.7, t$ of the torus and $τ9.7, r$ of the ring), and the dust clump size (Rclump). We perform a statistical analysis of the parameter space and find that all the models are able to explain the flat NIR SED of NGC 4151 with minor differences in the derived parameters. For the TO model, we get Rin,t = 0.1 pc, σ = 30°, i = 53°, $τ9.7, t = 10$, and the clumpsize, Rclump = 0.4 pc. For the smooth RAT model, $R_ in, r = 0.04$ pc and $τ9.7, total$ = 11 and for the clumpy RAT model, Rin,r = 0.04 pc/0.06 pc and $τ9.7, total = 20$. The Rin,t from the TO model does not agree with the NIR observations (∼0.04 pc). Hence, the most likely scenario is that a hot graphite ring is located at a distance 0.04 pc from the centre, composed of a smooth/clumpy distribution of dust followed by a dusty torus at 0.1 pc with interstellar medium type of grains.