The 230 GHz Variability of Numerical Models of Sagittarius A*. II. The Physical Origins of the Variability

We continue our previous work, H.-S. Chan et al., to investigate how variations in the electron temperature prescription parameter, R_Low, influence the 3 hr variability at 230 GHz, M_ΔT, in magnetically arrested disk (MAD) models of Sagittarius A* (Sgr A*), through analyzing a series of general-relativistic magnetohydrodynamics and ray-tracing simulations. For models with a black hole spin a > 0, we discovered that increasing R_Low renders the photon ring more optically thick, obscuring the varying accretion flows that contribute to the variability. However, as R_Low increases further, MAD flux eruptions become more pronounced, compensating for the decrease in M_ΔT. For models with spin a < 0, although a higher R_Low also increases the optical thickness of the fluid, voids within the optically thick gas fail to cover the entire photon ring. Similarly, flux eruptions become more prominent as R_Low increases further, contributing to the observed rise in M_ΔT relative to R_Low. For black holes with spin a = 0, although the effect of increasing optical depth is still present, their 230 GHz light curves, and hence M_ΔT, are insensitive to changes in R_Low. Furthermore, we found that the variability of the 230 GHz light curves at R_Low = 1 might correlate with fluctuations in the internal energy of the gas near the black hole, and we listed potential causes and solutions to the over-variability problem. Our findings highlight potential approaches for refining M_ΔT to better align with observations when modeling Sgr A*.