Two-temperature treatments in magnetically arrested disk GRMHD simulations more accurately predict light curves of Sagittarius A*

TL;DR

This study demonstrates that two-temperature GRMHD simulations of Sagittarius A* yield more accurate light curves and flux variability predictions than traditional single-temperature models, by accounting for microphysical dissipation and radiative cooling effects.

Contribution

It introduces two-temperature thermodynamics into MAD GRMHD simulations, improving the match with observed variability of Sgr A*'s emission.

Findings

2T simulations better match observed flux variability

Radiative cooling reduces electron temperature by about 10%

Including 2T effects decreases flux fluctuations

Abstract

The Event Horizon Telescope Collaboration (EHTC) observed the Galactic centre source Sagittarius A* (Sgr A*) and used emission models primarily based on single ion temperature (1T) general relativistic magnetohydrodynamic (GRMHD) simulations. This predicted emission is strongly dependent on a modelled prescription of the ion-to-electron temperature ratio. The most promising models are magnetically arrested disk (MAD) states. However, nearly all MAD models exhibit larger temporal fluctuations in radiative 230 GHz emission compared to observations. This limitation possibly stems from the fact that the actual temperature ratio depends on microphysical dissipation, radiative processes and other effects not captured in ideal fluid simulations. Therefore, we investigate the effects of two-temperature (2T) thermodynamics in MAD GRMHD simulations of Sgr A*, where the temperatures of both species are evolved. We find that the 230 GHz synchrotron flux variability more closely matches historical observations when we include the 2T treatment compared to 1T simulations. For the low accretion rates of Sgr A*, a common assumption is to neglect radiative cooling. However, we find that the radiative cooling of electrons-via synchrotron, inverse Compton, and bremsstrahlung processes-reduces the electron temperature in the inner disk, where the EHT observes, by about 10%, which, in turn, decreases both the (sub)millimetre synchrotron flux and its temporal fluctuations compared to uncooled simulations.