Radiative shock oscillation model for the long-term flares of Sgr A*

Abstract

ABSTRACT We examine time-dependent 2D relativistic radiation magnetohydrodynamic (MHD) flows to develop the shock oscillation model for the long-term flares of Sagittarius A* (Sgr A*). Adopting modified flow parameters in addition to the previous studies, we confirm quasi-periodic flares with periods of ∼5 and 10 d that are compatible with observations by Chandra, Swift, and XMM–Newton monitoring of Sgr A*. Using a simplified two-temperature model of ions and electrons, we find that the flare due to synchrotron emission lags that of bremsstrahlung emission by 1–2 h that are qualitatively comparable to the time lags of 1–5 h reported in several simultaneous observations of radio and X-ray variability in Sgr A*. The synchrotron emission is confined in a core region of 3Rg size with the strong magnetic field, while the bremsstrahlung emission mainly originates in a distant region of 10–20Rg behind the oscillating shock, where Rg is the Schwarzschild radius. The time lag is estimated as the transit time of the acoustic wave between the above two regions. The time-averaged distribution of radiation shows a strong anisotropic nature along the rotational axis but isotropic distribution in the radial direction. A high-velocity jet with ∼0.6c along the rotational axis is intermittently found in a narrow funnel region with a collimation angle ∼15°. The shock oscillating model explains well the flaring rate and the time lag between radio and X-ray emissions for the long-term flares of Sgr A*.