A possible model for the long-term flares of Sgr A*

Abstract

Abstract We examine the effects of magnetic field on low angular momentum flows with standing shock around black holes in two dimensions. The magnetic field brings change in behavior and location of the shock which results in regularly or chaotically oscillating phenomena of the flow. Adopting fiducial parameters like specific angular momentum, specific energy, and magnetic field strength for the flow around Sgr A*, we find that the shock moves back and forth in the range 60–$170\, R_{\rm g}$, irregularly recurring with a time-scale of ∼ 5 d with an accompanying, more rapid, small modulation with a period of 25 hr without fading, where Rg is the Schwarzschild radius. The time-variability associated with the two different periods is attributed to the oscillating outer strong shock, together with another rapidly oscillating inner weak shock. As a consequence of the variable shock location, the luminosities vary roughly by more than a factor of 3. The time-dependent behaviors of the flow are well compatible with luminous flares with a frequency of ∼ one per day and bright flares occurring every ∼ 5–10 d in the latest observations by Chandra, Swift, and XMM-Newton monitoring of Sgr A*.