No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2

摘要

The gas cloud G2 that falls toward Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, is assumed to provide valuable information on the physics of accretion flows and the environment of the black hole. We observed Sgr A* with four European stations of the Global Millimeter Very Long Baseline Interferometry Array (GMVA) at 86 GHz on 1 October 2013 when parts of G2 had already passed the pericenter. We searched for a possible transient asymmetric structure – such as jets or winds from hot accretion flows – around Sgr A* that might be caused by accretion of material from G2. The interferometric closure phases remained zero within errors during the observation time. We therefore conclude that Sgr A* did not show significant asymmetric (in the observer frame) outflows in late 2013. Using simulations, we constrain the size of the outflows that we could have missed to ≈2.5 mas along the major axis and ≈0.4 mas along the minor axis of the beam, corresponding to approximately 232 and 35 Schwarzschild radii, respectively; we thus probe spatial scales on which the jets of radio galaxies are thought to convert magnetic into kinetic energy. Because probably less than 0.2 Jy of the flux from Sgr A* can be attributed to accretion from G2, the effective accretion rate is η? ? 1.5 × 109 kg s-1 ≈ 7.7 × 10-9M⊕ yr-1 for material from G2. Exploiting the relation of kinetic jet power to accretion power of radio galaxies shows that the rate of accretion of matter that is finally deposited in jets is limited to ? ? 1017 kg s-1 ≈ 0.5 M⊕ yr-1. Accordingly, G2 appears to be mostly stable against loss of angular momentum and subsequent (partial) accretion at least on timescales ?1 yr.