Spectral energy distribution of the inner accretion flow around Sgr A* – clue for a weak outflow in the innermost region.

Sgr A* represents a unique laboratory for the detailed study of accretion processes around a low-luminosity supermassive black hole (SMBH). Recent X-ray observations have allowed for spatially resolved modelling of the emission from the accretion flow around the SMBH, placing tight constraints on the flux and spectral shape of the accretion from the inner region with r < 10³ R _g, where R _g ≡ GM _BH/ c ² is the gravitational radius of the black hole of mass M _BH. We present here the first modelling of the multiband spectral energy distribution (SED) of this inner region to better constrain the physical condition of the innermost accretion flow. Our modelling uses the Markov chain Monte Carlo method to fit the SED, accounting for the limitations on the accretion rate at the outer radius of 10³ R _g from the earlier works and the domination of the accretion flow within 30 R _g to the sub-mm bump. It is found that the fitting results of the outflow index could be very different. If only the most luminous part of the SED, the sub-mm bump, is considered, the outflow index is about 0, while if low-frequency radio data and X-ray data are also included, the outflow index could be 0.37 or even higher. The great difference of the fitting results indicates that the outflow index should be variable along radius, with a strong outflow in the outer region and a weak outflow in the innermost region. Such a weak outflow agrees with numerical simulations and makes it possible to explain the multiband SED even better. [ABSTRACT FROM AUTHOR]