Sagittarius A* in the Infrared

Abstract

The central black hole of the Milky Way, Sagittarius A* (SgrA*) is, in terms of Eddington luminosity, the weakest accreting object of its class accessible to detailed observations. It is therefore key to the refinement of theoretical models of radiatively inefficient accretion. Unfortunately, our knowledge of the mean SED of Sgr A* is very limited. Current models rely almost exclusively on cm to mm mean flux measurements and only on upper limits at infrared to soft X-ray wavelengths. Here, we present a new analysis of imaging data of the Galactic center (GC) at 2.2 to 8.6 microns, obtained with NACO and VISIR at the ESO VLT. We used the VISIR burst mode combined with a novel implementation of the holographic image reconstruction algorithm to obtain mid-infrared images with a Strehl ratio ≫ 90% even under conditions of ∼2 – 3" seeing in the visual. No counterpart of Sgr A* is detected at 8.6 microns. At this wavelength, Sgr A* is located right on top of a dust ridge, which considerably complicates the search for a potential point-source. Based on the available data, it is argued that Sgr A* cannot be detected in the MIR with currently available instruments, not even during flares. At 3.8 and 4.8 μm SgrA* is detected at all times. We measure the time-averaged mean fluxes of Sgr A* at these wavelengths. From the literature there is evidence that SgrA* is also detected at 2.2 μm most of the time. The new measurements of the mean, quiescent emission of SgrA* fill a gap of almost 6 orders of magnitude in its known mean SED and provide novel constraints on accretion/emission models.