The mean infrared emission of SagittariusA*
  (abridged) The massive black hole at the center of the Milky Way,
SagittariusA* is, in relative terms, the weakest accreting black hole
accessible to observations. At the moment, the mean SED of SgrA* is only known
reliably in the radio to mm regimes. The goal of this paper is to provide
constraints on the mean emission from SgrA* in the near-to-mid infrared.
Excellent imaging quality was reached in the MIR by using speckle imaging
combined with holographic image reconstruction, a novel technique for this kind
of data. No counterpart of SgrA* is detected at 8.6 microns. At this
wavelength, SgrA* is located atop a dust ridge, which considerably complicates
the search for a potential point source. An observed 3 sigma upper limit of ~10
mJy is estimated for the emission of SgrA* at 8.6 microns, a tighter limit at
this wavelength than in previous work. The de-reddened 3 sigma upper limit,
including the uncertainty of the extinction correction, is ~84 mJy . Based on
the available data, it is argued that, with currently available instruments,
SgrA* cannot be detected in the MIR, not even during flares. At 4.8 and 3.8
microns, on the other hand, SgrA* is detected at all times, at least when
considering timescales of a few up to 13 min. We derive well-defined
time-averaged, de-reddened flux densities of 3.8+-1.3 mJy at 4.8 microns and
5.0+-0.6 mJy at 3.8 microns. Observations with NIRC2/Keck and NaCo/VLT from the
literature provide good evidence that SgrA* also has a fairly well-defined
de-reddened mean flux of 0.5-2.5 mJy at wavelengths of 2.1-2.2 microns. We
present well-constrained anchor points for the SED of SgrA* on the
high-frequency side of the Terahertz peak. The new data are in general
agreement with published theoretical SEDs of the mean emission from SgrA*, but
we expect them to have an appreciable impact on the model parameters in future
theoretical work.
