  To date, mid-infrared properties of Galactic black hole binaries have barely
been investigated in the framework of multi-wavelength campaigns. Yet, studies
in this spectral domain are crucial to get complementary information on the
presence of dust and/or on the physical processes such as dust heating and
thermal bremsstrahlung. Here, we report a long-term multi-wavelength study of
the microquasar GRS 1915+105. On the one hand, we aimed at understanding the
origins of the mid-infrared emission, and on the other hand, at searching for
correlation with the high-energy and/or radio activities. We observed the
source at several epochs between 2004 and 2006 with the photometer IRAC and
spectrometer IRS, both mounted on the Spitzer Space Telescope. When available,
we completed our set of data with quasi-simultaneous RXTE and INTEGRAL
high-energy and/or Ryle radio observations from public archives. We then
studied the mid-infrared environment and activities of GRS 1915+105 through
spectral analysis and broad band fitting of its radio to X-ray spectral energy
distributions. We detected polycyclic aromatic hydrocarbon molecules in all but
one IRS spectra of GRS 1915+105 which unambiguously proves the presence of a
dust component, likely photoionised by the high-energy emission. We also argue
that this dust is distributed in a disc-like structure heated by the companion
star, as observed in some Herbig Ae/Be and isolated cool giant stars. Moreover,
we show that some of the soft X-ray emission emanating from the inner regions
of the accretion disc is reprocessed and thermalised in the outer part. This
leads to a mid-infrared excess that is very likely correlated to the soft X-ray
emission. We exclude thermal bremsstrahlung as contributing significantly in
this spectral domain.
