Multi-wavelength X-ray/mid-infrared observations of GRS 1915+105

We report preliminary results of mid-infrared and X-ray observations of GRS 1915+105 that we carried out between 2004 October 2 and 2006 June 5. Our main goals were to study its variability, to detect the presence of dust, and to investigate the possible links between mid-infrared and X-ray emissions. We performed photometric and spectroscopic observations of GRS 1915+105, using the IRAC photometer and the IRS spectrometer mounted on the Spitzer Space Telescope. We completed our set of MIR data with quasi-simultaneous high-energy data obtained with RXTE and INTEGRAL. In the hard state, we detect PAH emission features in the MIR spectrum of GRS 1915+105, which prove the presence of dust in the system. The dust is confirmed by the detection in the hard state of a warm MIR excess in the broadband spectral energy distribution of GRS 1915+105. This excess cannot be explained by the MIR synchrotron emission from the compact jets as GRS 1915+105 was not detected at 15 GHz with the Ryle telescope. We also show that the MIR emission of GRS 1915+105 is strongly variable; it is likely correlated to the soft X-ray emission as it increases in the soft state. We suggest that, beside the dust emission, part of the MIR excess in the soft state is non-thermal, and could be due either to free-free emission from an X-ray driven wind or X-ray reprocessing in the outer part of the accretion disc.