Long-term multi-wavelength studies of GRS 1915+105 I. A high-energy and mid-infrared focus with RXTE, INTEGRAL, and Spitzer

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.

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

We report preliminary results of mid-infrared (MIR) 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 MIR 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.