1998A&A...337..729W

We have observed the black hole candidate and Galactic superluminal source GRS 1915+105 with ISO LWS and SWS in March 1996 and March 1997 when the source was at fairly low X-ray and radio flux levels. If the source is associated with a dense molecular cloud, then corresponding far-IR continuum and line fluxes should be easily detectable and the data should constrain models for accretion from ISM (Maloney et al. 1997). The integrated fluxes in the SWS (2.3-45.2µm) and LWS (43.2-195.6µm) are 2.5x10^–17 and 8.9x10^–17W/cm², respectively. The continuum above 50µm is well represented by thermal black body radiation of T=(25.9±0.8)K, modified by a dust emissivity component ε~λ^–1.5. This can be indicative of a dust cloud with radius r_Dust∼0.3pc, and mass M_Dust∼5.5M_☉. However, we do not detect significant line emission in the background subtracted LWS spectrum of GRS 1915+105, as predicted by Maloney et al. (1997) for far-IR lines of the ambient ISM excited by compact objects, and the 3σ upper limits are below 7.5 to 9.6x10^–20W/cm² for prominent lines as [OI], [OIII], [NII] and [CII]. The non-detection of fine-strucure lines implies that either GRS 1915+105 is not embedded in a dense molecular cloud or, if the source would be indeed close to such a cloud, the source X-ray luminosity averaged over the past few hundred years is significantly below 10³⁷ erg/s so that line intensities are low. We conclude that the far-IR continuum emission from the source region, with T=26K not unusual for the ISM, is most likely not related to GRS 1915+105 and that most probably GRS 1915+105 is not surrounded by dense ISM, so that the lack of detected line emission is a consequence of the absence of material suitable for irradation.