ISO far - IR spectroscopy of the black hole candidate GRS 1915 + 105 ?

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 45.2 μm) and LWS (43.2 195.6 μm) are 2.5 × 10−17 and 8.9× 10−17 W/cm2, respectively. The continuum above 50 μm is well represented by thermal black body radiation of T = (25.9 ± 0.8) K, modified by a dust emissivity component ∼ λ−1.5. This can be indicative of a dust cloud with radius rDust ∼ 0.3 pc, and mass MDust ∼ 5.5 M . 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 9.6 × 10−20 W/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 = 26 K 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.