Astron. Astrophys. 337, 729-738 (1998)

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

Christoph Winkler ¹ and Norman Trams <sup>2

1</sup> Space Science Department of ESA, Astrophysics Division, ESTEC, 2200 AG Noordwijk, The Netherlands
² Space Science Department of ESA, Astrophysics Division, ISO Science Operations Centre, E-28080 Madrid, Spain

Received 2 March 1998 / Accepted 9 June 1998

Abstract

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 . This can be indicative of a dust cloud with radius r_Dust 0.3 pc, and mass M_Dust 5.5 . 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 3 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.

Key words: stars: individual: GRS 1915+105 ISM: clouds infrared: ISM: continuum infrared: ISM: lines and bands infrared: stars

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.

Send offprint requests to: C. Winkler, (cwinkler@astro.estec.esa.nl)

© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998

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