Contents of: VI/111/./abstract/AJCASTRO_G1915.abs

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GRS 1915+105 was discovered in Aug 1992 with the WATCH all-sky X-ray monitor 
on board the GRANAT satellite. The source has been since then one of the 
brightest objects in the X-ray sky, highly variable in all wavelenghts, from
gamma-rays to radio. At a distance of 12 kpc, the absorption towards the 
source is almost 30 magnitudes, putting the object well beyond the reach of 
optical telescopes.
The uniqueness of GRS 1915+105 is that it shows superluminal motion, 
following a bright radio outburst seen in March 1994 - the first such case 
in our Galaxy. GRS 1915+105 is a binary system in which a massive black hole 
(with perhaps 33 solar mases) is accreting mass from a nearby companion. The 
companion class is still unclear, but it could be a high-mass object, on the 
basis of observations performed in Aug 1995, that revealed the near-IR 
reverberation of an energetic radio-outburst. The jet was also observed in 
the K-band in July 1995. Near-IR spectroscopy indicates the presence of 
strong emission lines (He I 2.06 um and Br-gamma 2.16 um) during the high 
activity periods.
It has been recently noticed (Autumn 1997) that GRS 1915+105 is undergoing a 
new strong outburst in X-rays and radio frequencies. Taking into account that 
the outbursts observed so far do not last for more than 1-2 months, we request 
IR observations with ISO during discretionary time in order to observe the 
object in the course of this current outburst. The fact that ISO is in orbit 
provides a unique opportunity to observe this object during this active 
period, thus completing the ISO observations already performed during a more 
quiet state. 
We expect to find a variable source in the CAM energy range, brighter than 
in the previous ISO observation. We also intend to extend the ground-based 
JHK measurements (1.0-2.4 um) to 17 um in order to detect in a full CVF 
spectrum the H recombination lines, and the Ar II (7.05 um), Ne II (12.93 um) 
and Ne III (15.7 um) emission lines. A deep CAM observation at 15 um will 
allow us to search for faint mid-IR emission along the jet. The detection of 
the source in the far-IR with PHOT (up to 200 um), will give support to the 
hypothesis that the far infrared emission is the consequence of heating, by 
the high energy photons, of the surrounding dust ejected by the companion.  
The requested observations will help to clarify the poorly known nature of 
the emission mechanisms of GRS 1915+105, the most energetic source in the 
Galaxy. Only ISO has the necessary sensitivity to perform them.