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.
GRS 1915+105 underwent a new strong outburst in X-rays and radio frequencies
in late October 1997. Ejection of plasmoids at 0.92 c was also observed at
MERLIN in the radio domain. ISO observations, following the ToO, indicated
that the source varies by a factor of 30 (0.5-15 mJy) in the LW3 band on a
several day timescale. At that time, when the source displayed a strong
X-ray and radio activity, the source flux was much fainter than seen previous-
ly at 10 um (what is certainly surprising
does not have a good S/N ratio.
Therefore, and taking into account the strong 10-um variability seen in
GRS 1915+105, which appears not to follow the near-IR or radio behaviour, we
request further monitoring of the source during the last ISO observing window
(9/3-6/5), beginning on March 16 (rev. 852, not scheduled yet).