A transient low-frequency quasi-periodic oscillation from the black hole binary GRS 1915+105

We present the results of the timing analysis of five Rossi X-ray Timing Explorer observations of the black hole candidate GRS 1915+105 between 1996 September and 1997 December. The aim was to investigate the possible presence of a type B quasi-periodic oscillation (QPO). Since in other systems this QPO is found to appear during spectral transitions from hard to soft states, we analysed observations characterized by a fast and strong variability, in order to have a large number of transitions. In GRS 1915+105, transitions occur on very short time-scales (~s): to single them out we averaged power density spectra following the regular path covered by the source on a 3D hardness-hardness-intensity diagram. We identified both the type C and the type B QPOs: this is the first detection of a type B QPO in GRS 1915+105. As the spectral transitions have been associated to the emission and collimation of relativistic radio-jets, their presence in the prototypical Galactic jet source strengthens this connection.