A scenario for a periodic filling and emptying of the accretion disc of the microquasar GRS1915+105 is proposed , by estimating the mass transfer rate from the donor and comparing it with the observed accretion rate onto the primary black hole .
The mass of the Roche-lobe-filling donor ( 1.2 \pm { 0.2 } M _ { \sun } ) , the primary black hole mass ( 14 \pm { 4 } M _ { \sun } ) and the binary orbital period of 33.5 d ( Greiner et al .
2001b ) predict for the donor spectral type and K-magnitude around K6 III and -2.6 , respectively .
The He-core of 0.28 M _ { \sun } of such a giant leads to evolutionary expansion along the giant branch with a conservative mass transfer rate of \dot { M } _ { d } = ( 1.5 \pm { 0.5 } ) \times 10 ^ { -8 } M _ { \sun } /year .
On the other hand , the average observed accretion rate onto the primary is ten times larger : \dot { M } _ { obs } = 2.0 \times ( \eta /0.1 ) ^ { -1 } ( d/12.5kpc ) ^ { 2 } 10 ^ { -7 } M _ { \sun } /y , where \eta is the efficiency of converting accretion into radiation .
We propose a duty cycle with ( 5-10 ) ( \eta /0.1 ) per cent active ON-state .
The timescale of the ( recurrent ) OFF-state is identified as the viscosity time scale at the circularization radius ( 14 R _ { \sun } ) and equals t _ { visc } = 370 ( \alpha /0.001 ) ^ { -4 / 5 } years , where \alpha is the viscosity parameter in the \alpha -prescription of a classical disc .
If the viscosity at the outer edge of the disc is small and \eta is close to the maximum available potential energy ( per rest mass energy ) at the innermost stable orbit , the present activity phase may still last another 10 – 20 years .
We also discuss other solutions allowing a broader range of donor masses ( 0.6 – 2.4 M _ { \sun } ) .