Black hole spin in GRS 1915+105

Microquasars are galactic black hole binary systems with radio jets which can sometimes be spatially resolved to show superluminal motion. The first and best known of this class of objects is GRS 1915+105, the brightest accreting source in our Galaxy. There is persistent speculation that strong jet emission could be linked to black hole spin. If so, the high spin should also be evident in accretion disc spectra. We search the RXTE archive to find disc-dominated X-ray spectra from this object, as these are the only ones which can give reliable spin determinations by this method. Finding these is complicated by the rapid, unique limit cycle variability, but we are able to identify such spectra by going to the shortest possible time resolution (16 s). We fit them with a simple multicolour disc blackbody (DISKBB), and with the best current model which include full radiative transfer as well as relativistic effects (BHSPEC). Both these models show that the spin is intermediate, neither zero nor maximal. BHSPEC, the most physical model, gives a value for the dimensionless spin of a* ∼ 0.7 for a distance of 12.5 kpc and inclination of 66°. This, together with the range of spins 0.1 < a* < 0.8 derived using this method for other black holes, suggests that jet emission is probably fundamentally powered by gravity rather than spin, and implies that high-to-maximal spin is not a pre-requisite for powerful relativistic jets.