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Transitions to high mass accretion rates in black hole X-ray binaries areassociated with the ejection of powerful, relativistically-moving jets. Themechanism powering such events is thought to be linked to tapping of theangular momentum (spin) of the black hole, the rate of accretion through thedisc or some combination of the two. We can attempt to discriminate betweenthese possibilities by comparing proxies for jet power with spin estimates. Dueto the small number of sources reaching Eddington rates and have therefore beensuggested to act as 'standard candles', there has been much recent debate as towhether a significant correlation exists between jet power and spin. We performcontinuum fitting to the high-quality, disc-dominated XMM-Newton spectra of theextragalactic microquasar discovered in M31. Assuming prograde spin, we findthat, for sensible constraints the spin is always very low (a < 0.15 at3-sigma). When combined with a proxy for jet power derived from the maximum 5GHz radio luminosity during a bright flaring event, we find that the sourcesits well above the previously reported, rising correlation that would indicatethat spin tapping is the dominant mechanism for powering the jets. The notableexceptions require the inclination to be improbably small or the jet to be veryfast. We investigate whether this could be a by-product of selectingprograde-only spin, finding that the data statistically favour a substantiallyretrograde spin for the same constraints (a < -0.17 at 3-sigma). Althoughtheoretically improbable, this remarkable finding could be confirmation thatretrograde spin can power such jets via spin-tapping, as has been suggested forcertain radio quasars. In either case this work demonstrates the value ofstudying local extragalactic microquasars as a means to better understand thephysics of jet launching.

The low or retrograde spin of the first extragalactic microquasar: implications for Blandford-Znajek powering of jets