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

Transitions to high mass accretion rates in black hole X-ray binaries are associated with the ejection of powerful, relativistically-moving jets. The mechanism powering such events is thought to be linked to tapping of the angular momentum (spin) of the black hole, the rate of accretion through the disc or some combination of the two. We can attempt to discriminate between these possibilities by comparing proxies for jet power with spin estimates. Due to the small number of sources reaching Eddington rates and have therefore been suggested to act as 'standard candles', there has been much recent debate as to whether a significant correlation exists between jet power and spin. We perform continuum fitting to the high-quality, disc-dominated XMM-Newton spectra of the extragalactic microquasar discovered in M31. Assuming prograde spin, we find that, for sensible constraints the spin is always very low (a < 0.15 at 3-sigma). When combined with a proxy for jet power derived from the maximum 5 GHz radio luminosity during a bright flaring event, we find that the source sits well above the previously reported, rising correlation that would indicate that spin tapping is the dominant mechanism for powering the jets. The notable exceptions require the inclination to be improbably small or the jet to be very fast. We investigate whether this could be a by-product of selecting prograde-only spin, finding that the data statistically favour a substantially retrograde spin for the same constraints (a < -0.17 at 3-sigma). Although theoretically improbable, this remarkable finding could be confirmation that retrograde spin can power such jets via spin-tapping, as has been suggested for certain radio quasars. In either case this work demonstrates the value of studying local extragalactic microquasars as a means to better understand the physics of jet launching.