Transitions to high mass accretion rates in black hole X-ray binaries are associated with the ejection of powerful , relativistically-moving jets .
The mechanism that powers 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 different possibilities by comparing proxies for jet power with spin estimates .
Due to the small number of sources that reach Eddington mass accretion 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 black hole 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 _ { * } \leq 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 , i.e .
it is too ‘ radio loud ’ for such a low spin .
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 _ { * } \leq - 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 .