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.
