  The detections of both X-ray and radio emission from the cluster G1 in M31
have provided strong support for existing dynamical evidence for an
intermediate mass black hole (IMBH) of mass 1.8 +/- 0.5 x 10^4 solar masses at
the cluster center. However, given the relatively low significance and
astrometric accuracy of the radio detection, and the non-simultaneity of the
X-ray and radio measurements, this identification required further
confirmation. Here we present deep, high angular resolution, strictly
simultaneous X-ray and radio observations of G1. While the X-ray emission (L_X
= 1.74^{+0.53}_{-0.44} x 10^{36} (d/750 kpc)^2 erg/s in the 0.5-10 keV band)
remained fully consistent with previous observations, we detected no radio
emission from the cluster center down to a 3-sigma upper limit of 4.7
microJy/beam. Our favored explanation for the previous radio detection is
flaring activity from a black hole low mass X-ray binary (LMXB). We performed a
new regression of the Fundamental Plane of black hole activity, valid for
determining black hole mass from radio and X-ray observations of sub-Eddington
black holes, finding log M_{BH} = (1.638 +/- 0.070)log L_R - (1.136 +/-
0.077)log L_X - (6.863 +/- 0.790), with an empirically-determined uncertainty
of 0.44 dex. This constrains the mass of the X-ray source in G1, if a black
hole, to be <9700 solar masses at 95% confidence, suggesting that it is a
persistent LMXB. This annuls what was previously the most convincing evidence
from radiation for an IMBH in the Local Group, though the evidence for an IMBH
in G1 from velocity dispersion measurements remains unaffected by these
results.
