$n = 4$. Both models were convolved with the PSF, determined from two nearby stars, located at 61and 84away from, and with similar peak intensity to S2 0109+22. The fit was performed using pixels within 10.5of the center of S2 0109+22.

We used a Metropolis sampler [e.g. @2017arXiv170404629M] to map a posteriori distribution in three-dimensional parameter space. We employed 10 independent walkers, each completing 30 000 iteration steps and with flat priors. The walkers were initially distributed randomly over a fairly wide range of values, but they all quickly converged towards the same area in the parameter space corresponding to the maximum likelihood. The calculation of likelihood assumed that the pixel values had an uncertainty consisting of four components, each normally distributed: 1) Photon noise, 2) readout noise, 3) error in background determination and 4) error in the PSF model. The background uncertainty was determined by measuring the background around the source in 10 rectangular regions. For the PSF error, we subtracted the PSF from a star close to S2 0109+22 and examined the residuals. The residuals were the strongest near the center of the star, where they amounted to 2% of the local signal.

Figure \[hostplot\] shows the marginalized 