The orbital PDF: general inference of the gravitational potential from
  steady-state tracers
  We develop two general methods to infer the gravitational potential of a
system using steady-state tracers, i.e., tracers with a time-independent
phase-space distribution. Combined with the phase-space continuity equation,
the time independence implies a universal Orbital Probability Density Function
(oPDF) $\mathrm{d} P(\lambda|{\rm orbit})\propto \mathrm{d} t$, where $\lambda$
is the coordinate of the particle along the orbit. The oPDF is equivalent to
Jeans theorem, and is the key physical ingredient behind most dynamical
modelling of steady-state tracers. In the case of a spherical potential, we
develop a likelihood estimator that fits analytical potentials to the system,
and a non-parametric method ("phase-mark") that reconstructs the potential
profile, both assuming only the oPDF. The methods involve no extra assumptions
about the tracer distribution function and can be applied to tracers with any
arbitrary distribution of orbits, with possible extension to non-spherical
potentials. The methods are tested on Monte Carlo samples of steady-state
tracers in dark matter haloes to show that they are unbiased as well as
efficient. A fully documented \textsc{C/Python} code implementing our method is
freely available at a GitHub repository linked from
\url{http://icc.dur.ac.uk/data/#oPDF}.
