  We present 3-50 keV NuSTAR observations of the AGN Mrk 335 in a very low flux
state. The spectrum is dominated by very strong features at the energies of the
iron line at 5-7 keV and Compton hump from 10-30 keV. The source is variable
during the observation, with the variability concentrated at low energies,
which suggesting either a relativistic reflection or a variable absorption
scenario. In this work we focus on the reflection interpretation, making use of
new relativistic reflection models that self consistently calculate the
reflection fraction, relativistic blurring and angle-dependent reflection
spectrum for different coronal heights to model the spectra. We find that the
spectra can be well fit with relativistic reflection, and that the lowest flux
state spectrum is described by reflection alone, suggesting the effects of
extreme light-bending occurring within ~2 gravitational radii of the event
horizon. The reflection fraction decreases sharply with increasing flux,
consistent with a point source moving up to above 10 Rg as the source
brightens. We constrain the spin parameter to greater than 0.9 at the 3 sigma
confidence level. By adding a spin-dependent upper limit on the reflection
fraction to our models, we demonstrate that this can be a powerful way of
constraining the spin parameter, particularly in reflection dominated states.
We also calculate a detailed emissivity profile for the iron line, and find
that it closely matches theoretical predictions for a compact source within a
few Rg of the black hole.
