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We present high-energy (3--30 keV) {\it NuSTAR} observations of the nearestquasar, the ultraluminous infrared galaxy (ULIRG) Markarian 231 (Mrk 231),supplemented with new and simultaneous low-energy (0.5--8 keV) data from {\itChandra}. The source was detected, though at much fainter levels thanpreviously reported, likely due to contamination in the large apertures ofprevious non-focusing hard X-ray telescopes. The full band (0.5--30 keV) X-rayspectrum suggests the active galactic nucleus (AGN) in Mrk 231 is absorbed by apatchy and Compton-thin (N$_{\rm H} \sim1.2^{+0.3}_{-0.3}\times10^{23}$cm$^{-2}$) column. The intrinsic X-ray luminosity (L$_{\rm 0.5-30keV}\sim1.0\times10^{43}$ erg s$^{1}$) is extremely weak relative to thebolometric luminosity where the 2--10 keV to bolometric luminosity ratio is$\sim$0.03% compared to the typical values of 2--15%. Additionally, Mrk 231 hasa low X-ray-to-optical power law slope ($\alpha_{\rm OX}\sim-1.7$). It is alocal example of a low-ionization broad absorption line (LoBAL) quasar that isintrinsically X-ray weak. The weak ionizing continuum may explain the lack ofmid-infrared [O IV], [Ne V], and [Ne VI] fine-structure emission lines whichare present in sources with otherwise similar AGN properties. We argue that theintrinsic X-ray weakness may be a result of the super-Eddington accretionoccurring in the nucleus of this ULIRG, and may also be naturally related tothe powerful wind event seen in Mrk 231, a merger remnant escaping from itsdusty cocoon.

{\it NuSTAR} Reveals an Intrinsically X-ray Weak Broad Absorption Line Quasar in the Ultraluminous Infrared Galaxy Markarian 231