  We present Nustar 3-40 keV observations of the optically selected Type 2
quasar (QSO2) SDSS J1034+6001 or Mrk 34. The high-quality hard X-ray spectrum
and archival XMM-Newton data can be fitted self-consistently with a
reflection-dominated continuum and strong Fe Kalpha fluorescence line with
equivalent-width >1 keV. Prior X-ray spectral fitting below 10 keV showed the
source to be consistent with being obscured by Compton-thin column densities of
gas along the line-of-sight, despite evidence for much higher columns from
multiwavelength data. NuSTAR now enables a direct measurement of this column,
and shows that Nh lies in the Compton-thick (CT) regime. The new data also show
a high intrinsic 2-10 keV luminosity of L_{2-10}~10^{44} erg/s, in contrast to
previous low-energy X-ray measurements for which L_{2-10}<~10^{43} erg/s (i.e.
X-ray selection below 10 keV does not pick up this source as an intrinsically
luminous obscured quasar). Both the obscuring column and the intrinsic power
are about an order of magnitude (or more) larger than inferred from pre-NuSTAR
X-ray spectral fitting. Mrk34 is thus a 'gold standard' CT QSO2 and is the
nearest non-merging system in this class, in contrast to the other local CT
quasar NGC6240 which is currently undergoing a major merger coupled with strong
star-formation. For typical X-ray bolometric correction factors, the accretion
luminosity of Mrk34 is high enough to potentially power the total infrared
luminosity. X-ray spectral fitting also shows that thermal emission related to
star-formation is unlikely to drive the observed bright soft component below ~3
keV, favoring photionization instead.
