We report on the first hard X-ray detection of the Geminga pulsar above 10 keV using a 150 ks observation with the NuSTAR observatory .
The double-peaked pulse profile of non-thermal emission seen in the soft X-ray band persists at higher energies .
Broadband phase-integrated spectra over the 0.2–20 keV band with NuSTAR and archival XMM-Newton data do not fit to a conventional two-component model of a blackbody plus power-law , but instead exhibit spectral hardening above \sim 5 keV .
We find two spectral models fit the data well : ( 1 ) a blackbody ( kT _ { 1 } \sim 42 eV ) with a broken power-law ( \Gamma _ { 1 } \sim 2.0 , \Gamma _ { 2 } \sim 1.4 and E _ { break } \sim 3.4 keV ) , and ( 2 ) two blackbody components ( kT _ { 1 } \sim 44 eV and kT _ { 2 } \sim 195 eV ) with a power-law component ( \Gamma \sim 1.7 ) .
In both cases , the extrapolation of the Rayleigh-Jeans tail of the thermal component is consistent with the UV data , while the non-thermal component overpredicts the near-infrared data , requiring a spectral flattening at E \sim 0.01 - 1 keV .
While strong phase variation of the power-law index is present below \sim 5 keV , our phase-resolved spectroscopy with NuSTAR indicates that another hard non-thermal component with \Gamma \sim 1.3 emerges above \sim 5 keV .
The spectral hardening in non-thermal X-ray emission as well as spectral flattening between the optical and X-ray bands argue against the conjecture that a single power-law may account for multi-wavelength non-thermal spectra of middle-aged pulsars .