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We report on the first hard X-ray detection of the Geminga pulsar above 10keV using a 150 ks observation with the NuSTAR observatory. The double-peakedpulse profile of non-thermal emission seen in the soft X-ray band persists athigher energies. Broadband phase-integrated spectra over the 0.2-20 keV bandwith NuSTAR and archival XMM-Newton data do not fit to a conventionaltwo-component model of a blackbody plus power-law, but instead exhibit spectralhardening above ~5 keV. We find two spectral models fit the data well: (1) ablackbody (kT1 ~ 42 eV) with a broken power-law (Gamma1 ~ 2.0, Gamma2 ~ 1.4 andEbreak ~ 3.4 keV), and (2) two blackbody components (kT1 ~ 44 eV and kT2 ~ 195eV) with a power-law component (Gamma ~ 1.7). In both cases, the extrapolationof the Rayleigh-Jeans tail of the thermal component is consistent with the UVdata, while the non-thermal component overpredicts the near-infrared data,requiring a spectral flattening at E ~ 0.01 - 1 keV. While strong phasevariation of the power-law index is present below ~5 keV, our phase-resolvedspectroscopy with NuSTAR indicates that another hard non-thermal component withGamma ~ 1.3 emerges above ~5 keV. The spectral hardening in non-thermal X-rayemission as well as spectral flattening between the optical and X-ray bandsargue against the conjecture that a single power-law may account formulti-wavelength non-thermal spectra of middle-aged pulsars.

A broadband X-ray study of the Geminga pulsar with NuSTAR and XMM-Newton