A Broadband X-ray Study of the Geminga Pulsar with Nustar and Xmm-newton

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 twocomponent model of a blackbody plus power-law, but instead exhibit spectral hardening above ∼ 5 keV. We find two spectral models fit the data well: (1) a blackbody (kT1 ∼ 42 eV) with a broken power-law (Γ1 ∼ 2.0, Γ2 ∼ 1.4 and Ebreak ∼ 3.4 keV), and (2) two blackbody components (kT1 ∼ 44 eV and kT2 ∼ 195 eV) with a power-law component (Γ ∼ 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 ∼ 0.01 − 1 keV. While strong phase variation of the power-law index is present below ∼ 5 keV, our phase-resolved spectroscopy with NuSTAR indicates that another hard non-thermal component with Γ ∼ 1.3 emerges above ∼ 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. Subject headings: X-rays: Individual: Geminga