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

Kaya Mori, Eric V. Gotthelf, Francois Dufour, Victoria M. Kaspi, Jules P. Halpern, Andrei M. Beloborodov, Hongjun An, Matteo Bachetti, Steven E. Boggs, Finn Erland Christensen, William W. Craig, Charles J. Hailey, Fiona A. Harrison, Chryssa Kouveliotou, Michael J. Pivovaroff, Daniel Stern, William W. Zhang

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We report on the first hard X-ray detection of the Geminga pulsar above 10 keV using a 150 ks observation with the Nuclear Spectroscopic Telescope Array (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 ~5 keV. We find that 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 ~ 0.05-0.5 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.
Original languageEnglish
Article number88
JournalAstrophysical Journal
Issue number2
Number of pages11
Publication statusPublished - 2014

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