Hard X-ray Excess from the Magnificent Seven Neutron Stars

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We report significant hard X-ray excesses in the energy range 2-8 keV for two nearby isolated neutron stars RX J1856.6-3754 and RX J0420.0-5022.

• These neutron stars have previously been observed in soft X-rays to have nearly thermal spectra at temperatures ~100 eV, which are thought to arise from the warm neutron star surfaces.
• We find nontrivial hard X-ray spectra well above the thermal surface predictions with archival data from the XMM-Newton and Chandra X-ray telescopes.
• We analyze possible systematic effects that could generate such spurious signals, such as nearby X-ray point sources and pileup of soft X-rays, but we find that the hard X-ray excesses are robust to these systematics.