Discovery of a keV-X-ray excess in RX J1856.5-3754

RX J1856.5-3754 is the brightest and nearest (∼120 pc) source among thermally emitting isolated neutron stars. Its spectra observed with XMM-Newton and Chandra satellites are well-fitted with the two-temperature (kT^∞ ∼ 32 and 63 eV) blackbody model. Fitting ten sets of the data from Suzaku XIS 0, XIS 1, XIS 3, and XMM-Newton EPIC-pn with the two-temperature blackbody model, we discover an excess emission, 16%-26% in 0.8-1.2 keV. We examine possible causes of this keV-X-ray excess; uncertainty in the background, pile-up of the low-energy photons, and confusion of other sources. None of them succeeds in explaining the keV-X-ray excess observed with different instruments. We thus consider that this keV-X-ray excess most likely originates in RX J1856.5-3754. However, it is difficult to constrain the spectral shape of the keV-X-ray excess. A third blackbody component with kT^∞ = 137^{+18}_{-14}eV, an additional power-law component with a photon index Γ = 3.4^{+0.5}_{-0.6}, or Comptonization of blackbody seed photons into a power law with a photon index Γ _c = 4.3^{+0.8}_{-0.8} can reproduce the keV-X-ray excess. We also search for the periodicity of 0.8-1.2 keV data, since 7.055 s pulsation is discovered in the 0.15-1.2 keV band in the XMM Newton EPIC-pn data (∼1.5%). We only obtain the upper limit of pulsed fraction <3% in the keV-X-ray excess. We briefly discuss the possible origin of the keV-X-ray excess, such as synchrotron radiation and Comptonization of blackbody photons.