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The typical accreting neutron star, Aquila X-1, was observed with Suzakuseven times in the decay phase of an outburst in 2007 September-October. Amongthem, the second to the fourth observations were performed 10 to 22 days afterthe outburst peak, when the source was in the hard state with a luminosity of2x10^36 erg/sec. A unified spectral model for this type of objectsapproximately reproduced the 0.8--100~keV spectra obtained in these 3observations. However, the spectra all exhibited an enigmatic hump-like excessaround 30 keV, above the hard X-ray continuum which is interpreted as arisingvia Comptonization. The excess feature was confirmed to be significant againststatistical and systematic uncertainties. It was successfully represented by aGaussian centered at ~32 keV, with a width (sigma) of ~6 keV and an equivalentwidth of ~8.6 keV. Alternatively, the feature can also be explained by arecombination edge model, that produces a quasi-continuum above an edge energyof ~27 keV with an electron temperature of ~11 keV and an equivalent width of~6.3 keV. These results are discussed in the context of atomic features ofheavy elements synthesized via rapid-proton capture process duringthermonuclear flashes.

An enigmatic hump around 30 keV in Suzaku spectra of Aquila X-1 in the Hard State