An enigmatic hump around 30 keV in Suzaku spectra of Aquila X-1 in the hard state

Abstract

Abstract The typical accreting neutron star, Aquila X-1, was observed with Suzaku seven times in the decay phase of an outburst in 2007 September–October. Among them, the second to the fourth observations were performed 10 to 22 days after the outburst peak, when the source was in the hard state with a luminosity of 2 × 1036 erg s−1. A unified spectral model for this type of objects approximately reproduced the 0.8–100 keV spectra obtained in these three observations. However, the spectra all exhibited an enigmatic hump-like excess around 30 keV, above the hard X-ray continuum which is interpreted as arising via Comptonization. The excess feature was confirmed to be significant against statistical and systematic uncertainties. It was successfully represented by a Gaussian centered at ∼32 keV, with a width (sigma) of ∼6 keV and an equivalent width of ∼8.6 keV. Alternatively, the feature can also be explained by a recombination edge model, which produces a quasi-continuum above an edge energy of ∼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 the atomic features of heavy elements synthesized via a rapid-proton capture process during thermonuclear flashes.