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

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.