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

In Neutron Star Low Mass X-ray Binaries (NS-LMXBs), the matters accreting on the NS are compressed, heated, and eventually undergo thermonuclear flashes called Type-I bursts. Because the accreting materials mainly consist of hydrogen, the NS atmosphere is expected to be in a proton-rich environment. When type-I bursts ignite under such a condition, the rapid-proton capture process (rp-process) should occur. The NS atmosphere is hence a promising site of the rp-process. The typical NS-LMXB, 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 Low/Hard State with a luminosity of 10^{36} erg/sec. In general, the Low/Hard state spectrum of an NS-LMXB is represented by a hard X-ray continuum up to 100 keV arising when blackbody photons from the NS surface is Comptonized, plus an optically thick disk emission appearing below a few keV (Sakurai et al. 2014). This model was able to approximately reproduce the Aqlila X-1 spectra obtained in the 2nd to the 4th observations. However, these three spectra all exhibited an enigmatic hump-like excess around 30 keV above the Comptonized continuum. A similar feature is found in an RXTE spectrum of this object observed in the Low/Hard state (Lin et al. 2007). We quantify this feature, and attempt to interpret it in the context of rp-process products.The excess feature was successfully represented by a Gaussian centered at 29-34 keV, with a width (sigma) of 3-11 keV and an equivalent width of about 8 keV. Alternatively, the feature can also be explained by a recombination edge (redge) model, that produces a quasi-continuum above an edge energy of 26 ± 1 keV with a plasma temperature of 6-32 keV. Considering that the feature is K-shell structures of highly-ionized heavy elements produced via the rp-process, and taking into account the gravitational redshift on the NS surface, the Gaussian centroid energy indicates the atomic number of Z = 59 - 63, or Pr to Eu. Whereas the redge model results indicate Z = 47 and 48, namely, Ag and Cd. We discuss these results in the context of rp-process in X-ray bursts, and examine whether this scenario if feasible or not.