Evaluating Spectral Models and the X-ray States of Neutron-star X-ray Transients

We analyze the X-ray spectra of the neutron-star (NS) X-ray transients Aql X-1 and 4U 160852,obtained with RXTE during more than twenty outbursts. Our aim is to properly decompose the spectral components and to study their evolution across the hard and soft X-ray states. We test the commonly used spectral models and evaluate their performance against desirability criteria, including LX ∝ T 4 evolution for thermal components, and similarity to black holes for correlated timing/spectral behavior. None of the classical models for thermal emission plus Comptonization perform well in the soft state. Instead, we devise a hybrid model: the hard state is a single-temperature blackbody (BB) plus a broken power law (BPL), while the soft state is two thermal components (multicolor disk and BB), plus a constrained BPL. This model produces LX ∝ T 4 tracks for both the disk and BB, and it aligns the spectral/timing correlations of these NSs with the properties of accreting black holes. The BB emission area is very small (∼ 1/16 of the NS surface), but it remains nearly constant over a wide range of LX that spans both hard and soft states. A small and constant accretion boundary layer may further suggest that the NS radius is smaller than its inner-most stable circular orbit. Finally, if the BB luminosity traces the accretion rate at the NS surface, we find that the hard state is surprisingly luminous compared to the disk, when both are evaluated at the BB luminosity of state transitions. This result is difficult to understand within the framework of jet models for the hard state. Subject headings: accretion, accretion disks — stars: neutron — X-rays: binaries — X-rays: bursts — X-ray: stars