Evaluating Spectral Models and the X‐Ray States of Neutron Star X‐Ray Transients

Abstract

We analyze the X-ray spectra of the neutron star (NS) X-ray transients Aql X-1 and 4U 1608-52, obtained with RXTE during more than 20 outbursts. We test commonly used spectral models and evaluate their performance against desirability criteria, including LX ∝ T4 evolution for the multicolor disk (MCD) component and similarity to black holes (BHs) 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: for the hard state, a single-temperature blackbody (BB) plus a broken power law (BPL), and for the soft state, two thermal components (MCD and BB) plus a constrained BPL. This model produces LX ∝ T4 tracks for both the MCD and BB, and it aligns the spectral/timing correlations of these NSs with the properties of accreting BHs. The visible BB emission area is very small (~1/16 of the NS surface), but it remains roughly constant over a wide range of LX that spans both the hard and soft states. We discuss implications of a small and constant boundary layer in terms of the presence of an innermost stable circular orbit that lies outside the NS. Finally, if the BB luminosity tracks the overall accretion rate, then we find that the Comptonization in the hard state has surprisingly high radiative efficiency compared to MCD emission in the soft state. Alternatively, if we assume that the radiative efficiency of a jet in the hard state must be less than the MCD efficiency in the soft state, while relaxing presumptions about the accretion rate, then our results may suggest substantial mass outflow in the jet.