Neutral versus ionized absorber as an explanation of the X-ray dippers

TL;DR

This study analyzes X-ray dips in a low mass X-ray binary, showing that absorption by the outer disk bulge explains spectral evolution better than ionized absorber models, supported by broad energy range data and scattering evidence.

Contribution

The paper demonstrates that spectral dips are caused by absorption in the outer disk bulge, challenging ionized absorber explanations and emphasizing the extended nature of the accretion disk corona.

Findings

Spectral evolution during dips is explained by absorption in the outer disk bulge.

Energy-independent flux decrease above 20 keV is due to Thomson scattering.

Ionized absorber models are inconsistent with the extended accretion disk corona evidence.

Abstract

We present results of the Suzaku observation of the dipping, periodically bursting low mass X-ray binary XB 1323-619 in which we concentrate of the spectral evolution in dipping in the energy range 0.8 - 70 keV. It is shown that spectral evolution in dipping is well-described by absorption on the bulge in the outer accretion disk of two continuum components: emission of the neutron star plus the dominant, extended Comptonized emission of the accretion disk corona (ADC). This model is further supported by detection of a relatively small, energy-independent decrease of flux above 20 keV due to Thomson scattering. It is shown that this is consistent with the electron scattering expected of the bulge plasma. We address the recent proposal that the dip sources may be explained by an ionized absorber model giving a number of physical arguments against this model. In particular, that model is inconsistent with the extended nature of the ADC for which the evidence is now overwhelming.