The long stare at Hercules X-1 -- I. Emission lines from the outer disk, the magnetosphere boundary and the accretion curtain

TL;DR

This study provides a detailed spectral analysis of Hercules X-1 across its precession cycle, revealing complex emission line structures from different regions of the accretion flow, advancing understanding of neutron star accretion environments.

Contribution

It presents one of the most comprehensive datasets and analyses of Her X-1, identifying multiple emission line components from distinct accretion regions, and tracks their evolution with precession phase.

Findings

Identification of three emission line groups from different accretion regions.

Evidence of spectral component evolution with precession phase.

Detection of emission lines originating from the outer disk, boundary layer, and near the neutron star surface.

Abstract

Hercules X-1 is a nearly edge-on accreting X-ray pulsar with a warped accretion disk, precessing with a period of about 35 days. The disk precession allows for unique and changing sightlines towards the X-ray source. To investigate the accretion flow at a variety of sightlines, we obtained a large observational campaign on Her X-1 with XMM-Newton (380 ks exposure) and Chandra (50 ks exposure) for a significant fraction of a single disk precession cycle, resulting in one of the best datasets taken to date on a neutron star X-ray binary. Here we present the spectral analysis of the High State high-resolution grating and CCD datasets, including the extensive archival data available for this famous system. The observations reveal a complex Fe K region structure, with three emission line components of different velocity widths. Similarly, the high-resolution soft X-ray spectra reveal a number of emission lines of various widths. We correct for the uncertain gain of the EPIC-pn Timing mode spectra, and track the evolution of these spectral components with Her X-1 precession phase and observed luminosity. We find evidence for three groups of emission lines: one originates in the outer accretion disk (10^5 RG from the neutron star). The second line group plausibly originates at the boundary between the inner disk and the pulsar magnetosphere (10^3 RG). The last group is too broad to arise in the magnetically-truncated disk and instead must originate very close to the neutron star surface, likely from X-ray reflection from the accretion curtain (~10^2 RG).