Accretion geometry in neutron star low-mass X-ray binaries during the hard spectral state

TL;DR

Neutron-star LMXBs in the hard spectral state exhibit relativistically broadened Fe lines implying discs near the ISCO, challenging the pure ADAF picture. The authors extend the disc–corona condensation framework by including irradiation from the neutron star surface, and compute the resulting inner-disc extension and emergent spectra using Monte Carlo methods, then compare with reflection observations. They find that NS surface irradiation enhances condensation, producing a weak inner disc extending from $R_{ m ISCO}$ outward to tens of $R_S$ (up to $\sim 70\,R_S$ at low $\dot m$) and connecting to the outer disc around $L/L_{\rm Edd}\sim 0.02$, consistent with hard-state reflection features. They predict disappearance of broad reflection lines at very low accretion rates or when magnetic truncation dominates, providing a cohesive interpretation of NS LMXB hard-state phenomenology and constraints on magnetic fields.

Abstract

We investigate the accretion geometry in neutron star low-mass X-ray binaries (LMXBs) in the hard spectral state. It is commonly accepted that, for low mass transfer rates, an advection-dominated accretion flow (ADAF) is present in the inner region. But the observed relativistically broadened emission lines in the reflection spectra clearly indicate the existence of discs near the innermost stable circular orbit $(R_{\rm{ISCO}})$. We investigate the interaction between the coronal flow and the disc in neutron star LMXBs, and find that gas condensation from the dominant, coronal accretion flow to an inner disc is enhanced as compared to that in black hole LMXBs as a consequence of irradiation of the corona by the neutron star surface. Computations show that for low mass transfer rates ($\sim 0.005-0.02$ Eddington rate) a persistent weak disc can coexist with a coronal flow in the innermost region, where a pure ADAF would have been expected. The inner disc extends outwards from $R_{\rm{ISCO}}$ to $\sim 10 R_{\rm{ISCO}}$ for Eddington ratios ($L/L_{\rm{Edd}}$) as low as $\sim 0.002$, covers a larger region for higher Eddington ratios, and eventually connects to the outer disc at $L/L_{\rm{Edd}} \sim 0.02$, thereby transiting to a soft state. We demonstrate that the observationally inferred region of the broad iron lines in the hard-state sources generally lies within the extension of the inner discs predicted by the condensation model. Disappearance of the broad iron lines is predicted at very low luminosities, either caused by very low accretion rates or disc truncation by strong magnetic fields.

Figures (5)

• Figure 1: Extension of the inner disc as a function of the mass transfer rate for different values of $\lambda$, the fraction of the accretion energy released at the surface of the neutron star to be thermalised, assumed parameter $\alpha=0.3$. The curve with $\lambda=0$ corresponds to the case of black holes. At very low $\dot m$ the curves for different $\lambda$ overlap, indicating that the density is too low for Coulomb coupling.
• Figure 2: Spectra of the two-phase accretion flow towards the neutron star for different accretion rates $\dot{m}$, assumed parameters $\alpha=0.3$, $\lambda=0.01$. A new component contributed by the neutron star surface is apparently dominant at very low accretion rates, e.g. $\dot{m}=0.003,0.005$, becoming comparable with that from the accretion flows at higher $\dot{m}$, and eventually negligible when the accretion is dominantly via a thin disc.
• Figure 3: Extension of the inner disc as a function of the luminosity $L_{0.5-50{\rm keV}}$ in units of Eddington luminosity, assumed parameter $\alpha=0.3$. A general trend is shown that the disc extension increases with the luminosity. The effect of $\lambda$ is only obvious at very low luminosity as it is significantly contributed by the surface of neutron star.
• Figure 4: Radiative efficiency of the accretion flow to the neutron star, including recondensation, as a function of $\dot{m}$, assumed parameter $\alpha=0.3$. The efficiency increases with accretion rate as a consequence of increasing Coulomb coupling; The efficiency is higher for larger $\lambda$ because of more radiation from the neutron star surface and the Compton scattering.
• Figure 5: Extension of the inner disc as a function of the viscosity $\alpha$ for different values of $\lambda$, assumed $\dot m=0.015$. The disc extension decreases with increasing $\alpha$ as consequences of the increasing heating and decreasing cooling.