NICER / ISS Science Nugget
for February 20, 2020

Modeling "reflected" X-rays in NICER data for Low Mass X-ray Binaries

Low-mass X-ray binaries (LMXBs), in which a neutron star or a black hole attracts gas from a low-mass companion star, are key tools for the study of the physics of accretion. Their X-ray luminosity scales with the rate at which matter is accreted by the compact object. The population of transient sources alternates outbursts of accretion with quiescent episodes and can therefore be studied over a wide range of X-ray luminosities, from the Eddington limit (L_Edd, at which radiation pressure balances the pressure of the accreting matter) down to a very small fraction of it. It is well known that at high X-ray luminosities (> 1% L_Edd), the accreted gas spirals in a thin disk that typically extends close to the compact object. At very low accretion rates (∼ 0.01% L_Edd), on the other hand, the accretion disk is likely truncated further away from the black hole or the neutron star. Nevertheless, it is currently unclear at what accretion luminosity the disk begins to move away, how this truncation proceeds with changing luminosity, and if this process differs for neutron stars and black holes. Standard accretion theory suggests that with decreasing accretion rates, a large part of the inner disk evaporates into a radiatively inefficient hot flow. In neutron star LMXBs, a second mechanism that can lead to disk truncation may be at play: the stellar magnetic field may push the inner disk out, but the interaction between the accretion disk and the magnetosphere, while clearly important, remains poorly understood.

X-ray reflection modeling is a powerful tool for constraining the geometry of accretion flows in LMXBs. This X-ray emission – originating from close to the accretor and then reprocessed or "reflected" toward the observer by the accretion disk – manifests most prominently as an ionized iron emission line (Figure 1) at 6‒7 keV and a "Compton hump" at 20‒40 keV. The shape of these features is modified by Doppler and gravitational redshift effects as the gas in the disk moves in high-velocity orbits inside the gravitational well of the compact accretor. The reflection spectrum thus encodes information about the morphology of the accretion flow. Comparing the reflection spectrum at different fractions of the Eddington accretion rate in the same source can reveal changes in the accretion flow structure and properties as the X-ray luminosity decays.

NICER observed the 2018 accretion outburst of the neutron star LMXB 4U 1608–52, to study changes in the reflection spectrum. A team led by Univ. of Amsterdam graduate student Jakob van den Eijnden finds that the broad iron line, clearly seen during the peak of the outburst when the X-ray luminosity is high (5% L_Edd), disappear during the decay of the outburst when the source luminosity drops to ∼0.2% L_Edd. They show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise at lower flux, but is instead caused by physical changes in the accretion flow, in a manner consistent with the inner flow evaporating from a thin disk into a geometrically thicker configuration, such as the theoretically expected formation of a radiatively inefficient hot flow at low mass-accretion rates.

This work was recently accepted for publication by the British journal Monthly Notices of the Royal Astronomical Society.