A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608-52

Abstract

It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below $\sim 10^{-2}$ of the Eddington limit ($L_{\mathrm{Edd}}$). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608-52, to study changes in the reflection spectrum. We find that the broad Fe-K$\alpha$ line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is $\sim 10^{37}$ erg/s ($\sim 0.05$ $L_{\mathrm{Edd}}$), disappear during the decay of the outburst when the source luminosity drops to $\sim 4.5\times 10^{35}$ erg/s ($\sim 0.002$ $L_{\mathrm{Edd}}$). We 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. Simulating synthetic NuSTAR observations on a grid of inner disk radius, disk ionisation, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disk ionisation ($\log \xi \ge 4.1$) or a much decreased reflection fraction. A changing disk truncation alone, however, cannot account for the lack of reprocessed Fe-K$\alpha$ emission. The required increase in ionisation parameter could occur if the inner accretion flow evaporates from a thin disk into a geometrically thicker flow, such as the commonly assumed formation of an radiatively inefficient accretion flow at lower mass accretion rates.

Accepted for publication in MNRAS

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• Catalog of outbursts of neutron star LMXBs
• Phase resolved spectrum of the Crab pulsar from NICER
• Multi-band observations of Swift J0840.7-3516: a new transient ultra-compact X-ray binary candidate
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