Abstract
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 ∼10−2 of the Eddington limit (LEdd). 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–Kα line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is ∼1037 erg s−1 (∼0.05 LEdd), disappear during the decay of the outburst when the source luminosity drops to ∼4.5 × 1035 erg s−1 (∼0.002 LEdd). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise ratio at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disc radius, disc ionization, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disc ionization (log ξ ≳ 4.1) or a much decreased reflection fraction. A changing disc truncation alone, however, cannot account for the lack of reprocessed Fe–Kα emission. The required increase in ionization parameter could occur if the inner accretion flow evaporates from a thin disc into a geometrically thicker flow, such as the commonly assumed formation of a radiatively inefficient accretion flow at lower mass accretion rates.
Highlights
Low-mass X-ray binaries (LMXBs), in which a neutron star or a black hole attracts gas from a low-mass companion star, are prime tools to study the physics of accretion
We report on our efforts to use reflection spectroscopy to constrain the changing accreting morphology in the neutron star LMXB 4U 1608−52 in its 2018 outburst, using the NuSTAR, NICER, and Swift X-ray observatories
The 0.5–10 keV unabsorbed flux measured by Swift was (2.9 ± 0.15) × 10−10 erg cm−2 s−1. This translates into a luminosity of (4.5 ± 0.2) × 1035 erg s−1 which is a factor ∼10 lower, as intended, than during the NuSTAR observation that was obtained during the 2014 outburst (Degenaar et al 2015)
Summary
Low-mass X-ray binaries (LMXBs), in which a neutron star or a black hole attracts gas from a low-mass companion star, are prime tools to study the physics of accretion. The relation between X-ray luminosity and disc inner radius has been determined in detail, showing a general trend of decreasing inner radius with Eddington fraction (Miller et al 2006; Reis et al 2008; Tomsick et al 2009; Petrucci et al 2014; Garcıa et al 2015; WangJi et al 2018; Garcıa et al 2019) This large set of measurements is possible due to high duty cycle of GX 339−4, yielding over 20 measurements of the reflection spectrum in the past 15 yr. Such systematic studies have to date been rare, especially in neutron stars, given the deep exposures required to detect reflection or otherwise obtain constraining upper limits at low luminosities (e.g. LX < 10−2 LEdd). We report on our efforts to use reflection spectroscopy to constrain the changing accreting morphology in the neutron star LMXB 4U 1608−52 in its 2018 outburst, using the NuSTAR, NICER, and Swift X-ray observatories
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