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A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608−52
Name
2002.04003v1.pdf
Description
Accepted version
Size
1.58 MB
Format
Adobe PDF
Checksum (MD5)
001bb6a362f91a1bddcdfa060f2ec8bf
Author(s)
van den Eijnden, J
Degenaar, N
Ludlam, RM
Parikh, AS
Miller, JM
Wijnands, R
Gendreau, KC
Arzoumanian, Z
Chakrabarty, D
Bult, P
Date Issued
2020
Journal
Monthly Notices of the Royal Astronomical Society
Publisher
Oxford University Press (OUP)
Version
Author's final manuscript
Abstract
© 2020 The Author(s). It is commonly assumed that the properties and geometry of the accretion flow in transient lowmass 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 signalto-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.
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DOI of Published Version
10.1093/MNRAS/STAA423
