A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608−52. Issue 1 (14th February 2020)

• Record Type: Journal Article
• Authors: van den Eijnden, J; Degenaar, N; Ludlam, R M; Parikh, A S; Miller, J M; Wijnands, R; Gendreau, K C; Arzoumanian, Z; Chakrabarty, D; Bult, P
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 493:Issue 1(2020)
• Page Start: 1318
• Page End: 1327
• Publication Date: 2020-02-14
• Subjects: accretion, accretion discs -- stars: individual: (4U 1608−52) -- stars: neutron -- X-rays: binaries; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa423
• Languages: English
• ISSNs: 0035-8711

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 ( L 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–Kα line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is ∼10 37 erg s −1 (∼0.05 L Edd ), disappear during the decay of the outburst when the source luminosity drops to ∼4.5 × 10 35 erg s −1 (∼0.002 L Edd ). 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.