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

Author:

van den Eijnden J1ORCID,Degenaar N1,Ludlam R M2,Parikh A S1,Miller J M3,Wijnands R1,Gendreau K C4,Arzoumanian Z4,Chakrabarty D5,Bult P46

Affiliation:

1. Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, NL-1098 XH, Amsterdam, the Netherlands

2. Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA

3. Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA

4. Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

5. MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

6. Department of Astronomy, University of Maryland, College Park, MD 20742, USA

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.

Funder

NWO

NASA

Jet Propulsion Laboratory

California Institute of Technology

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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