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

van den Eijnden, J. and Degenaar, N. and Ludlam, R. M. and Parikh, A. S. and Miller, J. M. and Wijnands, R. and Gendreau, K. C. and Arzoumanian, Z. and Chakrabarty, D. and Bult, P. (2020) A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608−52. Monthly Notices of the Royal Astronomical Society, 493 (1). pp. 1318-1327. ISSN 0035-8711.

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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 ∼10³⁷ erg s⁻¹ (∼0.05 L_(Edd)), disappear during the decay of the outburst when the source luminosity drops to ∼4.5 × 10³⁵ 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
van den Eijnden, J.0000-0002-5686-0611
Ludlam, R. M.0000-0002-8961-939X
Chakrabarty, D.0000-0001-8804-8946
Additional Information:© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Received: 17 December 2019; Revision received: 09 February 2020; Accepted: 09 February 2020; Published: 14 February 2020. The authors thank the anonymous referee for comments that improved the quality of this manuscript. JvdE, ND, and ASP are supported by an NWO Vidi grant awarded to ND. RML acknowledges the support of NASA through Hubble Fellowship Program grant HST-HF2-51440.001. PB was supported by an NPP fellowship at NASA Goddard Space Flight Center. This research has made use of the MAXI data provided by RIKEN, JAXA, and the MAXI team, together with data and software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC) and NASA’s Astrophysics Data System Bibliographic Services. This work made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). This work was supported in part by NASA through the NICER mission and the Astrophysics Explorers Program.
Funding AgencyGrant Number
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)UNSPECIFIED
NASA Hubble FellowshipHST-HF2-51440.001
NASA Postdoctoral ProgramUNSPECIFIED
Subject Keywords:accretion, accretion discs, stars: individual: (4U 1608−52), stars: neutron, X-rays: binaries
Issue or Number:1
Record Number:CaltechAUTHORS:20200326-134529476
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Official Citation:J van den Eijnden, N Degenaar, R M Ludlam, A S Parikh, J M Miller, R Wijnands, K C Gendreau, Z Arzoumanian, D Chakrabarty, P Bult, A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608−52, Monthly Notices of the Royal Astronomical Society, Volume 493, Issue 1, March 2020, Pages 1318–1327,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:102127
Deposited By: Tony Diaz
Deposited On:27 Mar 2020 03:05
Last Modified:27 Mar 2020 03:05

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