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Broad-band X-ray emission and the reality of the broad iron line from the neutron star–white dwarf X-ray binary 4U 1820−30

Mondal, Aditya S. and Dewangan, G. C. and Pahari, M. and Misra, R. and Kembhavi, A. K. and Raychaudhuri, B. (2016) Broad-band X-ray emission and the reality of the broad iron line from the neutron star–white dwarf X-ray binary 4U 1820−30. Monthly Notices of the Royal Astronomical Society, 461 (2). pp. 1917-1926. ISSN 0035-8711. doi:10.1093/mnras/stw1464.

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Broad relativistic iron lines from neutron star X-ray binaries are important probes of the inner accretion disc. The X-ray reflection features can be weakened due to strong magnetic fields or very low iron abundances such as is possible in X-ray binaries with low mass, first generation stars as companions. Here, we investigate the reality of the broad iron line detected earlier from the neutron-star low-mass X-ray binary 4U 1820−30 with a degenerate helium dwarf companion. We perform a comprehensive, systematic broad-band spectral study of the atoll source using Suzaku and simultaneous NuSTAR and Swift observations. We have used different continuum models involving accretion disc emission, thermal blackbody and thermal Comptonization of either disc or blackbody photons. The Suzaku data show positive and negative residuals in the region of Fe K band. These features are well described by two absorption edges at 7.67 ± 0.14 keV and 6.93 ± 0.07 keV or partial covering photoionized absorption or by blurred reflection. Though, the simultaneous Swift and NuSTAR data do not clearly reveal the emission or absorption features, the data are consistent with the presence of either absorption or emission features. Thus, the absorption based models provide an alternative to the broad iron line or reflection model. The absorption features may arise in winds from the inner accretion disc. The broad-band spectra appear to disfavour continuum models in which the blackbody emission from the neutron-star surface provides the seed photons for thermal Comptonization. Our results suggest emission from a thin accretion disc (kT_(disc) ∼ 1 keV), Comptonization of disc photons in a boundary layer most likely covering a large fraction of the neutron-star surface and innermost parts of the accretion disc, and blackbody emission (kT_(bb) ∼ 2 keV) from the polar regions.

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Alternate Title:Broadband X-ray emission and the reality of the broad iron line from the Neutron Star - White Dwarf X-ray binary 4U 1820-30
Additional Information:© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2016 June 16. Received 2016 June 16; in original form 2015 September 8. Published: 20 June 2016. We are thankful to the anonymous referee whose detail in-depth comments helped us in improving the paper. This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Centre (HEASARC). ASM would like to thank Inter-University Centre for Astronomy and Astrophysics (IUCAA) for hosting him during subsequent visits. BR likes to thank IUCAA for hospitality and other facilities extended to him during his visit under their visiting Associateship programme.
Subject Keywords:stars: individual: 4U 1820−30 – stars: neutron – X-rays: binaries
Issue or Number:2
Record Number:CaltechAUTHORS:20160620-070445495
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:68501
Deposited By: Joy Painter
Deposited On:20 Jun 2016 16:04
Last Modified:11 Nov 2021 04:00

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