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NuSTAR and Swift joint view of neutron star X-ray binary 4U 1728−34: disc reflection in the island and lower banana states

Mondal, Aditya S. and Pahari, Mayukh and Dewangan, G. C. and Misra, R. and Raychaudhuri, B. (2017) NuSTAR and Swift joint view of neutron star X-ray binary 4U 1728−34: disc reflection in the island and lower banana states. Monthly Notices of the Royal Astronomical Society, 466 (4). pp. 4991-5002. ISSN 0035-8711. doi:10.1093/mnras/stx039.

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We analyse two simultaneous NuSTAR and Swift data of the Atoll-type neutron star (NS) X-ray binary 4U 1728−34 observed on 2013 October 1 and 3. We infer that the first and the second observations belong to the island state and the lower banana state, respectively. During island state, four type-I X-ray bursts are observed within 60 ks exposure. From the time-resolved spectral analysis of each burst with NuSTAR, the blackbody temperature kT_(bb) are found to vary between 1.3 and 3.0 keV, while the blackbody normalizations (km/10 kpc)^2 vary in the range 20–200, which translates to blackbody radii of 3.5–7.4 km for an assumed distance of 5 kpc. The persistent, joint energy spectra from Swift and NuSTAR for both observations in the energy band 1–79 keV are well described with thermal emission from the NS surface (kT_(bb) ≃ 1–2.5 keV), Comptonized emission of thermal seed photons from the hot boundary layer/corona and the strong reflection component from the accretion disc. We detect a broad iron line in the 5–8 keV band and reflection hump in the 15–30 keV band modelled by the relxill reflection model. Joint spectral fitting constrains the inclination angle of the binary system and inner disc radius to be 22°-40° and (2.0–4.3) × R_(ISCO), respectively. We estimate the magnetic field to be (1.8–6.5) × 108 G. The X-ray luminosity of the source during the island and lower banana states are found to be LX = 1.1 and 1.6 × 10^(37) erg s^(−1), respectively, which correspond to ∼6 per cent and ∼9 per cent of the Eddington luminosity.

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Additional Information:© 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2017 January 6. Received 2017 January 6; in original form 2016 April 14. Published: 11 January 2017. We are thankful to the referee for constructive and useful suggestions that help to improve the manuscript. This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Centre (HEASARC). We also thank NuSTAR team for making NuSTAR data public. ASM would like to thank Inter-University Centre for Astronomy and Astrophysics (IUCAA) for hosting him during subsequent visits. MP is thankful to Diego Altamirano for useful and constructive discussions. BR likes to thank IUCAA for hospitality and other facilities extended to him during his visit.
Subject Keywords:accretion, accretion discs – stars: neutron – X-rays: binaries – X-rays: individual: 4U 1728–34
Issue or Number:4
Record Number:CaltechAUTHORS:20160419-141608451
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Official Citation:Aditya S. Mondal, Mayukh Pahari, G. C. Dewangan, R. Misra, B. Raychaudhuri; NuSTAR and Swift joint view of neutron star X-ray binary 4U 1728−34: disc reflection in the island and lower banana states, Monthly Notices of the Royal Astronomical Society, Volume 466, Issue 4, 1 May 2017, Pages 4991–5002,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:66283
Deposited By: Joy Painter
Deposited On:19 Apr 2016 23:26
Last Modified:10 Nov 2021 23:55

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