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The Comptonizing medium of the neutron star in 4U 1636 − 53 through its lower kilohertz quasi-periodic oscillations

The Comptonizing medium of the neutron star in 4U 1636 − 53 through its lower kilohertz quasi-periodic oscillations
The Comptonizing medium of the neutron star in 4U 1636 − 53 through its lower kilohertz quasi-periodic oscillations
Inverse Compton scattering dominates the high-energy part of the spectra of neutron star (NS) low-mass X-ray binaries (LMXBs). It has been proposed that inverse Compton scattering also drives the radiative properties of kilohertz quasi-periodic oscillations (kHz QPOs). In this work, we construct a model that predicts the energy dependence of the rms amplitude and time lag of the kHz QPOs. Using this model, we fit the rms amplitude and time lag energy spectra of the lower kHz QPO in the NS LMXB 4U 1636 − 53 over 11 frequency intervals of the QPO and report three important findings: (i) A medium that extends 1–8 km above the NS surface is required to fit the data; this medium can be sustained by the balance between gravity and radiation pressure, without forcing any equilibrium condition. (ii) We predict a time delay between the oscillating NS temperature, due to feedback, and the oscillating electron temperature of the medium, which, with the help of phase resolved spectroscopy, can be used as a probe of the geometry and the feedback mechanism. (iii) We show that the observed variability as a function of QPO frequency is mainly driven by the oscillating electron temperature of the medium. This provides strong evidence that the Comptonizing medium in LMXBs significantly affects, if not completely drives, the radiative properties of the lower kHz QPOs regardless of the nature of the dynamical mechanism that produces the QPO frequencies.
Stars: neutron, X-rays: binaries
0035-8711
1399-1415
Karpouzas, Konstantinos
d796fbbc-2dbb-4244-8c06-ad6466a9abc0
Méndez, Mariano
5cc6f759-dae2-488a-9cc2-cff89f4eca72
Ribeiro, Evandro M
13bb102b-b4c2-461d-b6eb-1cc765be516a
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Blaes, Omer
21ce0f1f-0f30-483e-ad0e-aa22da8b46a2
García, Federico
7c0fed66-dda4-4333-b38c-175f570820ce
Karpouzas, Konstantinos
d796fbbc-2dbb-4244-8c06-ad6466a9abc0
Méndez, Mariano
5cc6f759-dae2-488a-9cc2-cff89f4eca72
Ribeiro, Evandro M
13bb102b-b4c2-461d-b6eb-1cc765be516a
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Blaes, Omer
21ce0f1f-0f30-483e-ad0e-aa22da8b46a2
García, Federico
7c0fed66-dda4-4333-b38c-175f570820ce

Karpouzas, Konstantinos, Méndez, Mariano, Ribeiro, Evandro M, Altamirano, Diego, Blaes, Omer and García, Federico (2020) The Comptonizing medium of the neutron star in 4U 1636 − 53 through its lower kilohertz quasi-periodic oscillations. Monthly Notices of the Royal Astronomical Society, 492 (1), 1399-1415. (doi:10.1093/mnras/stz3502).

Record type: Article

Abstract

Inverse Compton scattering dominates the high-energy part of the spectra of neutron star (NS) low-mass X-ray binaries (LMXBs). It has been proposed that inverse Compton scattering also drives the radiative properties of kilohertz quasi-periodic oscillations (kHz QPOs). In this work, we construct a model that predicts the energy dependence of the rms amplitude and time lag of the kHz QPOs. Using this model, we fit the rms amplitude and time lag energy spectra of the lower kHz QPO in the NS LMXB 4U 1636 − 53 over 11 frequency intervals of the QPO and report three important findings: (i) A medium that extends 1–8 km above the NS surface is required to fit the data; this medium can be sustained by the balance between gravity and radiation pressure, without forcing any equilibrium condition. (ii) We predict a time delay between the oscillating NS temperature, due to feedback, and the oscillating electron temperature of the medium, which, with the help of phase resolved spectroscopy, can be used as a probe of the geometry and the feedback mechanism. (iii) We show that the observed variability as a function of QPO frequency is mainly driven by the oscillating electron temperature of the medium. This provides strong evidence that the Comptonizing medium in LMXBs significantly affects, if not completely drives, the radiative properties of the lower kHz QPOs regardless of the nature of the dynamical mechanism that produces the QPO frequencies.

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stz3502 - Accepted Manuscript
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Accepted/In Press date: 6 December 2019
e-pub ahead of print date: 16 December 2019
Published date: 11 February 2020
Keywords: Stars: neutron, X-rays: binaries

Identifiers

Local EPrints ID: 438163
URI: http://eprints.soton.ac.uk/id/eprint/438163
ISSN: 0035-8711
PURE UUID: d1f64e0e-5b51-40ea-9ac3-866329c73a17
ORCID for Diego Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

Catalogue record

Date deposited: 03 Mar 2020 17:45
Last modified: 18 Feb 2021 17:23

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