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The Lense-Thirring timing-accretion plane for ULXs

The Lense-Thirring timing-accretion plane for ULXs
The Lense-Thirring timing-accretion plane for ULXs
Identifying the compact object in ultraluminous X-ray sources (ULXs) has to date required detection of pulsations or a cyclotron resonance scattering feature (CRSF), indicating a magnetized neutron star. However, pulsations are observed to be transient and it is plausible that accretion on to the neutron star may have suppressed the surface magnetic field such that pulsations and CRSFs will be entirely absent. We may therefore lack direct means to identify neutron star systems whilst we presently lack an effective means by which to identify black hole ULXs. Here we present a possible method for separating the ULX population by assuming the X-ray, mHz quasi-periodic oscillations (QPOs), and day time-scale periods/QPOs are associated with Lense–Thirring precession of the inflow and outflowing wind, respectively. The precession time-scales combined with the temperature of the soft X-ray component produce planes where the accretor mass enters as a free parameter. Depending on the properties of the wind, use of these planes may be robust to a range in the angular momentum (spin) and, for high accretion rates, essentially independent of the neutron star’s surface dipole field strength. Our model also predicts the mHz QPO frequency and magnitude of the phase lag imprinted due to propagation through the optically thick wind; in the case of NGC 5408 X-1 we subsequently infer a black hole mass and moderate-to-high spin. Finally, we note that observing secular QPO evolution over sufficient baselines may indicate a neutron star, as the precession responds to spin-up which is not readily observable for black hole primaries.
0035-8711
282-296
Middleton, M.J.
f91b89d9-fd2e-42ec-aa99-1249f08a52ad
Fragile, P.C.
498628bc-53fd-4094-b60a-69faf2b698c9
Ingram, A.
01a02529-ad9f-4936-af5d-c200f88d4e53
Roberts, T.P.
4a31f81a-507d-45ea-ada9-ee3aa21ebb7d
Middleton, M.J.
f91b89d9-fd2e-42ec-aa99-1249f08a52ad
Fragile, P.C.
498628bc-53fd-4094-b60a-69faf2b698c9
Ingram, A.
01a02529-ad9f-4936-af5d-c200f88d4e53
Roberts, T.P.
4a31f81a-507d-45ea-ada9-ee3aa21ebb7d

Middleton, M.J., Fragile, P.C., Ingram, A. and Roberts, T.P. (2019) The Lense-Thirring timing-accretion plane for ULXs. Monthly Notices of the Royal Astronomical Society, 489 (1), 282-296. (doi:10.1093/mnras/stz2005).

Record type: Article

Abstract

Identifying the compact object in ultraluminous X-ray sources (ULXs) has to date required detection of pulsations or a cyclotron resonance scattering feature (CRSF), indicating a magnetized neutron star. However, pulsations are observed to be transient and it is plausible that accretion on to the neutron star may have suppressed the surface magnetic field such that pulsations and CRSFs will be entirely absent. We may therefore lack direct means to identify neutron star systems whilst we presently lack an effective means by which to identify black hole ULXs. Here we present a possible method for separating the ULX population by assuming the X-ray, mHz quasi-periodic oscillations (QPOs), and day time-scale periods/QPOs are associated with Lense–Thirring precession of the inflow and outflowing wind, respectively. The precession time-scales combined with the temperature of the soft X-ray component produce planes where the accretor mass enters as a free parameter. Depending on the properties of the wind, use of these planes may be robust to a range in the angular momentum (spin) and, for high accretion rates, essentially independent of the neutron star’s surface dipole field strength. Our model also predicts the mHz QPO frequency and magnitude of the phase lag imprinted due to propagation through the optically thick wind; in the case of NGC 5408 X-1 we subsequently infer a black hole mass and moderate-to-high spin. Finally, we note that observing secular QPO evolution over sufficient baselines may indicate a neutron star, as the precession responds to spin-up which is not readily observable for black hole primaries.

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Accepted/In Press date: 5 July 2019
e-pub ahead of print date: 25 July 2019
Published date: October 2019

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Local EPrints ID: 434119
URI: http://eprints.soton.ac.uk/id/eprint/434119
ISSN: 0035-8711
PURE UUID: ff368af2-b144-4cfb-824f-db562fd6aab6

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Date deposited: 13 Sep 2019 16:30
Last modified: 06 Oct 2020 19:14

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Contributors

Author: M.J. Middleton
Author: P.C. Fragile
Author: A. Ingram
Author: T.P. Roberts

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