The Lense–Thirring timing-accretion plane for ULXs

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 magnetised neutron star. However, pulsations are observed to be transient and it is plausible that accretion onto 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 timescale periods/QPOs are associated with Lense-Thirring precession of the inflow and outflowing wind respectively. The precession timescales 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 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.

The impact of precession on the observed population of ULXs

“…The precession of an accretion disc may be driven by a variety of external torques, including tidal effects, radiation pressure driven instabilities, Lense-Thirring precession, magnetic warping and free-free precession (Fragile et al 2007;Maloney & Begelman 1997;Maloney et al 1998;Pringle 1996;Lei et al 2013). Lense-Thirring (solid-body) precession of the large scale-height disc and wind has recently been proposed as the driving mechanism for the modulations (Middleton et al 2018(Middleton et al , 2019b and is somewhat compelling as it requires a misaligned spin and binary axis, the same requirement for the detection of pulsations (King & Lasota 2020).…”

Periodic Fast Radio Bursts from Luminous X-ray Binaries

“…Another source of periodicity can arise due to precession of the accretion funnel along which the FRB is beamed (see Section 2.3, and Figures 1 and 5). If the spin axis of the accreting BH or NS is misaligned with the angular momentum axis of the disk, then the Lens-Thirring (LT) torque applied by the rotating spacetime on the disk may cause the latter to precess (e.g., Middleton et al 2019). Numerical simulations have shown that for thick disks (with vertical aspect ratio h/r  0.05 and low effective α viscosity), the warp propagation timescale is shorter than the differential precession timescale, thereby allowing them to precess as rigid bodies with negligible warping (Fragner & Nelson 2010).…”

Swift Monitoring of M51: A 38 day Superorbital Period for the Pulsar ULX7 and a New Transient Ultraluminous X-Ray Source

“…Middleton et al (2018) suggest that the precession can be explained by the Lense-Thirring effect, a consequence of general relativity whereby a massive spinning object induces a precession of orbiting particles that are displaced vertically from the rotation axis, for example, a large-scale height accretion flow. Indeed, Middleton et al (2019) suggest that these timescale periodicities can be used to determine whether the compact object is a black hole or a NS.…”