The frequency of Kozai–Lidov disc oscillation driven giant outbursts in Be/X-ray binaries

Martin, Rebecca G.; Franchini, Alessia

doi:10.1093/mnras/stz2250

Cited by 18 publications

(6 citation statements)

References 83 publications

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“…Smoothed particle hydrodynamics simulations of Be HMXBs (Martin et al 2014;Martin & Franchini 2019, 2021 and equal-mass Be + B binaries (Suffak et al 2022) indicate that Be discs that are inclined with respect to the orbital plane may get tilted away from the equatorial plane, precess about the orbital axis, and undergo Kozai-Lidov oscillations. These dynamical interactions with the companion act on timescales that are an order of magnitude longer than the orbital period.…”

Section: Doppler Tomographymentioning

confidence: 99%

The X-ray Emission of γ Cassiopeiae During the 2020–2021 disc eruption

Rauw

Nazé

Motch

et al. 2022

A&A

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Context. γ Cas is known for its unusually hard and intense X-ray emission. This emission could trace accretion by a compact companion, wind interaction with a hot sub-dwarf companion, or magnetic interaction between the star and its Be decretion disc. Aims. These various scenarios should lead to diverse dependences of the hard X-ray emission on disc density. To test these scenarios, we collected X-ray observations of γ Cas during an episode of enhanced disc activity that took place around January 2021. Methods. We investigate the variations in the disc properties using time series of dedicated optical spectroscopy and existing broadband photometry. Equivalent widths and peak velocity separations are measured for a number of prominent emission lines. Epochdependent Doppler maps of the Hα, Hβ, and He  λ 5876 emission lines are built to characterise the emission regions in velocity space. We analyse four XMM-Newton observations obtained between January 2021 and January 2022 at key phases of the episode of enhanced disc activity. Archival XMM-Newton, Chandra, MAXI, and RXTE-ASM data are also used to study the long-term correlation between optical and X-ray emission. Results. Optical spectroscopy unveils a clear increase in the radial extent of the emission regions during the episode of enhanced disc activity, whilst no increase in the V-band flux is recorded. Our Doppler maps do not reveal any stable feature in the disc resulting from the putative action of the companion on the outer parts of the Be disc. Whilst the hard X-ray emission is found to display the usual level and type of variability, no specific increase in the hard emission is observed in relation to the enhanced disc activity. However, at two occasions, including at the maximum disc activity, the soft X-ray emission of γ Cas is strongly attenuated, suggesting more efficient obscuration by material from a large flaring Be disc. In addition, there is a strong correlation between the long-term variations in the X-ray flux and the optical variations in the V-band photometry. Conclusions. The observed behaviour of γ Cas suggests no direct link between the properties of the outer regions of the Be disc and the hard X-ray emission, but it favours a link between the level of X-ray emission and the properties of the inner part of the Be disc. These results thus disfavour an accretion or colliding wind scenario.

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Section: Doppler Tomographymentioning

confidence: 99%

The X-ray Emission of γ Cassiopeiae During the 2020–2021 disc eruption

Rauw

Nazé

Motch

et al. 2022

A&A

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“…Current models that explain giant (type II) X-ray outbursts in BeXBs are based on the idea that these outbursts occur when the neutron star captures a large amount of gas from a warped (Martin et al 2011;Okazaki et al 2013;Moritani et al 2013) or a highly eccentric disk (Martin et al 2014). The warping of the disk may be caused by the tidal interaction with the neutron star (Martin et al 2011) or by radiation from the central star (Porter 1998), whereas the main mechanism that has been identified to produce eccentricity growth is Kozai-Lidov oscillations (Martin & Franchini 2019, but see also Martin et al 2014). In either case, the unavoidable condition for these two processes (warping and eccentricity growth) to work is the misalignment of the decretion disk.…”

Section: Giant X-ray Outburstmentioning

confidence: 99%

Optical counterpart to Swift J0243.6+6124

Reig

Fabregat

Alfonso-Garzón

2020

A&A

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Context. Swift J0243.6+6124 is a unique system. It is the first and only ultra-luminous X-ray source in our Galaxy. It is the first and only high-mass Be X-ray pulsar showing radio jet emission. It was discovered during a giant X-ray outburst in October 2017. While there are numerous studies in the X-ray band, very little is known about the optical counterpart. Aims. Our aim is to characterize the variability timescales in the optical and infrared bands in order to understand the nature of this intriguing system. Methods. We performed optical spectroscopic observations to determine the spectral type. Long-term photometric light curves together with the equivalent width of the Hα line were used to monitor the state of the circumstellar disk. We used BVRI photometry to estimate the interstellar absorption and distance to the source. Continuous photometric monitoring in the B and V bands allowed us to search for intra-night variability. Results. The optical counterpart to Swift J0243.6+6124 is a V = 12.9, O9.5Ve star, located at a distance of ∼5 kpc. The optical extinction in the direction of the source is AV = 3.6 mag. The rotational velocity of the O-type star is 210 km s−1. The long-term optical variability agrees with the growth and subsequent dissipation of the Be circumstellar disk after the giant X-ray outburst. The optical and X-ray luminosity are strongly correlated during the outburst, suggesting a common origin. We did not detect short-term periodic variability that could be associated with nonradial pulsations from the Be star photosphere. Conclusions. The long-term optical and infrared pattern of variability of Swift J0243.6+6124 is typical of Be/X-ray binaries. However, the absence of nonradial pulsations is unusual and adds another peculiar trait to this unique source.

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“…Using the parameters of our simulation and the azimuthally averaged surface density at the start of disc dissipation, we calculate the KL timescales should be 2.7 orb from Equation 6, and 33 orb according to Equation 7. Martin & Franchini (2019) find that a test particle with an initial eccentricity of 0.2 has a KL timescale 2.7 times shorter than what is analytically predicted. This would bring the prediction of Equation 7 to 12.2 orb .…”

Section: Day 60°simulationmentioning

confidence: 56%

Growth and Dissipation of Be Star Discs in Misaligned Binary Systems

Suffak,

Jones,

Carciofi

2021

Preprint

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We use a three-dimensional smoothed particle hydrodynamics code to simulate growth and dissipation of Be star discs in systems where the binary orbit is misaligned with respect to the spin axis of the primary star. We investigate six different scenarios of varying orbital period and misalignment angle, feeding the disc at a constant rate for 100 orbital periods, and then letting the disc dissipate for 100 orbital periods. During the disc growth phase, we find that the binary companion tilts the disc away from its initial plane at the equator of the primary star before settling to a constant orientation after 40 to 50 orbital periods. While the mass-injection into the disc is ongoing, the tilting of the disc can cause material to reaccrete onto the primary star prematurely. Once disc dissipation begins, usually the disc precesses about the binary companion's orbital axis with precession periods ranging from 20 to 50 orbital periods. In special cases we detect phenomena of disc tearing, as well as Kozai-Lidov oscillations of the disc. These oscillations reach a maximum eccentricity of about 0.6, and a minimum inclination of about 20°w ith respect to the binary's orbit. We also find the disc material to have highly eccentric orbits beyond the transition radius, where the disc changes from being dominated by viscous forces, to heavily controlled by the companion star, in contrast to its nearly circular motion inward of the transition radius. Finally, we offer predictions to how these changes will affect Be star observables.

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The frequency of Kozai–Lidov disc oscillation driven giant outbursts in Be/X-ray binaries

Cited by 18 publications

References 83 publications

The X-ray Emission of γ Cassiopeiae During the 2020–2021 disc eruption

The X-ray Emission of γ Cassiopeiae During the 2020–2021 disc eruption

Optical counterpart to Swift J0243.6+6124

Growth and Dissipation of Be Star Discs in Misaligned Binary Systems

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