Andrew King

Radio pulse search and X-Ray monitoring of SAX J1808.4-3658: What causes its orbital evolution?

The accreting millisecond X-ray pulsar SAX J1808.4‑3658 shows a peculiar orbital evolution that proceeds at a very fast pace. It is important to identify the underlying mechanism responsible for this behavior because it can help to understand how this system evolves and which physical processes (such as mass loss or spin–orbit coupling) are occurring in the binary. It has also been suggested that, when in quiescence, SAX J1808.4‑3658 turns on as a radio pulsar, a circumstance that might provide a link between accreting millisecond pulsars and black-widow (BW) radio pulsars. In this work, we report the results of a deep radio pulsation search at 2 GHz using the Green Bank Telescope in 2014 August and an X-ray study of the 2015 outburst with Chandra, Swift XRT, and INTEGRAL. In quiescence, we detect no radio pulsations and place the strongest limit to date on the pulsed radio flux density of any accreting millisecond pulsar. We also find that the orbit of SAX J1808.4‑3658 continues evolving at a fast pace. We compare the orbital evolution of SAX J1808.4‑3658 to that of several other accreting and nonaccreting binaries, including BWs, redbacks, cataclysmic variables, black holes, and neutron stars in low-mass X-ray binaries. We discuss two possible scenarios: either the neutron star has a large moment of inertia and is ablating the donor, generating mass loss with an efficiency of 40%, or the donor star has a strong magnetic field of at least 1 kG and is undergoing quasi-cyclic variations due to spin–orbit coupling.

Electromagnetic Signals Following Stellar-mass Black Hole Mergers

S.d.M. acknowledges support by a Marie Sklodowska-Curie Action (H2020 MSCA-IF-2014, project id 661502) and National Science Foundation under Grant No. NSF PHY11-25915. Astrophysics research at the University of Leicester is supported by an STFC Consolidated Grant.

The small observed scale of AGN-driven outflows, and inside-out disc quenching

KZ is funded by the Research Council Lithuania through the National nScience Programme grant no. LAT-09/2016. Astrophysics nresearch at the University of Leicester is funded by an STFC Consolidated ngrant.

The Maximum Mass Solar Nebula and the early formation of planets

Current planet formation theories provide successful frameworks with which to interpret the array of new observational data in this field. However, each of the two main theories (core accretion, gravitational instability) is unable to explain some key aspects. In many planet formation calculations, it is usual to treat the initial properties of the planet-forming disc (mass, radius, etc.) as free parameters. In this paper, we stress the importance of setting the formation of planet-forming discs within the context of the formation of the central stars. By exploring the early stages of disc formation, we introduce the concept of the Maximum Mass Solar Nebula, as opposed to the oft-used minimum mass solar nebula. It is evident that almost all protoplanetary discs start their evolution in a strongly self-gravitating state. In agreement with almost all previous work in this area, we conclude that on the scales relevant to planet formation these discs are not gravitationally unstable to gas fragmentation, but instead form strong, transient spiral arms. These spiral arms can act as efficient dust traps allowing the accumulation and subsequent fragmentation of the dust (but not the gas). This phase is likely to populate the disc with relatively large planetesimals on short time-scales while the disc is still veiled by a dusty-gas envelope. Crucially, the early formation of large planetesimals overcomes the main barriers remaining within the core accretion model. A prediction of this picture is that essentially all observable protoplanetary discs are already planet hosting.

Black holes in stellar-mass binary systems: expiating original spin?

We investigate systematically whether accreting black hole systems are likely to reach global alignment of the black hole spin and its accretion disc with the binary plane. In low-mass X-ray binaries (LMXBs), there is only a modest tendency to reach such global alignment, and it is difficult to achieve fully: except for special initial conditions, we expect misalignment of the spin and orbital planes by ∼1 rad for most of the LMXB lifetime. The same is expected in high-mass X-ray binaries. A fairly close approach to global alignment is likely in most stellar-mass ultraluminous X-ray binary systems (ULXs) where the companion star fills its Roche lobe and transfers mass on a thermal or nuclear time-scale to a black hole of lower mass. These systems are unlikely to show orbital eclipses, as their emission cones are close to the holes spin axis. This offers a potential observational test, as models for ULXs invoking intermediate-mass black holes do predict eclipses for ensembles of ≳ 10 systems. Recent observational work shows that eclipses are either absent or extremely rare in ULXs, supporting the picture that most ULXs are stellar-mass binaries with companion stars more massive than the accretor.

The origin of the structure of large-scale magnetic fields in disc galaxies

The large-scale magnetic fields observed in spiral disc galaxies are often thought to result from dynamo action in the disc plane. However, the increasing importance of Faraday depolarization along any line of sight towards the galactic plane suggests that the strongest polarization signal may come from well above (∼0.3–1u2009kpc) this plane, from the vicinity of the warm interstellar medium (WIM)/halo interface. We propose (see also Henriksen & Irwin 2016) that the observed spiral fields (polarization patterns) result from the action of vertical shear on an initially poloidal field. We show that this simple model accounts for the main observed properties of large-scale fields. We speculate as to how current models of optical spiral structure may generate the observed arm/interarm spiral polarization patterns.

Geometrical beaming of stellar mass ULXs

MJM appreciates support from an Ernest nRutherford STFC fellowship. This work is based on observations nobtained with XMM–Newton, an ESA science mission with instruments nand contributions directly funded by ESA Member States nand NASA

Instability of warped discs

Accretion discs are generally warped. If a warp in a disc is too large, the disc can `break apart into two or more distinct planes, with only tenuous connections between them. Further, if an initially planar disc is subject to a strong differential precession, then it can be torn apart into discrete annuli that precess effectively independently. In previous investigations, torque-balance formulae have been used to predict where and when the disc breaks into distinct parts. In this work, focusing on discs with Keplerian rotation and where the shearing motions driving the radial communication of the warp are damped locally by turbulence (the `diffusive regime), we investigate the stability of warped discs to determine the precise criterion for an isolated warped disc to break. We find and solve the dispersion relation, which, in general, yields three roots. We provide a comprehensive analysis of this viscous-warp instability and the emergent growth rates and their dependence on disc parameters. The physics of the instability can be understood as a combination of (1) a term that would generally encapsulate the classical Lightman-Eardley instability in planar discs (given by ∂(νΣ)/∂Σ <0) but is here modified by the warp to include ∂(ν1|ψ|)/∂|ψ| <0, and (2) a similar condition acting on the diffusion of the warp amplitude given in simplified form by ∂(ν2|ψ|)/∂|ψ| <0. We discuss our findings in the context of discs with an imposed precession, and comment on the implications for different astrophysical systems.

An ultra-fast inflow in the luminous Seyfert PG1211+143

Blueshifted absorption lines in the X-ray spectra of an active galactic nucleus (AGN) show that ultrafast outflows with typical velocities v ∼ 0.1c are a common feature of these luminous objects. Such powerful AGN winds offer an explanation of the observed M–σ relation linking the mass of the supermassive black hole and the velocity dispersion in the galaxy’s stellar bulge. An extended XMM–Newton study of the luminous Seyfert galaxy PG1211+143 recently revealed a variable multivelocity wind. Here we report the detection of a short-lived, ultrafast inflow during the same observation. Previous reports of inflows used single absorption lines with uncertain identifications, but this new result identifies an array of resonance absorption lines of highly ionized Fe, Ca, Ar, S, and Si, sharing a common redshift when compared with a grid of realistic photoionization spectra. The redshifted absorption arises in a column of highly ionized matter close to the black hole, with a line-of-sight velocity, v ∼ 0.3c, inconsistent with the standard picture of a plane circular accretion disc. This may represent the first direct evidence for chaotic accretion in an AGN, where accretion discs are generally misaligned to the black hole spin. For sufficient inclinations, the Lense–Thirring effect can break the discs into discrete rings, which then precess, collide, and shock, causing near free-fall of gas towards the black hole. The observed accretion rate for the reported infall is comparable to the hard X-ray luminosity in PG1211+143, suggesting that direct infall may be a significant contributor to inner disc accretion.

Circumbinary discs around merging stellar-mass black holes

A circumbinary disc around a pair of merging stellar-mass black holes may be shocked and heated during the recoil of the merged hole, causing a near-simultaneous electromagnetic counterpart to the gravitational wave event. The shocks occur around the recoil radius, where the disc orbital velocity is equal to the recoil velocity. The amount of mass present near this radius at the time of the merger is critical in determining how much radiation is released. We explore the evolution of a circumbinary disc in two limits. First, we consider an accretion disc that feels no torque from the binary. The disc does not survive until the merger unless there is a dead zone, a region of low turbulence. Even with the dead zone, the surface density in this case may be small. Secondly, we consider a disc that feels a strong binary torque that prevents accretion on to the binary. In this case there is significantly more mass in regions of interest at the time of the merger. A dead zone in this disc increases the mass close to the recoil radius. For typical binary-disc parameters we expect accretion to be significantly slowed by the resonant torque from the binary, and for a dead zone to be present. We conclude that provided significant mass orbits the binary after the formation of the black hole binary and that the radiation produced in recoil shocks can escape the flow efficiently, there is likely to be an observable electromagnetic signal from black hole binary merger