arXiv subjects

Chris Nixon

Publications and source records attributed to Chris Nixon.

At least 19 recordsLinked to original sources

arXiv2026

On the origin of anomalous dissipation in simulations of tidal disruption events

In a tidal disruption event (TDE), a star is destroyed by the tidal field of a supermassive black hole. The stellar debris is initially placed on highly elliptical orbits, and a longstanding question in TDE theory is: How does the stellar debris circularize into a disc and accrete? The originally proposed answer to this question is self-intersection shocks, where relativistic apsidal precession results in a strong collision between the incoming and outgoing material. However, global simulations of TDEs tend to find enhanced hydrodynamical dissipation prior to any intersections of the debris orbits, with the material ``fanning out'' into a wide-angle and partially-unbound outflow upon passing through pericenter. We show that this dissipation is numerical in origin and arises from a combination of 1) the change in the kinematics of the debris as it passes through pericenter, with its velocity profile along the stream transitioning from strongly diverging pre-pericenter to strongly converging post-pericenter, and 2) the dependence of numerical algorithms (viscosity switches for particle-based methods and Riemann solvers for Godunov-based schemes) on the diverging vs. converging nature of the fluid. We support this conclusion with analytical and numerical modeling. We discuss possible resolutions to these issues as well as the implications of our findings in the context of observations.

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arXiv2026

On the isotropy of viscosity in accretion discs

Accretion discs are fundamental to many astrophysical systems, providing the conversion of gravitational potential energy into radiation that we can observe. In many systems there is evidence that discs are warped; from spatially-resolved observations of protoplanetary discs, to the features of lightcurves and line profiles from discs around supermassive black holes in galaxy centres. The dynamics of warped discs is largely controlled by the physical nature of the internal disc viscosity. While typically disc viscosity is hydromagnetic in origin, simulations of magnetized discs cannot match observed rates of angular momentum transport in planar discs and thus cannot be used to determine the ratio of the torques responsible for driving accretion to those responsible for evolving the disc warp. The analytic work of Ogilvie is the most comprehensive model for warped disc evolution, but makes assumptions that need to be tested. In particular, it assumes that the disc viscosity is Navier-Stokes, and therefore small-scale and isotropic. Here we attempt to test this model using the long periods of X-ray binaries that are due to precession of the disc. These systems have well-constrained estimates of the component of viscosity responsible for driving accretion, and by looking at systems with and without evidence for disc misalignment and precession we can constrain the component of viscosity responsible for flattening the disc. We conclude that the observational constraints suggest that the Ogilvie model provides an adequate description of the disc evolution, but that there are indications that the internal disc viscosity might be marginally non-isotropic.

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arXiv2025

The Double Tidal Disruption Event AT 2022dbl Implies That at Least Some "Standard" Optical TDEs are Partial Disruptions

Flares produced following the tidal disruption of stars by supermassive black holes can reveal the properties of the otherwise dormant majority of black holes and the physics of accretion. In the past decade, a class of optical-ultraviolet tidal disruption flares has been discovered whose emission properties do not match theoretical predictions. This has led to extensive efforts to model the dynamics and emission mechanisms of optical-ultraviolet tidal disruptions in order to establish them as probes of supermassive black holes. Here we present the optical-ultraviolet tidal disruption event AT 2022dbl, which showed a nearly identical repetition 700 days after the first flare. Ruling out gravitational lensing and two chance unrelated disruptions, we conclude that at least the first flare represents the partial disruption of a star, possibly captured through the Hills mechanism. Since both flares are typical of the optical-ultraviolet class of tidal disruptions in terms of their radiated energy, temperature, luminosity, and spectral features, it follows that either the entire class are partial rather than full stellar disruptions, contrary to the prevalent assumption, or that some members of the class are partial disruptions, having nearly the same observational characteristics as full disruptions. Whichever option is true, these findings could require revised models for the emission mechanisms of optical-ultraviolet tidal disruption flares and a reassessment of their expected rates.

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arXiv2024

Repeated Partial Tidal Disruptions and Quasi-Periodic Eruptions in SwJ023017.0+283603

SwJ023017.0+283603 (SwJ0230) exhibited soft X-ray (0.3-1.0 keV) eruptions recurring roughly every 22 days. We present results from an extended monitoring campaign of SwJ0230 using Swift, NICER, and deep XMM-Newton observations. Our main findings are: 1) SwJ0230 did not display any eruptions during two 80-day periods (June-September 2023 and July-September 2024) of high-cadence monitoring with NICER and Swift, suggesting that the eruptions have ceased, implying an eruption lifetime of less than 536 days; 2) quiescent/non-eruption emission is detected with XMM-Newton, with a 0.3-2.0 keV luminosity of 4$\times$10$^{40}$ erg/s (bolometric luminosity of $<$0.1% Eddington assuming a black hole mass of 10$^{6-7}$ M$_{\odot}$), that is consistent with a thermal disk spectrum peaking at 0.11$^{+0.06}_{-0.03}$ keV; 3) SwJ0230 exhibited multiple, rapid eruptions (duration$<$5 hours, similar to quasi-periodic eruptions; QPEs), and there is tentative evidence that they recur, on average, on roughly the same timescale of 22 days. \target therefore exhibited (when active) both rapid, QPE-like outbursts and longer-duration outbursts, more akin to those from repeating partial Tidal Disruption Event (rpTDE) candidates. These findings are difficult to explain with existing models that invoke an orbiter interacting with a persistent disk and those involving disk instabilities. We propose a hybrid model wherein an object of smaller mass (e.g., a Jupiter-sized planet) being repeatedly partially stripped and subsequently punching through its own, fallback-induced disk, can explain many of the observed properties, including the long-duration flares (from accretion), the short-duration outbursts (from the planet-disk interaction), and the turn-off of the flares (when the planet is totally stripped of gas).

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arXiv2024

A Potential Second Shutoff from AT2018fyk: An updated Orbital Ephemeris of the Surviving Star under the Repeating Partial Tidal Disruption Event Paradigm

The tidal disruption event (TDE) AT2018dyk/ASASSN-18UL showed a rapid dimming event 500 days after discovery, followed by a re-brightening roughly 700 days later. It has been hypothesized that this behavior results from a repeating partial TDE (rpTDE), such that prompt dimmings/shutoffs are coincident with the return of the star to pericenter and rebrightenings generated by the renewed supply of tidally stripped debris. This model predicted that the emission should shut off again around August of 2023. We report AT2018fyk's continued X-ray and UV monitoring, which shows an X-ray (UV) drop in flux by a factor of 10 (5) over a span of two months, starting 14 Aug 2023. This sudden change can be interpreted as the second emission shutoff, which 1) strengthens the rpTDE scenario for AT2018fyk, 2) allows us to constrain the orbital period to a more precise value of 1306$\pm$47 days, and 3) establishes that X-ray and UV/optical emission track the fallback rate onto this SMBH -- an often-made assumption that otherwise lacks observational verification -- and therefore the UV/optical lightcurve is powered predominantly by processes tied to X-rays. The second cutoff implies that another rebrightening should happen between May-Aug 2025, and if the star survived the second encounter, a third shutoff is predicted to occur between Jan-July 2027. Finally, low-level accretion from the less bound debris tail (which is completely unbound/does not contribute to accretion in a non-repeating TDE) can result in a faint X-ray plateau that could be detectable until the next rebrightening.

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arXiv2023

On the relative importance of shocks and self-gravity in modifying tidal disruption event debris streams

In a tidal disruption event (TDE), a star is destroyed by the gravitational field of a supermassive black hole (SMBH) to produce a stream of debris, some of which accretes onto the SMBH and creates a luminous flare. The distribution of mass along the stream has a direct impact on the accretion rate, and thus modeling the time-dependent evolution of this distribution provides insight into the relevant physical processes that drive the observable properties of TDEs. Analytic models that only account for the ballistic evolution of the debris do not capture salient and time-dependent features of the mass distribution, suggesting that fluid dynamical effects significantly modify the debris dynamics. Previous investigations have claimed that shocks are primarily responsible for these modifications, but here we show -- with high-resolution hydrodynamical simulations -- that self-gravity is the dominant physical mechanism responsible for the anomalous (i.e., not predicted by ballistic models) debris stream features and its time dependence. These high-resolution simulations also show that there is a specific length scale on which self-gravity modifies the debris mass distribution, and as such there is enhanced power in specific Fourier modes. Our results have implications for the stability of the debris stream under the influence of self-gravity, particularly at late times and the corresponding observational signatures of TDEs.

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arXiv2022

A simple and accurate prescription for the tidal disruption radius of a star and the peak accretion rate in tidal disruption events

A star destroyed by a supermassive black hole (SMBH) in a tidal disruption event (TDE) enables the study of SMBHs. We propose that the distance within which a star is completely destroyed by a SMBH, defined $r_{\rm t, c}$, is accurately estimated by equating the SMBH tidal field (including numerical factors) to the maximum gravitational field in the star. We demonstrate that this definition accurately reproduces the critical $β_{\rm c} = r_{\rm t}/r_{\rm t, c}$, where $r_{\rm t} = R_{\star}\left(M_{\bullet}/M_{\star}\right)^{1/3}$ is the standard tidal radius with $R_{\star}$ and $M_{\star}$ the stellar radius and mass and $M_{\bullet}$ the SMBH mass, for multiple stellar progenitors at various ages, and can be reasonably approximated by $β_{\rm c} \simeq \left[ρ_{\rm c}/(4ρ_{\star})\right]^{1/3}$, where $ρ_{\rm c}$ ($ρ_{\star}$) is the central (average) stellar density. We also calculate the peak fallback rate and time at which the fallback rate peaks, finding excellent agreement with hydrodynamical simulations, and also suggest that the partial disruption radius -- the distance at which any mass is successfully liberated from the star -- is $β_{\rm partial} \simeq 4^{-1/3} \simeq 0.6$. For given stellar and SMBH populations, this model yields, e.g., the fraction of partial TDEs, the peak luminosity distribution of TDEs, and the number of directly captured stars.

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arXiv2022

On the impact of relativistic gravity on the rate of tidal disruption events

The tidal disruption of stars by supermassive black holes (SMBHs) probes relativistic gravity. In the coming decade, the number of observed tidal disruption events (TDEs) will grow by several orders of magnitude, allowing statistical inferences of the properties of the SMBH and stellar populations. Here we analyse the probability distribution functions of the pericentre distances of stars that encounter an SMBH in the Schwarzschild geometry, where the results are completely analytic, and the Kerr metric. From this analysis we calculate the number of observable TDEs, defined to be those that come within the tidal radius $r_{\rm t}$ but outside the direct capture radius (which is, in general, larger than the horizon radius). We find that relativistic effects result in a steep decline in the number of stars that have pericenter distances $r_{\rm p} \lesssim 10\,r_{\rm g}$, where $r_{\rm g} = GM/c^2$, and that for maximally spinning SMBHs the distribution function of $r_{\rm p}$ at such distances scales as $f_{\rm r_{\rm p}}\propto r_{\rm p}^{4/3}$, or in terms of $β\equiv r_{\rm t}/r_{\rm p}$ scales as $f_β \propto β^{-10/3}$. We find that spin has little effect on the TDE fraction until the very high-mass end, where instead of being identically zero the rate is small ($\lesssim 1\%$ of the expected rate in the absence of relativistic effects). Effectively independent of spin, if the progenitors of TDEs reflect the predominantly low-mass stellar population and thus have masses $\lesssim 1M_{\odot}$, we expect a substantial reduction in the rate of TDEs above $10^{7}M_{\odot}$.

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arXiv2022

Stellar Revival and Repeated Flares in Deeply Plunging Tidal Disruption Events

Tidal disruption events with tidal radius $r_{\rm t}$ and pericenter distance $r_{\rm p}$ are characterized by the quantity $β= r_{\rm t}/r_{\rm p}$, and "deep encounters" have $β\gg 1$. It has been assumed that there is a critical $β\equiv β_{\rm c} \sim 1$ that differentiates between partial and full disruption: for $β< β_{\rm c}$ a fraction of the star survives the tidal interaction with the black hole, while for $β> β_{\rm c}$ the star is completely destroyed, and hence all deep encounters should be full. Here we show that this assumption is incorrect by providing an example of a $β= 16$ encounter between a $γ= 5/3$, solar-like polytrope and a $10^6 M_{\odot}$ black hole -- for which previous investigations have found $β_{\rm c} \simeq 0.9$ -- that results in the reformation of a stellar core post-disruption that comprises approximately 25\% of the original stellar mass. We propose that the core reforms under self-gravity, which remains important because of the compression of the gas both near pericenter, where the compression occurs out of the orbital plane, and substantially after pericenter, where compression is within the plane. We find that the core forms on a bound orbit about the black hole, and we discuss the corresponding implications of our findings in the context of recently observed, repeating nuclear transients.

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arXiv2021

Stars Crushed by Black Holes. I. On the Energy Distribution of Stellar Debris in Tidal Disruption Events

The distribution of orbital energies imparted into stellar debris following the close encounter of a star with a supermassive black hole is the principal factor in determining the rate of return of debris to the black hole, and thus in determining the properties of the resulting lightcurves from such events. We present simulations of tidal disruption events for a range of $β\equiv r_{\rm t}/r_{\rm p}$ where $r_{\rm p}$ is the pericentre distance and $r_{\rm t}$ the tidal radius. We perform these simulations at different spatial resolutions to determine the numerical convergence of our models. We compare simulations in which the heating due to shocks is included or excluded from the dynamics. For $β\lesssim 8$ the simulation results are well-converged at sufficiently moderate-to-high spatial resolution, while for $β\gtrsim 8$ the breadth of the energy distribution can be grossly exaggerated by insufficient spatial resolution. We find that shock heating plays a non-negligible role only for $β\gtrsim 4$, and that typically the effect of shock heating is mild. We show that self-gravity can modify the energy distribution over time after the debris has receded to large distances for all $β$. Primarily, our results show that across a range of impact parameters, while the shape of the energy distribution varies with $β$, the width of the energy spread imparted to the bulk of the debris is closely matched to the canonical spread, $ΔE = GM_\bullet R_\star/r_{\rm t}^2$, for the range of $β$ we have simulated.

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arXiv2021

Stars Crushed by Black Holes. II. A Physical Model of Adiabatic Compression and Shock Formation in Tidal Disruption Events

We develop a Newtonian model of a deep tidal disruption event (TDE), for which the pericenter distance of the star, $r_{\rm p}$, is well within the tidal radius of the black hole, $r_{\rm t}$, i.e., when $β\equiv r_{\rm t}/r_{\rm p} \gg 1$. We find that shocks form for $β\gtrsim 3$, but they are weak (with Mach numbers $\sim 1$) for all $β$, and that they reach the center of the star prior to the time of maximum adiabatic compression for $β\gtrsim 10$. The maximum density and temperature reached during the TDE follow much shallower relations with $β$ than the previously predicted $ρ_{\rm max} \propto β^3$ and $T_{\rm max} \propto β^2$ scalings. Below $β\simeq 10$, this shallower dependence occurs because the pressure gradient is dynamically significant before the pressure is comparable to the ram pressure of the freefalling gas, while above $β\simeq 10$ we find that shocks prematurely halt the compression and yield the scalings $ρ_{\rm max} \propto β^{1.62}$ and $T_{\rm max} \propto β^{1.12}$. We find excellent agreement between our results and high-resolution simulations. Our results demonstrate that, in the Newtonian limit, the compression experienced by the star is completely independent of the mass of the black hole. We discuss our results in the context of existing (affine) models, polytropic vs.~non-polytropic stars, and general relativistic effects, which become important when the pericenter of the star nears the direct capture radius, at $β\sim 12.5$ (2.7) for a solar-like star disrupted by a $10^6M_{\odot}$ ($10^{7}M_{\odot}$) supermassive black hole.

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arXiv2021

Partial, zombie, and full tidal disruption of stars by supermassive black holes

We present long-duration numerical simulations of the tidal disruption of stars modelled with accurate stellar structures and spanning a range of pericentre distances, corresponding to cases where the stars are partially and completely disrupted. We substantiate the prediction that the late-time power-law index of the fallback rate $n_{\infty} \simeq -5/3$ for full disruptions, while for partial disruptions---in which the central part of the star survives the tidal encounter intact---we show that $n_{\infty} \simeq -9/4$. For the subset of simulations where the pericenter distance is close to that which delineates full from partial disruption, we find that a stellar core can reform after the star has been completely destroyed; for these events the energy of the zombie core is slightly positive, which results in late-time evolution from $n \simeq -9/4$ to $n \simeq -5/3$. We find that self-gravity can generate an $n(t)$ that deviates from $n_{\infty}$ by a small but significant amount for several years post-disruption. In one specific case with the stellar pericenter near the critical value, we find self-gravity also drives the re-collapse of the central regions of the debris stream into a collection of several cores while the rest of the stream remains relatively smooth. We also show that it is possible for the surviving stellar core in a partial disruption to acquire a circumstellar disc that is shed from the rapidly rotating core. Finally, we provide a novel analytical fitting function for the fallback rates that may also be useful in a range of contexts beyond TDEs.

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arXiv2021

Disc tearing: numerical investigation of warped disc instability

We present numerical simulations of misaligned discs around a spinning black hole covering a range of parameters. Previous simulations have shown that discs that are strongly warped by a forced precession -- in this case the Lense-Thirring effect from the spinning black hole -- can break apart into discrete discs or rings that can behave quasi-independently for short timescales. With the simulations we present here, we confirm that thin and highly inclined discs are more susceptible to disc tearing than thicker or low inclination discs, and we show that lower values of the disc viscosity parameter lead to instability at lower warp amplitudes. This is consistent with detailed stability analysis of the warped disc equations. We find that the growth rates of the instability seen in the numerical simulations are similar across a broad range of parameters, and are of the same order as the predicted growth rates. However, we did not find the expected trend of growth rates with viscosity parameter. This may indicate that the growth rates are affected by numerical resolution, or that the wavelength of the fastest growing mode is a function of local disc parameters. Finally, we also find that disc tearing can occur for discs with a viscosity parameter that is higher than predicted by a local stability analysis of the warped disc equations. In this case, the instability manifests differently producing large changes in the disc tilt locally in the disc, rather than the large changes in disc twist that typically occur in lower viscosity discs.

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arXiv2021

Disc tearing: implications for black hole accretion and AGN variability

Accretion discs around black holes power some of the most luminous objects in the Universe. Discs that are misaligned to the black hole spin can become warped over time by Lense-Thirring precession. Recent work has shown that strongly warped discs can become unstable, causing the disc to break into discrete rings producing a more dynamic and variable accretion flow. In a companion paper, we present numerical simulations of this instability and the resulting dynamics. In this paper, we discuss the implications of this dynamics for accreting black hole systems, with particular focus on the variability of Active Galactic Nuclei (AGN). We discuss the timescales on which variability might manifest, and the impact of the observer orientation with respect to the black hole spin axis. When the disc warp is unstable near the inner edge of the disc, we find quasi periodic behaviour of the inner disc which may explain the recent quasi periodic eruptions observed in, for example, the Seyfert 2 galaxy GSN 069 and in the galactic nucleus of RX J1301.9+2747. These eruptions are thought to be similar to the `heartbeat' modes observed in some X-ray binaries (e.g. GRS 1915+105 and IGR J17091-3624). When the instability manifests at larger radii in the disc, we find that the central accretion rate can vary on timescales that may be commensurate with, e.g., changing-look AGN. We therefore suggest that some of the variability properties of accreting black hole systems may be explained by the disc being significantly warped, leading to disc tearing.

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arXiv2020

Be star discs: powered by a non-zero central torque

Be stars are rapidly rotating B stars with Balmer emission lines that indicate the presence of a Keplerian, rotationally supported, circumstellar gas disc. Current disc models, referred to as "decretion discs", make use of the zero torque inner boundary condition typically applied to accretion discs, with the 'decretion' modelled by adding mass to the disc at a radius of about two per cent larger than the inner disc boundary. We point out that, in this model, the rates at which mass and energy need to be added to the disc are implausibly large. What is required is that the disc has not only a source of mass but also a continuing source of angular momentum. We argue that the disc evolution may be more physically modelled by application of the non-zero torque inner boundary condition of Nixon & Pringle (2020), which determines the torque applied at the boundary as a fraction of the advected angular momentum flux there and approaches the accretion and decretion disc cases in the appropriate limits. We provide supporting arguments for the suggestion that the origin of the disc material is small-scale magnetic flaring events on the stellar surface, which, when combined with rapid rotation, can provide sufficient mass to form, and sufficient angular momentum to maintain, a Keplerian Be star disc. We discuss the origin of such small-scale magnetic fields in radiative stars with differential rotation. We conclude that small-scale magnetic fields on the stellar surface, may be able to provide the necessary mass flux and the necessary time-dependent torque on the disc inner regions to drive the observed disc evolution.

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arXiv2020

On the orbital evolution of binaries with circumbinary discs

Circumbinary discs are generally thought to take up angular momentum and energy from the binary orbit over time through gravitational torques mediated by orbital resonances. This process leads to the shrinkage of the binary orbit over time, and is important in a variety of astrophysical contexts including the orbital evolution of stellar binaries, the migration of planets in protoplanetary discs, and the evolution of black hole binaries (stellar and supermassive). The merger of compact object binaries provides a source of gravitational waves in the Universe. Recently, several groups have reported numerical simulations of circumbinary discs that yield the opposite result, finding that the binary expands with time. Here we argue that this result is primarily due to the choice of simulation parameters, made for numerical reasons, which differ from realistic disc parameters in many cases. We provide physical arguments, and then demonstrate with 3D hydrodynamical simulations, that thick (high pressure, high viscosity) discs drive sufficient accretion of high angular momentum material to force binary expansion, while in the more realistic case of thin (low pressure, low viscosity) discs there is less accretion and the binary shrinks. In the latter case, tides, which generally transfer angular momentum and energy from the more rapidly rotating object (the binary) to the less rapidly rotating object (the disc), are the dominant driver of disc-binary evolution. This causes the binary to shrink. We therefore conclude that for common circumbinary disc parameters, binaries with non-extreme mass ratios are expected to shrink over time. Expansion of the binary can occur if the disc viscosity is unusually high, which may occur in the very thick discs encountered in e.g. circumplanetary discs, super-Eddington AGN, or the outer regions of passive protostellar discs that are heated by the ... (abridged).

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arXiv2020

Instability of non-Keplerian warped discs

Many accretion discs are thought to be warped. Recent hydrodynamical simulations show that (i) discs can break into distinct planes when the amplitude of an imposed warp is sufficiently high and the viscosity sufficiently low, and that (ii) discs can tear up into discrete rings when an initially planar disc is subject to a forced precession. Previously, we investigated the local stability of isolated, Keplerian, warped discs in order to understand the physics causing an accretion disc to break into distinct planes, finding that anti-diffusion of the warp amplitude is the underlying cause. Here, we explore the behaviour of this instability in disc regions where the rotation profile deviates from Keplerian. We find that at small warp amplitudes non-Keplerian rotation can stabilize the disc by increasing the critical warp amplitude for instability, while at large warp amplitudes non-Keplerian rotation can lead to an increased growth rate for discs that are unstable. Tidal effects on discs in binary systems are typically weak enough such that the disc remains close to Keplerian rotation. However, the inner regions of discs around black holes are strongly affected, with the smallest radius at which the disc can break into discrete planes being a function of the black hole spin. We suggest that interpreting observed frequencies in the power spectra of light curves from accreting compact objects as nodal and apsidal precession of discrete orbits requires an instability that can break the disc into discrete rings such as the one explored here.

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arXiv2019

Ultra-deep tidal disruption events: prompt self-intersections and observables

A star approaching a supermassive black hole (SMBH) can be torn apart in a tidal disruption event (TDE). We examine ultra-deep TDEs, a new regime in which the disrupted debris approaches close to the black hole's Schwarzschild radius, and the leading part intersects the trailing part at the first pericenter passage. We calculate the range of penetration factors $β$ vs SMBH masses $M$ that produce these prompt self-intersections using a Newtonian analytic estimate and a general relativistic (GR) geodesic model. We find that significant self-intersection of Solar-type stars requires $β\sim 50 - 127$ for $M/M_\odot = 10^4$, down to $β\sim 5.6 - 5.9$ for $M/M_\odot = 10^6$. We run smoothed-particle hydrodynamic (SPH) simulations to corroborate our calculations and find close agreement, with a slightly shallower dependence on $M$. We predict that the shock from the collision emits an X-ray flare lasting $t \sim 2$ s with $L \sim 10^{47}$ ergs/s at $E \sim 2$ keV, and the debris has a prompt accretion episode lasting $t \sim$ several min. The events are rare and occur with a rate $\dot{N} \lesssim 10^{-7}$ Mpc$^{-3}$ yr$^{-1}$. Ultra-deep TDEs can probe the strong gravity and demographics of low-mass SMBHs.

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