arXiv subjects

Eric R. Coughlin

Publications and source records attributed to Eric R. Coughlin.

At least 55 records · Page 3Linked to original sources

arXiv2022

Simulated optical light curves of super-Eddington tidal disruption events with ZEBRA flows

We present simulated optical light curves of super-Eddington tidal disruption events (TDEs) using the zero-Bernoulli accretion (ZEBRA) flow model, which proposes that during the super-Eddington phase, the disc is quasi-spherical, radiation-pressure dominated, and accompanied by the production of strong jets. We construct light curves for both on- and off-axis (with respect to the jet) observers to account for the anisotropic nature of the jetted emission. We find that at optical wavelengths, emission from the accretion flow is orders of magnitude brighter than that produced by the jet, even with boosting from synchrotron self-Compton. Comparing to the observed jetted TDE Swift J2058.4+0516, we find that the ZEBRA model accurately captures the timescale for which accretion remains super-Eddington and reproduces the luminosity of the transient. However, we find the shape of the light curves deviate at early times and the radius and temperature of our modelled ZEBRA are $\sim2.7 - 4.1$ times smaller and $\sim1.4 - 2.3$ times larger, respectively, than observed. We suggest that this indicates the ZEBRA inflates more, and more rapidly, than currently predicted by the model, and we discuss possible extensions to the model to account for this. Such refinements, coupled with valuable new data from upcoming large scale surveys, could help to resolve the nature of super-Eddington TDEs and how they are powered.

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arXiv2022

Stars Crushed by Black Holes. III. Mild Compression of Radiative Stars by Supermassive Black Holes

A tidal disruption event (TDE) occurs when the gravitational field of a supermassive black hole (SMBH) destroys a star. For TDEs in which the star enters deep within the tidal radius, such that the ratio of the tidal radius to the pericenter distance $β$ satisfies $β\gg 1$, the star is tidally compressed and heated. It was predicted that the maximum density and temperature attained during deep TDEs scale as $\propto β^3$ and $\propto β^2$, respectively, and nuclear detonation triggered by $β\gtrsim 5$, but these predictions have been debated over the last four decades. We perform Newtonian smoothed-particle hydrodynamics (SPH) simulations of deep TDEs between a Sun-like star and a $10^6 M_\odot$ SMBH for $2 \le β\le 10$. We find that neither the maximum density nor temperature follow the $\propto β^3$ and $\propto β^2$ scalings or, for that matter, any power-law dependence, and that the maximum-achieved density and temperature are reduced by $\sim$ an order of magnitude compared to past predictions. We also perform simulations in the Schwarzschild metric, and find that relativistic effects modestly increase the maximum density (by a factor of $\lesssim 1.5$) and induce a time lag relative to the Newtonian simulations, which is induced by time dilation. We also confirm that the time the star spends at high density and temperature is a very small fraction of its dynamical time. We therefore predict that the amount of nuclear burning achieved by radiative stars during deep TDEs is minimal.

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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

Spherically Symmetric Accretion onto a Compact Object through a Standing Shock: The Effects of General Relativity in the Schwarzschild Geometry

A core-collapse supernova is generated by the passage of a shockwave through the envelope of a massive star, where the shock wave is initially launched from the ``bounce'' of the neutron star formed during the collapse of the stellar core. Instead of successfully exploding the star, however, numerical investigations of core-collapse supernovae find that this shock tends to ``stall'' at small radii ($\lesssim$ 10 neutron star radii), with stellar material accreting onto the central object through the standing shock. Here, we present time-steady, adiabatic solutions for the density, pressure, and velocity of the shocked fluid that accretes onto the compact object through the stalled shock, and we include the effects of general relativity in the Schwarzschild metric. Similar to previous works that were carried out in the Newtonian limit, we find that the gas ``settles'' interior to the stalled shock; in the relativistic regime analyzed here, the velocity asymptotically approaches zero near the Schwarzschild radius. These solutions can represent accretion onto a material surface if the radius of the compact object is outside of its event horizon, such as a neutron star; we also discuss the possibility that these solutions can approximately represent the accretion of gas onto a newly formed black hole following a core-collapse event. Our findings and solutions are particularly relevant in weak and failed supernovae, where the shock is pushed to small radii and relativistic effects are large.

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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

Using the Hills Mechanism to Generate Repeating Partial Tidal Disruption Events and ASASSN-14ko

Periodic nuclear transients have been detected with increasing frequency, with one such system -- ASASSN-14ko -- exhibiting highly regular outbursts on a timescale of $114 \pm 1$ days. It has been postulated that the outbursts from this source are generated by the repeated partial disruption of a star, but how the star was placed onto such a tightly bound orbit about the supermassive black hole remains unclear. Here we use analytic arguments and three-body integrations to demonstrate that the Hills mechanism, where a binary system is destroyed by the tides of the black hole, can lead to the capture of a star on a $\sim 114$ day orbit and with a pericenter distance that is comparable to the tidal radius of one of the stars within the binary. Thus, Hills capture can produce stars on tightly bound orbits that undergo repeated partial disruption, leading to a viable mechanism for generating not only the outbursts detected from ASASSN-14ko, but for periodic nuclear transients in general. We also show that the rate of change of the period of the captured star due to gravitational-wave emission is likely too small to produce the observed value for ASASSN-14ko, indicating that in this system there must be additional effects that contribute to the decay of the orbit. In general, however, gravitational-wave emission can be important for limiting the lifetimes of these systems, and could produce observable period decay rates in future events.

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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

A Physical Model of Delayed Rebrightenings in Shock-interacting Supernovae without Narrow Line Emission

Core-collapse supernovae can display evidence of interaction with pre-existing, circumstellar shells of material by rebrightening and forming spectral lines, and can even change types as Hydrogen appears in previously Hydrogen-poor spectra. However, a recently observed core-collapse supernova -- SN 2019tsf -- was found to brighten after roughly 100 days after it was first observed, suggesting that the supernova ejecta was interacting with surrounding material, but it lacked any observable emission lines and thereby challenged the standard supernova-interaction picture. We show through linear perturbation theory that delayed rebrightenings without the formation of spectral lines are generated as a consequence of the finite sound crossing time of the post-shock gas left in the wake of a supernova explosion. In particular, we demonstrate that sound waves -- generated in the post-shock flow as a consequence of the interaction between a shock and a density enhancement -- traverse the shocked ejecta and impinge upon the shock from behind in a finite time, generating sudden changes in the shock properties in the absence of ambient density enhancements. We also show that a blastwave dominated by gas pressure and propagating in a wind-fed medium is unstable from the standpoint that small perturbations lead to the formation of reverse shocks within the post-shock flow, implying that the gas within the inner regions of these blastwaves should be highly turbulent.

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arXiv2021

Tidal Disruption Disks Formed and Fed by Stream-Stream and Stream-Disk Interactions in Global GRHD Simulations

When a star passes close to a supermassive black hole (BH), the BH's tidal forces rip it apart into a thin stream, leading to a tidal disruption event (TDE). In this work, we study the post-disruption phase of TDEs in general relativistic hydrodynamics (GRHD) using our GPU-accelerated code H-AMR. We carry out the first grid-based simulation of a deep-penetration TDE ($β$=7) with realistic system parameters: a black-hole-to-star mass ratio of $10^6$, a parabolic stellar trajectory, and a nonzero BH spin. We also carry out a simulation of a tilted TDE whose stellar orbit is inclined relative to the BH midplane. We show that for our aligned TDE, an accretion disk forms due to the dissipation of orbital energy with $\sim$20 percent of the infalling material reaching the BH. The dissipation is initially dominated by violent self-intersections and later by stream-disk interactions near the pericenter. The self-intersections completely disrupt the incoming stream, resulting in five distinct self-intersection events separated by approximately 12 hours and a flaring in the accretion rate. We also find that the disk is eccentric with mean eccentricity e$\approx$0.88. For our tilted TDE, we find only partial self-intersections due to nodal precession near pericenter. Although these partial intersections eject gas out of the orbital plane, an accretion disk still forms with a similar accreted fraction of the material to the aligned case. These results have important implications for disk formation in realistic tidal disruptions. For instance, the periodicity in accretion rate induced by the complete stream disruption may explain the flaring events from Swift J1644+57.

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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

Dynamical stability of giant planets: the critical adiabatic index in the presence of a solid core

The dissociation and ionization of hydrogen, during the formation of giant planets via core accretion, reduces the effective adiabatic index $γ$ of the gas and could trigger dynamical instability. We generalize the analysis of Chandrasekhar, who determined that the threshold for instability of a self-gravitating hydrostatic body lies at $γ=4/3$, to account for the presence of a planetary core, which we model as an incompressible fluid. We show that the dominant effect of the core is to stabilize the envelope to radial perturbations, in some cases completely (i.e. for all $γ> 1$). When instability is possible, unstable planetary configurations occupy a strip of $γ$ values whose upper boundary falls below $γ=4/3$. Fiducial evolutionary tracks of giant planets forming through core accretion appear unlikely to cross the dynamical instability strip that we define.

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arXiv2020

Nonthermal filaments from the tidal destruction of clouds in the Galactic center

Synchrotron-emitting, nonthermal filaments (NTFs) have been observed near the Galactic center for nearly four decades, yet their physical origin remains unclear. Here we investigate the possibility that NTFs are produced by the destruction of molecular clouds by the gravitational potential of the Galactic center. We show that this model predicts the formation of a filamentary structure with length on the order of tens to hundreds of pc, a highly ordered magnetic field along the axis of the filament, and conditions conducive to magnetic reconnection that result in particle acceleration. This model therefore yields the observed magnetic properties of NTFs and a population of relativistic electrons, without the need to appeal to a dipolar, $\sim$ mG, Galactic magnetic field. As the clouds can be both completely or partially disrupted, this model provides a means of establishing the connection between filamentary structures and molecular clouds that is observed in some, but not all, cases.

astro-ph.GA↗

arXiv2020

Structured, relativistic jets driven by radiation

Relativistic jets, or highly collimated and fast-moving outflows, are endemic to many astrophysical phenomena. The jets produced by gamma-ray bursts and tidal disruption events are accompanied by the accretion of material onto a black hole or neutron star, with the accretion rate exceeding the Eddington limit of the compact object by orders of magnitude. In such systems, radiation dominates the energy-momentum budget of the outflow, and the dynamical evolution of the jet is governed by the equations of radiation hydrodynamics. Here we show that there are analytic solutions to the equations of radiation hydrodynamics in the viscous (i.e., diffusive) regime that describe structured, relativistic jets, which consist of a fast-moving, highly relativistic core surrounded by a slower-moving, less relativistic sheath. In these solutions, the slower-moving, outer sheath contains most of the mass, and the jet structure is mediated by local anisotropies in the radiation field. We show that, depending on the pressure and density profile of the ambient medium, the angular profile of the jet Lorentz factor is Gaussian or falls off even more steeply with angle. These solutions have implications for the nature of jet production and evolution in hyperaccreting systems, and demonstrate that such jets -- and the corresponding jet structure -- can be sustained entirely by radiative processes. We discuss the implications of these findings in the context of jetted tidal disruption events and short and long gamma-ray bursts.

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arXiv2020

The persistence of pancakes and the revival of self-gravity in tidal disruption events

The destruction of a star by the tides of a supermassive black hole (SMBH) powers a bright accretion flare, and the theoretical modeling of such tidal disruption events (TDEs) can provide a direct means of inferring SMBH properties from observations. Previously it has been shown that TDEs with $β= r_{\rm t}/r_{\rm p} = 1$, where $r_{\rm t}$ is the tidal disruption radius and $r_{\rm p}$ is the pericenter distance of the star, form an in-plane caustic, or ``pancake,'' where the tidally disrupted debris is compressed into a one-dimensional line within the orbital plane of the star. Here we show that this result applies generally to all TDEs for which the star is fully disrupted, i.e., that satisfy $β\gtrsim 1$. We show that the location of this caustic is always outside of the tidal disruption radius of the star and the compression of the gas near the caustic is at most mildly supersonic, which results in an adiabatic increase in the gas density above the tidal density of the black hole. As such, this in-plane pancake revitalizes the influence of self-gravity even for large $β$, in agreement with recent simulations. This finding suggests that for all TDEs in which the star is fully disrupted, self-gravity is revived post-pericenter, keeps the stream of debris narrowly confined in its transverse directions, and renders the debris prone to gravitational instability.

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arXiv2020

Variability in Short Gamma-ray Bursts: Gravitationally Unstable Tidal Tails

Short gamma-ray bursts are thought to result from the mergers of two neutron stars or a neutron star and stellar mass black hole. The final stages of the merger are generally accompanied by the production of one or more tidal "tails" of ejecta, which fall back onto the remnant-disc system at late times. Using the results of a linear stability analysis, we show that if the material comprising these tails is modeled as adiabatic and the effective adiabatic index satisfies $γ\ge 5/3$, then the tails are gravitationally unstable and collapse to form small-scale knots. We analytically estimate the properties of these knots, including their spacing along the tidal tail and the total number produced, and their effect on the mass return rate to the merger remnant. We perform hydrodynamical simulations of the disruption of a polytropic (with the polytropic and adiabatic indices $γ$ equal), $γ=2$ neutron star by a black hole, and find agreement between the predictions of the linear stability analysis and the distribution of knots that collapse out of the instability. The return of these knots to the black hole induces variability in the fallback rate, which can manifest as variability in the lightcurve of the GRB and -- depending on how rapidly the instability operates -- the prompt emission. The late-time variability induced by the return of these knots is also consistent with the extended emission observed in some GRBs.

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arXiv2020

Fallback Rates from Partial Tidal Disruption Events

A tidal disruption event (TDE) occurs when a star plunges through a supermassive black hole's tidal radius, at which point the star's self-gravity is overwhelmed by the tidal gravity of the black hole. In a partial TDE, where the star does not reach the full disruption radius, only a fraction of the star's mass is tidally stripped while the rest remains intact in the form of a surviving core. Analytical arguments have recently suggested that the temporal scaling of the fallback rate of debris to the black hole asymptotes to $t^{-9/4}$ for partial disruptions, effectively independently of the mass of the intact core. We present hydrodynamical simulations that verify the existence of this predicted, $t^{-9/4}$ scaling. We also define a break timescale -- the time at which the fallback rate transitions from a $t^{-5/3}$ scaling to the characteristic $t^{-9/4}$ scaling -- and measure this break timescale as a function of the impact parameter and the surviving core mass. These results deepen our understanding of the properties and breadth of possible fallback curves expected from TDEs and will therefore facilitate more accurate interpretation of data from wide-field surveys.

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arXiv2020

The structure of nearly isothermal, adiabatic shockwaves

An explosively generated shockwave with time-dependent radius $R(t)$ is characterized by a phase in which the shocked gas becomes radiative with an effective adiabatic index $γ\simeq 1$. Using the result that the post-shock gas is compressed into a shell of width $ΔR/R \simeq δ$, where $δ= γ-1$, we show that a choice of self-similar variable that exploits this compressive behavior in the limit that $γ\rightarrow 1$ naturally leads to a series expansion of the post-shock fluid density, pressure, and velocity in the small quantity $δ$. We demonstrate that the leading-order (in $δ$) solutions, which are increasingly accurate as $γ\rightarrow 1$, can be written in simple, closed forms when the fluid is still approximated to be in the energy-conserving regime (i.e., the Sedov-Taylor limit), and that the density declines exponentially rapidly with distance behind the shock. We also analyze the solutions for the bubble surrounding a stellar or galactic wind that interacts with its surroundings, and derive expressions for the location of the contact discontinuity that separates the shocked ambient gas from the shocked wind. We discuss the implications of our findings in the context of the dynamical stability of nearly isothermal shocks.

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