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Evgeni Grishin

Publications and source records attributed to Evgeni Grishin.

At least 19 recordsLinked to original sources

Semi-Analytical Model for the Evolution of Stellar Binaries in the Empty Loss Cone of Massive Black Holes

Binary star systems orbiting close to a supermassive black hole (SMBH) evolve through encounters with other stars, the SMBH's tidal forces, and the binary's internal dynamics, including general relativistic precession and tides. Many are driven onto highly eccentric inner binary orbits, potentially leading to stellar mergers; other possible outcomes include hypervelocity star ejections or tidal disruption events. We study the evolution of binaries in the empty loss cone regime, where the outer orbit's angular momentum change per orbit due to scattering off other stars is smaller than the outer angular momentum at the tidal separation radius. We build on the work of Hamers \& Samsing to develop a computationally efficient semi-analytical model that captures the long term evolution of binaries in perturbative regimes where the ratio of the binary tidal separation radius to the pericenter around the SMBH is smaller than 0.15. Crucially, we apply corrections to preserve the orthogonality between the binary's eccentricity and angular momentum vectors, which prevents unphysical eccentricity growth. From these simulations, we find analytical fits for the probability distributions of the final orbital parameters of binaries approaching the SMBH. We find that general relativistic precession efficiently suppresses von-Zeipel-Lidov-Kozai-like eccentricity oscillations and reduces the fraction of merging binaries from $84\%$ with Newtonian physics only, to $3\%$ with precession included. Stellar tides further reduce the merger fraction to $0.4\%$.

astro-ph.HE

Modern tidal interaction models for rapid binary population synthesis: II. Binary black hole formation, mergers, and spins

We present predictions for the merger rates and effective spin ($χ_{\rm eff}$) distribution of binary black holes (BBHs) from isolated binary evolution, using a new self-consistent tidal dissipation implementation in the rapid binary population synthesis code COMPAS. Most of the first-born black holes (BHs) in our simulated merging BBHs are formed with zero spins, with the exception of BBHs formed from chemically homogeneous evolution. The spins of the second-born BHs with the new model depend significantly on the efficiency of tidal dissipation and mass transfer history, and crucially, are not always consistent with pre-supernova synchronization. High-$χ_{\rm eff}$ binaries preferentially merge at high redshift due to smaller binary separations at BBH formation and shorter coalescence times, thus rendering them largely inaccessible to current gravitational wave (GW) detectors. We expect the intrinsic spin distribution of merging BBHs formed from isolated evolution to be strongly biased toward low $χ_{\rm eff}$ with current detectors, with a third of systems having $χ_{\rm eff} < 0.05$ and only $\sim 3\%$ with $χ_{\rm eff}>0.5$. However, $χ_{\rm eff}$ will increase as GW detectors become sensitive to higher redshift sources, with up to $\sim 15\%$ of systems having $χ_{\rm eff}>0.5$.

astro-ph.HE

Modern tidal interaction models for rapid binary population synthesis: I. Methods

In this work, we present an updated prescription of contemporary tidal dissipation theory adapted for rapid binary population synthesis. Our simplified expressions encode the dependence of tidal dissipation on stellar structure, stratification, and tidal forcing frequency, while remaining computationally efficient. We implement these prescriptions in the rapid population synthesis code COMPAS, and demonstrate the self-consistent coupling of tides with stellar evolution and binary properties such as orbital periods, spins, and eccentricities for several representative binary systems. When compared with commonly used tidal prescriptions, our equilibrium tidal dissipation efficiencies can be stronger by 1-2 orders of magnitude for low mass main sequence and giant type stars, and dynamical tides can be stronger by 1-7 orders of magnitude due to the explicit dependence on internal stellar structure and the presence of inertial wave dissipation. Despite our simplistic approach, our models agree with detailed stellar simulations to within an order of magnitude across tidal dissipation mechanisms.

astro-ph.SR

Extensions of Brown Hamiltonian-III. Applications to irregular satellites of giant planets

Irregular satellites, orbiting at large distances from their host planets, are subject to strong solar perturbations that render long-term orbital predictions particularly challenging. Building upon the extended Brown Hamiltonian framework developed in Paper I, we introduce the modified Lidov integral ($C_{\rm ZLK}$) as a practical diagnostic index to characterize the dynamical modes. We demonstrate that a satellite is trapped inside the von Zeipel--Lidov--Kozai (ZLK) resonance when $C_{\rm ZLK} < 0$. Applying this criterion to the known population of irregular satellites, we identify 27 candidates in libration. Direct $N$-body simulations confirm 26 of these predictions, with the sole exception of S/2019 S1, whose discrepancy is attributed to its proximity to the separatrix. These results establish $C_{\rm ZLK}$ as a decisive parameter for identifying the ZLK resonance, providing an efficient tool for analyzing the secular dynamics in weakly hierarchical three-body systems.

astro-ph.EP

A benchmark for binary star interaction with a supermassive black hole in general relativity

Most galaxies have supermassive black holes (SMBH) at their centres, surrounded by stars with binary systems also present in this environment. We use two schemes - post-Newtonian (PN) and a scalar perturbation to a background metric to numerically solve the three-body problem of a binary with a SMBH. We test three different PN formulations for the PN scheme: The Einstein-Infeld-Hoffman equation, pair-wise implementation of two-body PN-terms for three bodies and the Arnowitt-Deser-Misner Hamiltonian. We compare these approaches for one million solar mass and one billion solar mass black holes, and find a statistical match between the two approximations for stellar mass binary interacting with a million solar mass black hole. We also perform a statistical study for encounters with this black hole, and find that the higher order PN formulation matches with metric-with-perturbation scheme. However, we find a decrease in separation of the binary, and eccentricity variations between different schemes around the billion solar mass black hole. This behaviour is not present if binary has a large separation or is further away from the black hole due to decreased general-relativistic effects. We find that the pair-wise PN method results in a decrease in separation at pericentre in all test cases irrespective of the distance from the black hole or mass of the black hole, making this the least reliable method for solving this problem. Our work highlights the need for caution when interpreting the results in different formulations around SMBHs. This also shows that when understanding extreme mass ratio inspirals (EMRIs) using simulations, one should beware as the binary gets closer to the black hole.

astro-ph.IM

Sub-Snowline Formation of Gas-Giant Planets in Binary Systems

Gas-giant planets are thought to require conditions beyond the water snow line to build solid cores efficiently. In close binary star systems, the companion's gravity additionally limits the region of stable orbits, potentially excluding the zone where giants should form.} We aim to identify binary systems in which gas giants exist despite the snow line lying in the dynamically unstable zone, and to develop a physically motivated formation channel that explains and predicts their observed locations. We analyse a catalogue of 811 circumstellar binary systems from \citet{Thebault2025}, identifying those hosting gas giants. ($M_p \geq 0.15\,M_\mathrm{Jup}$) with snow lines larger than $0.8\,a_c$ as defined by \citet{Quarles_2020}. We compare their metallicity and eccentricity distributions with the background population, model snow-line evolution with MESA, and fit a linear relation between observed planet semi-major axes and the tidal truncation radius from \citet{Pichardo2005}.} Among 393 gas-giant hosts, we identify 17 systems whose snow line lies in the dynamically unstable zone. Their metallicity and eccentricity distributions are consistent with the background population. We propose that a dust trap formed near the tidal truncation radius of the protoplanetary disc can explain sub-snowline giant formation. The observed planet positions follow $a_\mathrm{planet} = (0.569 \pm 0.05)\,r_t$ ($R^2 = 0.94$), enabling system-by-system predictive power. Evolved systems deviate from this relation, independently supporting a second-generation planet origin for those cases. The tidal truncation of a protoplanetary disc by the stellar companion provides a natural mechanism for sub-snowline gas-giant formation in binaries. The resulting empirical relation yields testable predictions for binary eccentricities in systems lacking direct orbital measurements.

astro-ph.EP

Gravitational-Wave Signatures of Highly Eccentric Stellar-Mass Binary Black Holes in Galactic Nuclei

A significant fraction of compact-object mergers in galactic nuclei are expected to be eccentric in the Laser Interferometer Space Antenna (LISA) frequency sensitivity range, $10^{-4} - 10^{-1}\ \rm Hz$. Several compact binaries detected by the LIGO-Virgo-KAGRA Collaboration may retain hints of residual eccentricity at $\sim 10$ Hz, suggesting dynamical or triple origins for a significant fraction of the gravitational-wave-observable population. In triple systems, von-Zeipel-Lidov-Kozai oscillations perturb both the eccentricity and the argument of pericentre, $ω$, of the inner black hole binary. The latter could be fully \textit{circulating}, where $ω$ cycles through $2π$, or may \textit{librate}, with $ω$ ranges about a fixed value with small or large variation. We use \texttt{TSUNAMI}, a regularised N-body code with up to 3.5 post-Newtonian (PN) term corrections, to identify four different families of orbits: (i) circulating, (ii) small and (iii) large amplitude librating, and (iv) merging orbits. We develop and demonstrate a new method to construct gravitational waveforms using the quadrupole formula utilising the instantaneous {\it total} acceleration of each binary component in \texttt{TSUNAMI}. We show that the four orbital families have distinct waveform phenomenologies, enabling them to be distinguished if observed in LISA. The orbits are also distinguishable from an isolated binary or from a binary perturbed by a different tertiary orbit, even if the secular timescale is the same. Future burst timing models will be able to distinguish the different orbital configurations. For efficient binary formation, about $\sim 1000$ binaries can have highly eccentric, librating orbits in the Galactic Centre.

astro-ph.HE

Hot Jupiters in Old Wide-Binary Systems

Hot Jupiters (HJs) are giant planets with orbital periods shorter than $10$ days, found around $\sim 0.5$-$1\%$ of Sun-like stars. Their origins remain debated despite decades of study. The high prevalence of stellar companions, the eccentricity distribution of 'Cold' Jupiters on longer orbits, and the wide range of stellar spin-orbit misalignments support high-eccentricity migration: planets are excited to eccentric orbits and subsequently circularised via tidal dissipation. Existing high-eccentricity migration models, however, are inefficient in converting the initial population of Cold Jupiters to HJs. Current models reproduce at most $\lesssim 30\%$ of observed HJs, while the resulting Cold/Hot Jupiter ratios ($\gtrsim 30$) overproduce the observed values of $10$-$15$. These models also fail to form HJs around old stars ($\gtrsim 3$ Gyr) on short tidal decay timescales (e.g., $<40$ Myr). Here we show that wide binaries ($a > 10^3$ au) perturbed by the Galactic tidal field produce $1.8\pm 0.14$ more HJs compared to isolated binary systems, accounting for $26$-$40\%$ of the observed population under conservative assumptions. Wide-binaries predominantly produce Gyr-old systems, consistent with the host-age distribution for $t \ge 2.5\ \rm Gyr$. In $\sim 20\%$ of cases, wide-binary perturbations eject giant planets entirely, resolving the Cold/Hot Jupiter ratio discrepancy while naturally seeding the population of free-floating giant planets. In our dynamical framework, wide binaries emerge as active agents that reshape planetary demographics across billions of years. These results will be decisively tested by forthcoming exoplanet and microlensing surveys.

astro-ph.EP

Not all roads lead to merger: AGN disc properties influence the interactions of highly unequal mass black holes

As the number of gravitational-wave detections of black hole binaries grows, so does the diversity of proposed formation channels. The growing sample of systems with highly unequal masses, such as GW190814 with $m_1=23.2\,M_{\odot}$ and $m_2=2.59\,M_{\odot}$ -- corresponding to a mass ratio $q=0.112$ -- cannot be readily explained by isolated binary evolution and may originate through dynamical assembly in an active galactic nucleus (AGN). We investigate AGN discs capable of producing GW190814-like mergers using \texttt{pAGN} to model self-consistent AGN torques, coupled with \texttt{TSUNAMI}, a regularised N-body code including post-Newtonian terms up to 3.5 order. Suites of N-body simulations reveal possible outcomes of binary capture and merger, mean-motion resonance interactions, and other novel dynamical pathways. We develop analytical models linking the branching ratios of captures and mergers to local disc properties, applicable to black hole populations across all mass ratios. Capture probability is primarily governed by $\mathscr{B}$, the ratio of libration time to resonance-width crossing, and is well-described by a log-Gaussian, $P(\rm{capture}|\mathscr{B}) = A \exp[-(\ln \mathscr{B}-μ)^2/2σ^2]$, with $A = 0.41^{+0.04}_{-0.04}$, $μ= 1.09^{+0.08}_{-0.07}$, $σ= 1.05^{+0.08}_{-0.07}$. This fit, while an upper limit, is useful for simplified population synthesis. Finally, we explore the mass ratio AGN luminosity parameter space and find that GW190814 may be formed in a low luminosity AGN of $L_{\rm AGN}\approx 10^{43.5}\ \rm erg\ s^{-1}$. A more systematic parameter space exploration and future population studies will further test our predictions.

astro-ph.HE

Hierarchical Triples vs. Globular Clusters: Binary black hole merger eccentricity distributions compete and evolve with redshift

The formation mechanisms of merging binary black holes (BBHs) observed by the LIGO-Virgo-KAGRA collaboration remain uncertain. Detectable eccentricity provides a powerful diagnostic for distinguishing between different formation channels, but resolving their eccentricity distributions requires the detection of a large number of eccentric mergers. Future gravitational wave detectors such as the Einstein Telescope and Cosmic Explorer will detect tens of thousands of BBH mergers out to redshifts $z \ge 10$, making it critical to understand the redshift-dependent evolution of eccentricity distributions. We simulate this evolution for two key channels: dynamical assembly in globular clusters (GCs), which leads to rapid, eccentric mergers; and hierarchical triples in the field, where three-body dynamics can induce eccentricity in the inner binary. When considering all BBH mergers, the GC channel dominates overall, consistent with previous studies. However, when focusing on mergers with detectable eccentricity in next-generation detectors, we find that hierarchical triples dominate the eccentric merger rate at $0\le z \le 4$, with GC mergers becoming competitive at higher redshifts. Across all model variations, eccentric mergers in the local Universe ($z\lesssim 1$) have significant contributions from field triples, challenging the common view that such systems primarily form in dense environments. We show that, regardless of cluster and stellar evolution uncertainties, hierarchical triples contribute at least 30 per cent of eccentric mergers across a large range of redshifts.

astro-ph.GA

Gravitational-Wave Signatures of Highly Eccentric Stellar Binary Black-Holes in Galactic Nuclei

A significant fraction of gravitational-wave mergers are expected to be eccentric in the Laser-Interferometer-Space-Antenna (LISA) frequency band, $10^{-4} - 10^{-1}$ Hz. Several LIGO-Virgo-KAGRA events show potential hints of residual eccentricity at 10 Hz, pointing to dynamical or triple origins for part of the population, where von-Zeipel-Lidov-Kozai oscillations can perturb both the eccentricity and the inclination of the binary. Moreover, the argument of pericentre, $ω$ could be fully circulating, or librating, with a limited range for $ω$. We use TSUNAMI, a regularised N-body code with 3.5PN corrections to identify four different orbital families: (i) circulating, (ii) small-amplitude and (iii) large-amplitude librating, and (iv) merging. We develop a new method to construct gravitational-wave waveforms using the quadrupole formula from the instantaneous acceleration in TSUNAMI. The four orbital families have distinct waveform phenomenologies, enabling them to be distinguished if observed in LISA. In particular, the properties of the tertiary companion can be inferred and serve as an independent mass measurement and distinguish field triple dynamics from galactic dynamics.

astro-ph.HE

Extensions of Brown Hamiltonian-II. Analytical study on the modified von Zeipel-Lidov-Kozai effects

In triple systems of weak hierarchies, nonlinear perturbations arising from the periodic oscillations associated with the inner and outer binaries play a crucial role in shaping their long-term dynamical evolution. In this context, we have developed an extended Brown Hamiltonian in Paper I, which serves as a fundamental model for describing the modified von Zeipel-Lidov-Kozai (ZLK) oscillations. The present work aims to analyze the characteristics of ZLK oscillations within this extended framework, focusing on phase-space structures, the location of ZLK center, the maximum eccentricity reached, the boundaries of librating cycles, and the critical inclination required to trigger ZLK resonance. Under the extended Hamiltonian, we introduce the Lidov integral C_ZLK, which is a combination of the Hamiltonian and the z-component of angular momentum, to characterize the modified ZLK properties. It is found that the librating and circulating cycles are separated by C_ZLK=0, which is consistent with the classical theory. Furthermore, we derive analytical expressions of these ZLK properties using perturbation techniques. Analytical predictions are compared to numerical results, showing an excellent agreement between them. Notably, the results reveal that ZLK characteristics in prograde and retrograde regimes are no longer symmetric under the influence of Brown corrections. At last, we conduct $N$-body integrations about millions of orbits to generate dynamical maps, where the numerical structures are well captured by the analytical solutions derived from the extended model.

astro-ph.EP

Prompt stellar and binary black hole mergers in tight triples: Insights from chemically homogeneous evolution

Short-period massive binary stars are predicted to undergo chemically homogeneous evolution (CHE), making them prime candidates for producing binary black holes (BBHs) that may merge within the age of the Universe. Most of these binaries have a tertiary companion, and here we explore how a nearby third body possibly influences this evolutionary channel. Our analysis combines analytic treatments of triple dynamics with insights from detailed stellar evolution models, focusing on the role of the von Zeipel-Lidov-Kozai mechanism while also accounting for tidal and general relativistic apsidal precession. We examine the dynamics of triples at three critical evolutionary stages: the zero-age main sequence, shortly after the main sequence, and at the time of BBH formation. We find that, for triples having outer orbital periods less than 70 d(120 d), the inner binary can merge during(or after) the main-sequence stage, leading to a hydrogen-rich(helium-rich) stellar merger. If a stellar merger is avoided, the inner binary may eventually form a BBH. In mildly hierarchical triples, with outer periods of around 100 d, the tertiary component can trigger a rapid merger of the BBH on timescales comparable to the outer orbital period. Stellar tides play a crucial role in determining the fate of the inner binary in such tight triple systems, as they can suppress the perturbative effects of the third star. When tidal forces damp the oscillations induced by the tertiary, the BBH merger may occur soon after stellar collapse. Notably, these outcomes are not restricted to CHE binaries but can also apply to any BBH formed from stars in tight orbits. Mergers in these systems are characterized by the proximity of a tertiary companion and the presence of recently ejected gas, making them promising candidates for electromagnetic counterparts and gravitational-wave signals influenced by nearby tertiary objects.

astro-ph.SR

Extensions of Brown Hamiltonian-I. A high-accuracy model for von Zeipel-Lidov-Kozai oscillations

Triple systems with low hierarchical structure are common throughout the Universe, including examples such as high-altitude lunar satellites influenced by the Earth, planetary satellites perturbed by the Sun, and stellar binaries affected by a supermassive black hole. In these systems, nonlinear perturbations are significant, making classical double-averaged models (even those incorporating the Brown Hamiltonian correction) insufficient for accurately capturing long-term dynamics. To overcome this limitation, the current study develops a high-precision dynamical model that incorporates the nonlinear effects of the quadrupole-order potential arising from both the inner and outer bodies, referred to as the extended Brown Hamiltonian model. This framework specifically expresses the Hamiltonian function and the transformation between mean and osculating orbital elements in elegant, closed forms with respect to the eccentricities of the inner and outer orbits. Practical applications to Jupiter's irregular satellites show that the long-term evolutions predicted by the extended Brown Hamiltonian model align well with the results of direct N-body simulations. The developed Hamiltonian offers a fundamental dynamical model, which is particularly well suited for describing von Zeipel-Lidov-Kozai oscillations in low-hierarchy three-body systems.

astro-ph.EP

Planet formation and long-term stability in a very eccentric stellar binary

Planets orbiting one of the two stars in a binary are vulnerable to gravitational perturbations from the other star. Particularly, highly eccentric companion stars risk disrupting planetary orbits, such as in the extreme system TOI 4633 where close encounters between the companion and a gas giant planet in the habitable zone make it one of the most fragile systems discovered so far. Here, we report that TOI 4633's planet likely survived these encounters throughout the system's age by orbiting retrograde relative to the binary, stabilised by the Coriolis force. Using direct $N$-body simulations, we show it otherwise tends to collide with the binary stars or becomes free-floating after getting ejected. A retrograde planetary orbit has profound implications for TOI 4633's formation and evolution, suggesting an extraordinary history where its eccentric companion was likely randomly captured after planet formation in a single-star system. Alternatively, if stars and planet are born in situ from the same gas clump, we show the planet must have formed at sub-snow-line distances, contrary to the conventional core-accretion model. Our study highlights the importance of considering the long-term stability ($\gtrsim\rm Gyr$) of planets in eccentric binaries and demonstrates that the mere existence in such dynamically hostile environments places strong constraints on their orbital configuration and formation.

astro-ph.EP

Irregular Fixation: I. Fixed points and librating orbits of the Brown Hamiltonian

In hierarchical triple systems, the inner binary is slowly perturbed by a distant companion, giving rise to large-scale oscillations in eccentricity and inclination, known as von-Zeipel-Lidov-Kozai (ZLK) oscillations. Stable systems with a mild hierarchy, where the period ratio is not too small, require an additional corrective term, known as the Brown Hamiltonian, to adequately account for their long-term evolution. Although the Brown Hamiltonian has been used to accurately describe the highly eccentric systems on circulating orbits where the periapse completes a complete revolution, the analysis near its elliptical fixed points had been overlooked. We derive analytically the modified fixed points including the Brown Hamiltonian and analyse its librating orbits (where the periapse motion is limited in range). We compare our result to the direct three-body integrations of millions of orbits and discuss the regimes of validity. We numerically discover the regions of orbital instability, allowed and forbidden librating zones with a complex, fractal, structure. The retrograde orbits, where the mutual inclination is $ι> 90\ \rm deg$, are more stable and allowed to librate for larger areas of the parameter space. We find numerical fits for the librating-circulating boundary. Finally, we discuss the astrophysical implications for systems of satellites, stars and compact objects. In a companion paper (paper II), we apply our formalism to the orbits of irregular satellites around giant planets.

astro-ph.EP

How to Escape from a Trap: Outcomes of Repeated Black Hole Mergers in AGN

Stellar-mass black holes (BHs) embedded in active galactic nuclei (AGN) may be major sources of astrophysical gravitational waves (GWs), contributing both to the observed LIGO-Virgo-KAGRA population of binary BH mergers and to future populations of LISA-band extreme mass ratio inspirals (EMRIs). The ability of these BHs to pair up into binaries, inspiral, and produce GWs will be shaped by the existence of migration traps, regions in the AGN where hydrodynamic torques vanish. Previous works have studied the existence and location of migration traps in AGN disks. Here, we investigate how individual BHs may escape such traps as an outcome of mergers, potentially suppressing hierarchical growth. We find that while GW recoil kicks are strong enough to kick merged BHs onto inclined orbits, gas drag quickly realigns them into the AGN disk. A more robust escape mechanism is gap opening: once a BH grows above a critical mass, its gravity disturbs the AGN gas sufficiently to eliminate the trap. In low-mass AGN relevant for LISA, gaps open easily and the resulting ``wet EMRI'' masses are unlikely to reflect protracted hierarchical mergers. In combination with our previous work, we find that migration traps only exist in a relatively narrow range of AGN luminosities between [10^{43.5},10^{45.5}] erg/s. We identify an even narrower AGN luminosity range for which stellar mass BHs can grow into the pair instability mass gap and beyond. This characteristic luminosity scale may assist in indirect tests of the ``AGN channel'' for binary BH mergers.

astro-ph.HE

Detecting gravitational-wave bursts from black hole binaries in the Galactic Center with LISA

Stellar-mass black hole binaries (BHBs) in galactic nuclei are gravitationally perturbed by the central supermassive black hole (SMBH) of the host galaxy, potentially inducing strong eccentricity oscillations through the eccentric Kozai-Lidov (EKL) mechanism. These highly eccentric binaries emit a train of gravitational-wave (GW) bursts detectable by the Laser Interferometer Space Antenna (LISA) -- a planned space-based GW detector -- with signal-to-noise ratios (SNRs) up to ${\sim}100$ per burst. In this work, we study the GW signature of BHBs orbiting our galaxy's SMBH, Sgr A$^*$, which are consequently driven to very high eccentricities. We demonstrate that an unmodeled approach using a wavelet decomposition of the data effectively yields the time-frequency properties of each burst, provided that the GW frequency peaks between $10^{-3}\,\,\mathrm{Hz}$--$10^{-1}\,\,\mathrm{Hz}$. The wavelet parameters may be used to infer the eccentricity of the binary, measuring $\log_{10}(1-e)$ within an error of $20\%$. Our proposed search method can thus constrain the parameter space to be sampled by complementary Bayesian inference methods, which use waveform templates or orthogonal wavelets to reconstruct and subtract the signal from LISA data.

astro-ph.HE