arXiv subjects

Yuri Levin

Publications and source records attributed to Yuri Levin.

At least 19 recordsLinked to original sources

arXiv2026

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↗

arXiv2026

Searching for a waveform-agnostic gravitational wave signal in pulsar timing arrays

Pulsar timing arrays have recently provided compelling evidence for a nanohertz stochastic gravitational wave background, motivating searches for gravitational waves from localized sources. Most existing searches assume specific waveform templates, which can be computationally demanding and potentially insensitive to unexpected signals. We introduce a waveform-agnostic framework that models signal-induced timing residuals via a Fourier expansion. A Lorentzian hyperprior is imposed on the variances of the Fourier coefficients, providing a flexible spectral envelope that captures the signal's dominant frequency and bandwidth while remaining agnostic to its exact shape. Analytical marginalization over the Fourier coefficients then yields a Bayesian hierarchical framework that concurrently infers the source sky location, its frequency content, and the stochastic background. To mitigate contamination from unmodeled pulsar noise, we further allow for additional flat-spectrum features for each pulsar. Tests on simulated datasets show that the method is robust and provides a flexible tool for future PTA searches, with sensitivity to both expected and unexpected gravitational wave phenomena.

gr-qc↗

arXiv2025

Vortex Dynamics in the Neutron Star Inner Crust

We study the superfluid vortex motion in the neutron star inner crust through direct three-dimensional simulations of the coupled dynamics of the vortex and the nuclear lattice. We demonstrate the pinning of an initially moving vortex to the lattice through excitation of lattice vibrations, and show that the efficiency of this process is higher for attractive than for repulsive nucleus-vortex interactions. We explore the unpinning of a vortex under the action of the applied Magnus force, and find that it is influenced by multiple parameters, including the sign of the pinning force, the lattice orientation, composition, temperature, and the energy of pinning to individual nucleus. In lattices with multiple grains, the unpinning transition is triggered inside the grains with weaker pinning, propagates along the vortex (mediated by the excited Kelvin waves) and crosses into grains with stronger pinning. This is likely to effectively decrease the critical force at which vortices unpin and to produce extended regions of unpinned vorticity. Shearing of the crust lattice (e.g., by a starquake) initiates the unpinning of the vortices that are crossing the slip plane. A close encounter of an unpinned vortex with a pinned vortex would cause the latter to unpin, perhaps initiating an unpinning avalanche of many vortices.

astro-ph.HE↗

arXiv2025

Hyperactive Magnetar Eruptions: Giant Flares, Baryon Ejections, and Fast Radio Bursts

Young neutron stars born with magnetic fields $B\gtrsim 10^{16}$ G become hyperactive as the field inside the star evolves through ambipolar diffusion on a timescale $\sim 10^9$ s. We simulate this process numerically and find that it can eject magnetic loops from the star. The internal magnetic field first diffuses to the crust surrounding the liquid core and then erupts from the surface, taking a significant amount of crustal material with it. The eruption involves magnetic reconnection, generating a giant gamma-ray flare. A significant fraction of the eruption energy is carried by the neutron-rich crustal material, which must go through a phase of decompression and nuclear heating. The massive ejecta should produce additional emission components after the giant flare, including radioactively powered gamma-rays, optical emission, and much later a radio afterglow. The predicted eruptions may rarely happen in observed magnetars in our galaxy, which are relatively old and rarely produce giant flares. The model can, however, explain the extremely powerful flare from SGR 1806-20 in December 2004, its ejecta mass, and afterglow. More active, younger magnetars may produce frequent crustal eruptions and form unusual nebulae. Such hyperactive magnetars are candidates for the central engines of cosmological fast radio bursts (FRBs). We argue that each eruption launches an ultrarelativistic magnetosonic pulse leading the ejecta and steepening into a relativistic shock capable of emitting an FRB.

astro-ph.HE↗

arXiv2025

Pulsar Rockets and Gaia Neutron Star Binaries

We prove that the spin-aligned electromagnetic recoil force acting on a pulsar vanishes identically in Force-Free Electrodynamics. This contrasts with Hirai et al.'s recent argument that the rocket effect was important for explaining the eccentricity distribution of wide neutron star binaries found by Gaia. Our detailed analysis confirms that for a broad range of initial conditions and natal kick distributions, the rocket velocities of $v_r \gtrsim 30\, \mathrm{km/s}$ are required to account for the observed eccentricities. However, we find that in scenarios where the common envelope phase does not significantly shrink the initial orbit, and the natal kicks are drawn from Paczynski-type distribution, these eccentricities may arise without the influence of an EM rocket. If the natal kicks are perpendicular to the initial orbits, then explaining the Gaia neutron star eccentricities without invoking rockets additionally requires that the pre-supernova binaries avoid significant circularization, and that the mass loss during the supernova is minimal. The rocket effect can only be substantial in "weak" pulsars, where pair production near the light cylinder is suppressed and ${\bf E}\cdot{\bf B} \neq 0$ in the outer magnetosphere. We derive a rough estimate for a rocket force in a weak pulsar and relate it to the pulsar's radiative efficiency; simulations are needed to obtain numerically reliable expressions. If future observations prove that rockets are required to explain the data, this would imply that Gaia neutron stars are $\gtrsim $ Myr old, were previously rapidly spinning and are weakly magnetized, with a dipole field $\lesssim 10^{10}$ G and a relatively strong quadrupole component.

astro-ph.HE↗

arXiv2024

Giant Hall Waves Launched by Superconducting Phase Transition in Pulsars

The cores of pulsars are expected to become superconducting soon after birth. The transition to type-II superconductivity is associated with the bunching of magnetic field lines into discrete superconducting flux tubes which possess enormous tension. The coupling of the crust to the flux tubes implies the existence of huge tangential magnetic fields at the crust-core interface. We show that the transition to superconductivity triggers a highly non-linear response in the Hall drift of the crustal magnetic field, an effect which was neglected in previous numerical modelling. We argue that at the time of the phase transition giant Hall waves are launched from the crust-core interface toward the surface. Our models show that if the crust contains a multipolar magnetic field $\sim 10^{13}$ G, the amplitude of the Hall waves is $\sim 10^{15}$ G. The elastic deformation of the lattice is included in our models, which allows us to track the time-dependent shear stresses everywhere in the crust. The simulations indicate that the Hall waves may be strong enough to break the crust, and could cause star quakes which trigger rotation glitches and changes in the radio pulse profile. The Hall waves also couple to slow magnetospheric changes which cause anomalous braking indices. The emission of the giant Hall waves from the crust-core interface facilitates fast flux expulsion from the superconducting core, provided that the flux tubes in the core are themselves sufficiently mobile. For all of the flux tube mobility prescriptions implemented in this work, the core approaches the Meissner state with B=0 at late times.

astro-ph.HE↗

arXiv2024

In LIGO's Sight? Vigorous Coherent Gravitational Waves from Cooled Collapsar Disks

We present the first numerical study of gravitational waves (GWs) from collapsar disks, using state-of-the-art 3D general relativistic magnetohydrodynamic simulations of collapsing stars. These simulations incorporate a fixed Kerr metric for the central black hole (BH) and employ simplified prescriptions for disk cooling. We find that cooled disks with an expected scale height ratio of $H/R\gtrsim0.1$ at $\sim10$ gravitational radii induce Rossby instability in compact, high-density rings. The trapped Rossby vortices generate vigorous coherent emission regardless of disk magnetization and BH spin. For BH mass of $\sim10\,M_\odot$, the GW spectrum peaks at $\sim100\,{\rm Hz}$ with some breadth due to various nonaxisymmetric modes. The spectrum shifts toward lower frequencies as the disk viscously spreads and the circularization radius of the infalling gas increases. Weaker-cooled disks with $H/R\gtrsim0.3$ form a low-density extended structure of spiral arms, resulting in a broader, lower-amplitude spectrum. Assuming an optimistic detection threshold with a matched-filter signal-to-noise ratio of 20 and a rate similar to Type Ib/c supernovae, LIGO-Virgo-KAGRA (LVK) could detect $\lesssim1$ event annually, suggesting that GW events may already be hidden in observed data. Third-generation GW detectors could detect dozens to hundreds of collapsar disks annually, depending on the cooling strength and the disk formation rate. The GW amplitudes from collapsar disks are $\gtrsim100$ times higher with a substantially greater event rate than those from core-collapse supernovae, making them potentially the most promising burst-type GW class for LVK and Cosmic Explorer. This highlights the importance of further exploration and modeling of disk-powered GWs, promising insights into collapsing star physics.

astro-ph.HE↗

arXiv2024

Gravitational Wave Driven Inspirals of Binaries Connected by Cosmic Strings

We consider gravitational waves from a pair of monopoles or black holes that are moving non-relativistically and are connected by a cosmic string. Shortly after the binary's formation, the connecting string straightens due the direct coupling of its motion to gravitational radiation. Afterwards, the motion of the binary can be well-approximated by a non-relativistic motion of its components that have an additional constant mutual attraction force due to the tension of the straight string that connects them. The orbit shrinks due to the gravitational radiation backreacting on the binary's components. We find that if the binary's semimajor axis $a\gg \sqrt{R_1 R_2/{\mu}}$, its eccentricity grows on the inspiral's timescale; here $R_1$ and $R_2$ are the gravitational radii of the binary components, and $\mu$ is the dimensionless tension of the string. When the eccentricity is high, it approaches unity super-exponentially. If the binary's components are monopole-antimonopole pair, this leads to the physical collision that would likely destroy the string and annihilate the monopoles when the semimajor axis is still many orders of magnitude greater than the string thickness. If the binary's components are black holes, then the eccentricity reaches its peak when $a\sim \sqrt{R_1 R_2/\mu}$, and then decays according to the standard Peter's formula. The black-hole spins initially become locked to the orbital motion, but then lag behind as the inspiral proceeds. We estimate the string-tension-induced dimensionless spins just prior to the merger and find them to be $\sim\mu^{3/8}\ll 1$.

gr-qc↗

arXiv2024

Eddington envelopes: The fate of stars on parabolic orbits tidally disrupted by supermassive black holes

Stars falling too close to massive black holes in the centres of galaxies can be torn apart by the strong tidal forces. Simulating the subsequent feeding of the black hole with disrupted material has proved challenging because of the range of timescales involved. Here we report a set of simulations that capture the relativistic disruption of the star, followed by one year of evolution of the returning debris stream. These reveal the formation of an expanding asymmetric bubble of material extending to hundreds of astronomical units -- an outflowing Eddington envelope with an optically thick inner region. Such envelopes have been hypothesised as the reprocessing layer needed to explain optical/UV emission in tidal disruption events, but never produced self-consistently in a simulation. Our model broadly matches the observed light curves with low temperatures, faint luminosities, and line widths of 10,000--20,000 km/s.

astro-ph.HE↗

arXiv2023

Search for an isotropic gravitational-wave background with the Parkes Pulsar Timing Array

Pulsar timing arrays aim to detect nanohertz-frequency gravitational waves (GWs). A background of GWs modulates pulsar arrival times and manifests as a stochastic process, common to all pulsars, with a signature spatial correlation. Here we describe a search for an isotropic stochastic gravitational-wave background (GWB) using observations of 30 millisecond pulsars from the third data release of the Parkes Pulsar Timing Array (PPTA), which spans 18 years. Using current Bayesian inference techniques we recover and characterize a common-spectrum noise process. Represented as a strain spectrum $h_c = A(f/1 {\rm yr}^{-1})^{\alpha}$, we measure $A=3.1^{+1.3}_{-0.9} \times 10^{-15}$ and $\alpha=-0.45 \pm 0.20$ respectively (median and 68% credible interval). For a spectral index of $\alpha=-2/3$, corresponding to an isotropic background of GWs radiated by inspiraling supermassive black hole binaries, we recover an amplitude of $A=2.04^{+0.25}_{-0.22} \times 10^{-15}$. However, we demonstrate that the apparent signal strength is time-dependent, as the first half of our data set can be used to place an upper limit on $A$ that is in tension with the inferred common-spectrum amplitude using the complete data set. We search for spatial correlations in the observations by hierarchically analyzing individual pulsar pairs, which also allows for significance validation through randomizing pulsar positions on the sky. For a process with $\alpha=-2/3$, we measure spatial correlations consistent with a GWB, with an estimated false-alarm probability of $p \lesssim 0.02$ (approx. $2\sigma$). The long timing baselines of the PPTA and the access to southern pulsars will continue to play an important role in the International Pulsar Timing Array.

astro-ph.HE↗

arXiv2023

Superfluid Rivers in Spinning-down Neutron Stars

We study the motion of neutron superfluid vortices in a spinning-down neutron star, assuming axisymmetry of the flow and ignoring motion of vortices about the rotation axis. We find that the vortex array, if initially rectilinear, is soon substantially deformed as the star spins down; vortices are swept outward by the Magnus force, accumulating in regions of the inner crust where they pin, accompanied by significant bending of the vortex array. As the star spins down to below a spin rate of ~20 Hz (twice the spin rate of the Vela pulsar), the Magnus and pinning forces gradually compress the vortex array into dense sheets that follow spherical shells. In some cases, the vortex array bends on itself and reconnects, forming one or more tori of vortex rings that contain superfluid ``rivers" with significant angular momentum. Vortex sheets are likely to form near the base of the inner crust, in the regime of nuclear pasta.

astro-ph.HE↗

arXiv2023

Simulating cosmic string loop captured by a rotating black hole

We study the dynamics of a cosmic string loop captured by a rotating black hole, ignoring string reconnections. A loop is numerically evolved in Kerr spacetime, with the result that it turns into one or more growing or contracting double-lines rotating around the black hole in the equatorial plane. This is in good agreement with the approximate analytical treatment of the problem investigated by Xing et al., who studied the evolution of the auxiliary curve associated with the string loop. We confirm that the auxiliary curve deformation can indeed describe the string motion in realistic physical scenarios to a reasonable accuracy, and can thus be used to further study other phenomena such as superradiance and reconnections of the captured loop.

gr-qc↗

arXiv2022

Resonant friction on discs in galactic nuclei

We argue that resonant friction has a dramatic effect on a disc whose rotation direction is misaligned with that of its host nuclear star cluster. The disc's gravity causes gravitational perturbation of the cluster that in turn exerts a strong torque back onto the disc. We argue that this torque may be responsible for the observed disruption of the clockwise disc of young stars in the Galactic Center, and show in numerical experiments that it produces the observed features in the distribution of the stars' angular momenta. More generally, we speculate that the rotation of nuclear star clusters has a stabilizing effect on the orientation of transient massive accretion discs around the supermassive black holes residing in their centers, and thus on the directions and magnitudes of the black-hole spins.

astro-ph.GA↗

arXiv2022

Radio Emission and Electric Gaps in Pulsar Magnetospheres

The origin of pulsar radio emission is one of the old puzzles in theoretical astrophysics. In this Letter we present a global kinetic plasma simulation which shows from first-principles how and where radio emission can be produced in pulsar magnetospheres. We observe the self-consistent formation of electric gaps which periodically ignite electron-positron discharge. The gaps form above the polar-cap, and in the bulk return-current. Discharge of the gaps excites electromagnetic modes which share several features with the radio emission of real pulsars. We also observe the excitation of plasma waves and charge bunches by streaming instabilities in the outer magnetosphere. Our numerical experiment demonstrates that global kinetic models can provide deep insight into the emission physics of pulsars, and may help interpret their multi-wavelength observations.

astro-ph.HE↗

arXiv2022

Revisiting Thermal Charge Carrier Refractive Noise in Semiconductor Optics for Gravitational-Wave Interferometers

The test masses in next-generation gravitational-wave interferometers may have a semiconductor substrate, most likely silicon. The stochastic motion of charge carriers within the semiconductor will cause random fluctuations in the material's index of refraction, introducing a noise source called Thermal Charge Carrier Refractive (TCCR) noise. TCCR noise was previously studied in 2020 by Bruns et al., using a Langevin force approach. Here we compute the power spectral density of TCCR noise by both using the Fluctuation-Dissipation theorem (FDT) and accounting for previously neglected effects of the standing wave of laser light which is produced inside the input test mass by its high-reflecting coatings. We quantify our results with parameters from Einstein Telescope, and show that at temperatures of 10 K the amplitude of TCCR noise is up to a factor of $\sqrt{2}$ times greater than what was previously claimed, and from 77 K to 300 K the amplitude is around 5 to 7 orders of magnitude lower than previously claimed when we choose to neglect the standing wave, and is up to a factor of 6 times lower if the standing wave is included. Despite these differences, we still conclude like Bruns et al. that TCCR noise should not be a limiting noise source for next-generation gravitational-wave interferometers.

gr-qc↗

arXiv2022

Vortex Pinning in Neutron Stars, Slip-stick Dynamics, and the Origin of Spin Glitches

We study pinning and unpinning of superfluid vortices in the inner crust of a neutron star using 3-dimensional dynamical simulations. Strong pinning occurs for certain lattice orientations of an idealized, body-centered cubic lattice, and occurs generally in an amorphous or impure nuclear lattice. The pinning force per unit length is $\sim 10^{16}$ dyn cm$^{-1}$ for a vortex-nucleus interaction that is repulsive, and $\sim 10^{17}$ dyn cm$^{-1}$ for an attractive interaction. The pinning force is strong enough to account for observed spin jumps (glitches). Vortices forced through the lattice move with a slip-stick character; for a range of superfluid velocities, the vortex can be in either a cold, pinned state or a hot unpinned state, with strong excitation of Kelvin waves on the vortex. This two-state nature of vortex motion sets the stage for large-scale vortex movement that creates an observable spin glitch. We argue that the vortex array is likely to become tangled as a result of repeated unpinnings and repinnings. We conjecture that during a glitch, the Kelvin-wave excitation spreads rapidly along the direction of the mean superfluid vorticity and slower in the direction perpendicular to it, akin to an anisotropic deflagration.

astro-ph.HE↗

arXiv2022

Magnetar bursts due to Alfv\'{e}n wave nonlinear breakout

The most common form of magnetar activity is short X-ray bursts, with durations from milliseconds to seconds, and luminosities ranging from $10^{36}$ to $10^{43}\ {\rm erg}\,{\rm s}^{-1}$. Recently, an X-ray burst from the galactic magnetar SGR 1935+2154 was detected to be coincident with two fast radio burst (FRB) like events from the same source, providing evidence that FRBs may be linked to magnetar bursts. Using fully 3D force-free electrodynamics simulations, we show that such magnetar bursts may be produced by Alfv\'{e}n waves launched from localized magnetar quakes: a wave packet propagates to the outer magnetosphere, becomes nonlinear, and escapes the magnetosphere, forming an ultra-relativistic ejecta. The ejecta pushes open the magnetospheric field lines, creating current sheets behind it. Magnetic reconnection can happen at these current sheets, leading to plasma energization and X-ray emission. The angular size of the ejecta can be compact, $\lesssim 1$ sr if the quake launching region is small, $\lesssim 0.01$ sr at the stellar surface. We discuss implications for the FRBs and the coincident X-ray burst from SGR 1935+2154.

astro-ph.HE↗

arXiv2020

Constraining temperature distribution inside LIGO test masses from frequencies of their vibrational modes

Thermal distortion of test masses, as well as thermal drift of their vibrational mode frequencies, present a major challenge for operation of the Advanced LIGO and Advanced VIRGO interferometers, reducing optical efficiency, which limits sensitivity and potentially causing instabilities which reduce duty-cycle. In this paper, we demonstrate that test-mass vibrational mode frequency data can be used to overcome some of these difficulties. First, we derive a general expression for the change in a mode frequency as a function of temperature distribution inside the test mass. Then we show how the mode frequency dependence on temperature distribution can be used to identify the wavefunction of observed vibrational modes. We then show how monitoring the frequencies of multiple vibrational modes allows the temperature distribution inside the test mass to be strongly constrained. Finally, we demonstrate using simulations, the potential to improve the thermal model of the test mass, providing independent and improved estimates of important parameters such as the coating absorption coefficient and the location of point absorbers.

astro-ph.IM↗