arXiv subjects

Andrei Gruzinov

Publications and source records attributed to Andrei Gruzinov.

At least 19 recordsLinked to original sources

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↗

arXiv2025

The collisionless hydrodynamics: On the nonexistence of collisionless shocks

Collisionless shocks, essential for astrophysics, perhaps do not exist as statistically stationary solutions. If so, any quantitative statement about a collisionless shock should be qualified by the age of the shock. A theoretical description of the upstream of the 1+1 dimensional electrostatic collisionless shock is developed -- collisionless hydrodynamics. Peculiarities of collisionless hydrodynamics prevent a shock formation when a piston is driven into cold plasma. An exact self-similar solution is found instead; the spatial extent of the solution grows linearly in time. Direct numerical simulations of plasma kinetics in 1+1 dimensions confirm the hydrodynamic result -- a statistically steady collisionless shock doesn't exist. Instead, at each fixed time, there is a continuous succession in space of marginally stable velocity distribution functions. The spatial support of this continuous succession grows linearly in time.

astro-ph.HE↗

arXiv2024

Strong Bow Shocks: Turbulence and An Exact Self-Similar Asymptotic

We show that strong bow shocks are turbulent and non-universal near the head, but asymptote to a universal steady, self-similar, and analytically solvable flow in the downstream. The turbulence is essentially 3D, and has been confirmed by a 3D simulation. The asymptotic behavior is confirmed with high resolution 2D and 3D simulations of a cold uniform wind encountering both a solid spherical obstacle and stellar wind. This solution is relevant in the context of: (i) probing the kinematic properties of observed high-velocity compact bodies -- e.g., runaway stars and/or supernova ejecta blobs -- flying through the interstellar medium; and (ii) constraining stellar bow shock luminosities invoked by some quasi-periodic eruption (QPE) models.

astro-ph.HE↗

arXiv2024

The Density of Relic Neutrinos Near the Surface of Earth

It has been claimed that matter effects cause an asymmetry in the density of relic neutrinos versus antineutrinos near the surface of Earth, of order $O(G_F^{1/2})\sim 10^{-4}$, with the vertical extent $\sim 10$m. We argue that the effect is of order $O(G_F)\sim 10^{-8}$, with the vertical extent $\sim 1$mm.

hep-ph↗

arXiv2024

Self-similarity of the third type in ultra relativistic blastwaves

A new type of self-similarity is found in the problem of a plane-parallel, ultra-relativistic blast wave, propagating in a powerlaw density profile of the form $ρ\propto z^{-k}$. Self-similar solutions of the first kind can be found for $k<7/4$ using dimensional considerations. For steeper density gradients with $k>2$, second type solutions are obtained by eliminating a singularity from the equations. However, for intermediate powerlaw indices $7/4<k<2$ the flow does not obey any of the known types of self-similarity. Instead, the solutions belong to a new class in which the self-similar dynamics are dictated by the non self-similar part of the flow. We obtain an exact solution to the ultra-relativistic fluid equations and find that the non self-similar flow is described by relativistic expansion into vacuum, composed of (1) an accelerating piston that contains most of the energy and (2) a leading edge of fast material that coincides with the interiors of the blastwave and terminates at the shock. The dynamics of the piston itself are self-similar and universal, and do not depend on the external medium. The exact solution of the non self-similar flow is used to solve for the shock in the new class of solutions.

astro-ph.HE↗

arXiv2023

How Pulsars Shine II: TeV emission

Recent discovery of 20 TeV radiation from the Vela pulsar confirms (tentatively, at the level of crude estimates) the Aristotelian Electrodynamics picture of pulsar radiation: pulsars shine, mostly in GeV, by annihilating colliding Poynting fluxes into curvature radiation near the light cylinder. The observed GeV photons are the curvature radiation of electrons/positrons with Lorentz factors ~10^8. These "super-ultra-relativistic" electrons/positrons must also produce TeV radiation by inverse Compton if low-energy target photons are available, as they are in the Vela pulsar.

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↗

arXiv2023

Shapes of non-Gaussianity in warm inflation

Sphaleron heating has been recently proposed as a mechanism to realize warm inflation when inflaton is an axion coupled to pure Yang-Mills. As a result of heating, there is a friction coefficient $γ\propto T^3$ in the equation of motion for the inflaton, and a thermal contribution to cosmological fluctuations. Without the knowledge of the inflaton potential, non-Gaussianity is the most promising way of searching for the signatures of this model. Building on an earlier work by Bastero-Gil, Berera, Moss and Ramos, we compute the scalar three-point correlation function and point out some distinct features in the squeezed and folded limits. As a detection strategy, we show that the combination of the equilateral template and one new template has a large overlap with the shape of non-Gaussianity over the range $0.01 \leqγ/H\leq 1000$, and in this range $0.7 <|f_{\rm NL}| < 50$.

astro-ph.CO↗

arXiv2021

Subsonic Wind Accretion

The rate of subsonic wind accretion (accretion on a point Newtonian mass moving through uniform gas) is shown to be independent of the wind velocity and equal to the spherical Bondi rate -- for the adiabatic index equal to 5/3. A (very accurate) numerical calculation of the accretion flow, confirming this result, is also presented.

astro-ph.HE↗

arXiv2021

Fast Afterglows of Fast Radio Bursts

The main FRB event may leave behind a clump of relativistic plasma with high ``free energy'' density. As the plasma undergoes collisionless relaxation, it emits coherent electromagnetic waves. These electromagnetic waves may be observable as a fast radio afterglow, with decreasing frequency and intensity. We demonstrate the fast coherent afterglow in a numerical experiment. We tentatively predict the peak afterglow frequency decreasing with time as $ν\propto t^{-3/2}$.

astro-ph.HE↗

arXiv2020

Spinning black holes as cosmic string factories

We consider the evolution of a cosmic string loop that is captured by a much more massive and compact black hole. We show that after several reconnections that produce ejections of smaller loops, the loop that remains bound to the black hole moves on a nearly-periodic non-self-intersecting trajectory, "the orbit". The orbit evolves due to an energy and angular momentum exchange between the loop and the spinning black hole. We show that such evolution is mathematically equivalent to a certain continuous deformation of an auxiliary closed curve in a 3-dimensional space; for zero black-hole spin this deformation is curve-shortening that has been extensively studied by mathematicians. The evolution features competing effects of loop growth by the superradiant extraction of the black-hole spin energy, and loop decay by the friction of the moving string against the horizon. A self-intersection of an auxiliary curve corresponds to a capture by the black hole of a new string segment and thus an addition of a new captured loop. Possible asymptotic states of such evolution are shown to be strong emitters of gravitational waves. Whether reconnections prevent reaching the asymptotic states remains to be explored. Additionally, the orbit's shape also evolves due to an emission of gravitational waves, and a recoil of the black hole that changes the orbit and likely leads to self-intersections. We argue that for a significant range of the dimensionless tension $μ$, string loops are captured by supermassive black holes at the centers of galaxies. This strongly motivates further study of interaction between string loops and black holes, especially the influence of this process on the black hole spindown and on the production of gravitational waves by strings created in galactic nuclei. We also discuss potential loop captures by primordial black holes.

astro-ph.HE↗

arXiv2020

The Thermodynamics of Rotating Black-Hole Star Clusters

Rotating star clusters near supermassive black holes are studied using Touma-Tremaine thermodynamics of gravitationally interacting orbital ellipses. A simple numerical procedure for calculating thermodynamic equilibrium states for an arbitrary distribution of stars over masses and semimajor axes is described. Spontaneous symmetry breaking and breakdown of thermodynamics at low positive temperatures are rigorously proven for non-rotating clusters. Rotation is introduced through a second temperature-like parameter. Both axially symmetric and lopsided rotational equilibria are found; the lopsided equilibria precess with the angular velocity that is given by the ratio of the two temperatures. Eccentric stellar disc in the nucleus of Andromeda galaxy may be an example of a lopsided thermodynamic equilibrium of a rotating black hole star cluster. Stellar-mass black holes occupy highly eccentric orbits in broken-symmetry star clusters, and form flattened disc-like configurations in rotating star clusters. They are attracted to orbits that are stationary in the frame of reference rotating with the angular velocity of the cluster. In spherical clusters, stellar-mass black holes' orbits are significantly more eccentric than those of the lighter stars if the temperature is negative, and more circular if the temperature is positive. Finally we note that planets, comets, dark matter particles and other light bodies tend to form a spherically symmetric non-rotating sub-cluster with maximum-entropy eccentricity distribution $\mathscr{P}(e)=2e$, even if their host cluster is rotating and lopsided.

astro-ph.GA↗

arXiv2020

Spin of Primordial Black Holes

Primordial black holes, formed from rare peaks in the primordial fluctuations $ζ$, are non-rotating at zeroth order in $ζ_{\rm rms}$. We show that the spin also vanishes at first order in $ζ_{\rm rms}$, suggesting the dimensionless spin parameter $a_{\rm rms} \sim ζ_{\rm rms}^2$. We identify one quadratic contribution to the spin by calculating (and extrapolating to the formation time) the torque on a black hole due to ambient acoustic waves. For a reasonable density of primordial black holes this implies a percent level spin parameter.

astro-ph.CO↗

arXiv2020

Nonlinear scattering of Fast Radio Bursts

Nonlinear radio waves modulate the plasma, scatter on the modulations, and develop an intermittent power spectrum -- perhaps. Rudiments of theory, numerical simulations, and qualitative modeling of nonlinear scattering are presented. Nonlinear radio waves in electron-positron plasma are shown to be unstable, the instability growth rate is calculated. The long-term outcome of the instability is studied numerically; an intermittent power spectrum is found. The frequency coherence scale of the intermittent power spectrum is interpreted as a measure of the nonlinear scattering screen.

astro-ph.HE↗

arXiv2019

Negative Dynamical Friction on compact objects moving through dense gas

An over-dense wake is created by a gravitating object moving through a gaseous medium, and this wake pulls back on the object and slows it down. This is conventional dynamical friction in a gaseous medium. We argue that if the object drives a sufficiently powerful outflow, the wake is destroyed and instead an extended under-dense region is created behind the object. In this case the overall gravitational force is applied in the direction of the object's motion, producing a negative dynamical friction (NDF). Black holes in dense gas drive powerful outflows and may experience the NDF, although extensive numerical work is probably needed to demonstrate or refute this conclusively. NDF may be important for stellar-mass black holes and neutron stars inside `common envelopes' in binary systems, for stellar mass black holes inside AGN discs, or for massive black holes growing through super-Eddington accretion in early Universe.

astro-ph.HE↗

arXiv2019

Conversion Measure of Faraday Rotation-Conversion with application to Fast Radio Bursts

Faraday Rotation-Conversion is the simultaneous rotation of all three Stokes polarization parameters $Q$, $U$, $V$ as an electromagnetic wave propagates through a magnetized plasma. In this regime the Faraday plasma screen is characterized by more than just a Rotation Measure. We define the Conversion Measure that characterizes the wavelength-dependent conversion between the linear and circular polarization. In a cold plasma, the conversion occurs at the localized regions along the wave's path, where the large-scale magnetic field is perpendicular to the propagation direction. The number of these regions along the line of sight through the screen, and their individual contributions to the conversion measure, can be inferred from the polarization measurements. We argue that the simultaneous measurement of wavelength-dependent linear and circular polarization might give an important insight into the magnetic-field geometry of the Faraday screen in FRB121102 and other repeating Fast Radio Bursts.

astro-ph.HE↗

arXiv2019

Maser and other instabilities in a weakly magnetized relativistic plasma: Theory and the astrophysical relevance of the maser

A sufficient condition for maser instability in a weakly magnetized relativistic plasma with an isotropic particle distribution function is given. The maser growth rates and polarizations are computed starting from the exact dielectric permittivity tensor of a magnetized plasma. For very weak magnetic fields, our results confirm the approximate validity of the 'standard maser theory', which is based on the Einstein coefficients method, with one significant exception. For inclined propagation and realistic (small but finite) field, the growth rates of the two (nearly circular) polarizations differ significantly, while the standard theory predicts two (nearly circular) polarizations with similar growth rates. We show that this deviation is due to circularly polarized synchrotron emission, which is neglected in the standard theory. The maser is shown to grow slower than Langmuir waves. Nevertheless, significant generation of EM waves is seen in (highly simplified) direct numerical simulations. We study the nonlinear saturation of the maser instability and find that it offers a mechanism for the conversion of a significant fraction of the plasma energy into radio waves. We briefly discuss the conditions under which the maser instability may operate in astrophysical sources, and provide rough estimates that may be used as a guidance when studying particular astrophysical sources/phenomena.

astro-ph.HE↗

arXiv2018

Cosmic strings and primordial black holes

Cosmic strings and primordial black holes (PBHs) commonly and naturally form in many scenarios describing the early universe. Here we show that if both cosmic strings and PBHs are present, their interaction leads to a range of interesting consequences. At the time of their formation, the PBHs get attached to the strings and influence their evolution, leading to the formation of black-hole-string networks and commonly to the suppression of loop production in a range of redshifts. Subsequently, reconnections within the network give rise to small nets made of several black holes and connecting strings. The number of black holes in the network as well as the stability of the nets depend on the topological properties of the strings. The nets oscillate and shrink exponentially due to the emission of gravitational waves. This leads to potentially observable string-driven mergers of PBHs. The strings can keep PBHs from galactic halos, making the current bounds on PBHs not generally applicable. Alternatively, heavy PBHs can drag low-tension strings into the centers of galaxies. The superconducting strings can appear as radio filaments pointing towards supermassive black holes.

astro-ph.CO↗