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Andrei Gruzinov

Publications and source records attributed to Andrei Gruzinov.

At least 37 records · Page 2Linked to original sources
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↗
arXiv2018

Are 10 EeV cosmic rays extragalactic? Theory of cosmic ray diffusion at high energy

Auger Collaboration has reported a large-scale anisotropy in the arrival directions of cosmic rays above 8 EeV. The dipole direction, at 125$^\circ$ from the Galactic center, is taken as an indication of an extragalactic origin of these cosmic rays. We show, both theoretically and by direct numerical simulations, that this is not necessarily true. Intermediate mass nuclei originating in the Galaxy and diffusing in the Galactic magnetic field can have a dipole anisotropy pointing away from the Galactic center. Our theory predicts a quadrupole anisotropy of the same order of magnitude as the dipole.

astro-ph.HE↗
arXiv2016

Fireballs from Superconducting Cosmic Strings

Thermalized fireballs should be created by cusp events on superconducting cosmic strings. This simple notion allows to reliably estimate particle emission from the cusps in a given background magnetic field. With plausible assumptions about intergalactic magnetic fields, the cusp events can produce observable fluxes of high-energy photons and neutrinos with unique signatures.

astro-ph.HE↗
arXiv2016

Black Hole Spindown by Light Bosons

The saturation mechanism for the fastest-growing instability of massive scalar field in Kerr metric is identified, assuming saturation by cubic or quartic nonlinearities of the field potential. The resulting spindown rate of the black hole is calculated. The (rather involved) saturation scenario is confirmed by numerical simulations.

astro-ph.HE↗
arXiv2016

Gravitational Radiation from Massless Particle Collisions

We compute classical gravitational bremsstrahlung from the gravitational scattering of two massless particles at leading order in the (center of mass) deflection angle $θ\sim 4 G \sqrt{s}/b = 8 G E/b \ll 1$. The calculation, although non-perturbative in the gravitational constant, is surprisingly simple and yields explicit formulae --in terms of multidimensional integrals-- for the frequency and angular distribution of the radiation. In the range $ b^{-1} < ω< (GE)^{-1}$, the GW spectrum behaves like $ \log (1/GEω) d ω$, is confined to cones of angular sizes (around the deflected particle trajectories) ranging from $O(θ)$ to $O(1/ωb)$, and exactly reproduces, at its lower end, a well-known zero-frequency limit. At $ω> (GE)^{-1}$ the radiation is confined to cones of angular size of order $θ(GEω)^{-1/2}$ resulting in a scale-invariant ($dω/ω$) spectrum. The total efficiency in GW production is dominated by this "high frequency" region and is formally logarithmically divergent in the UV. If the spectrum is cutoff at the limit of validity of our approximations (where a conjectured bound on GW power is also saturated), the fraction of incoming energy radiated away turns out to be $\frac{1}{2 π} θ^2 \log θ^{-2}$ at leading logarithmic accuracy.

gr-qc↗
arXiv2015

No-Hair Theorem for Weak Pulsar

It is proposed that there exists a class of pulsars, called weak pulsars, for which the large-scale magnetosphere, and hence the gamma-ray emission, are independent of the detailed pattern of plasma production. The weak pulsar magnetosphere and its gamma-ray emission are uniquely determined by just three parameters: spin, dipole, and the spin-dipole angle. We calculate this supposedly unique pulsar magnetosphere in the axisymmetric case. The magnetosphere is found to be very close to (although interestingly not fully identical with) the magnetosphere we have previously calculated, explaining the phenomenological success of the old calculation. We offer only a highly tentative proof of this "Pulsar No-Hair Theorem". Our analytics, while convincing in its non-triviality, is incomplete, and counts only as a plausibility argument. Our numerics, while complete, is dubious. The plasma flow in the weak pulsar magnetosphere turns out to be even more intricate than what we have previously proposed: some particles, after being created near the star, move beyond the light cylinder and then return to the star.

astro-ph.HE↗
arXiv2014

Laboratory gamma-ray pulsar

The mechanism by which gamma-ray pulsars shine might be reproducible in a laboratory. This claim is supported by three observations: (i) properly focusing a few PW optical laser gives an electromagnetic field in the so-called Aristotelian regime, when a test electron is radiation-overdamped; (ii) the Goldreich-Julian number density of this electromagnetic field (the number density of elementary charges needed for a nearly full conversion of optical power into gamma-rays) is of order the electron number density in a solid; (iii) above about $50$PW, the external source of electrons is not needed -- charges will be created by a pair production avalanche.

astro-ph.HE↗
arXiv2014

How Pulsars Shine: Poynting Flux Annihilation

Recent solution of a pulsar (although likely correct, as agreeing with phenomenology of weak pulsars without a single adjustable) is somewhat unsatisfactory on account of being heavily numerical. Here the main features of a pulsar -- (i) the existence of a force-free zone, (ii) bounded by a non-force-free radiation zone, (iii) where colliding Poynting fluxes annihilate into curvature radiation -- are reproduced as an exact solution of the same pulsar equations but in a much simpler geometry.

astro-ph.HE↗
arXiv2013

1/2, 1, and 3/2--Law Non-Radiative Accretion Flows

Assuming self-similarity of the first kind, we get three possible values p=1/2, 1, 3/2 for the exponent describing the density profile, rho r^{-p}, of a non-radiative (and hence quasi-spherical) accretion flow. The high and low p cases are known as Bondi and Convection-Dominated accretion flows. The 1-law flow we tentatively identify with the so-called Magnetically-Frustrated accretion flow. If our interpretation is correct, the accretion flow must be, roughly speaking, a collection of Prandtl's turbulent jets. The 1-law flow, being a first-kind self-similar solution, may actually occur in nature (in collisionless plasma).

astro-ph.HE↗
arXiv2013

Fermi Pulsars and the Cosmic Ray Knee

Nuclei, if lifted into the radiation zone of a weak axisymmetric pulsar, are accelerated to quasi-monoenergetic spectrum, with more than half of the nuclear luminosity emitted nearly monoenergetically, at the energy E approximately = 4Ze(L/c)^(1/2), where L is the spin-down power. With a number of (not obviously wrong) assumptions, this turns Fermi (-like) weak pulsars into prominent accelerators at energies near the cosmic ray knee. The main uncertainty is purely theoretical: one needs to understand if nuclei are major positive charge carries in some weak pulsar magnetospheres.

astro-ph.HE↗
arXiv2013

Pulsars as Standard Candles

To illustrate the standard candle property of gamma-ray pulsars (and also to thereby confirm the recent first-principle calculation of pulsar gamma-ray emission), we "measure", via the lightcurve fitting, three distances and one moment of inertia of some weak pulsars. We are not sure what the three distances are good for, but the measurement of the moment of inertia must be of interest for nuclear physics. Although we must state that the quality of the numerical program which we use to calculate the lightcurves and efficiencies is inadequate (as are the author's qualifications as a numericist and data analyst), in good hands, and upon an easily doable extension to non-weak pulsars, the method's yield should be impressive.

astro-ph.HE↗
arXiv2013

Distances and moments of inertia of Fermi Pulsars

Measurement of distances and moments of inertia of pulsars must be useful, for instance, for three-dimensional mapping of the dispersion and rotation measures, constraining the nuclear equation of state, etc. The distances and moments of inertia can be measured by fitting the gamma-ray lightcurves of pulsars, because the gamma-ray emission seems to be governed by easily calculable physics. The (first-principle) theoretical lightcurves have been computed only for weak pulsars (pair production near the light cylinder much smaller than Goldreich-Julian [1] per rotation), and at insufficient accuracy; but, since this computation has been done by a self-taught numericist, it must be possible to improve the theoretical accuracy for weak pulsars, and also to extend the computation to non-weak pulsars. To invite the computational effort of better-equipped researchers, we describe an (entirely obvious) procedure for measuring the distances and moments of inertia for weak pulsars.

astro-ph.HE↗
arXiv2013

Pulsar Emission Spectrum

Emission spectrum is calculated for a weak axisymmetric pulsar. Also calculated are the observed spectrum, efficiency, and the observed efficiency. The underlying flow of electrons and positrons turns out to be curiously intricate.

astro-ph.HE↗
arXiv2013

The radiation belt of the Sun

For a given solar magnetic field, the near-Sun (phase-space) density of cosmic ray electrons and positrons of energy above about 10GeV can be calculated from first principles, without any assumptions about the cosmic ray diffusion. This is because the sunlight Compton drag must be more important than diffusion. If the solar magnetic field has an appreciable dipole component, the electron/positron density should have a belt-like dent, perhaps extending to several solar radii. The belt structure appears because the quasi-bound orbits are depopulated by the sunlight Compton drag.

astro-ph.HE↗
arXiv2013

Aristotelian Electrodynamics solves the Pulsar: Lower Efficiency of Strong Pulsars

In Aristotelian Electrodynamics (AE), due to radiation over-damping, the velocity rather than the acceleration of a charge is determined by the local electromagnetic field. Treating electrons and positrons separately, AE seems to give a faithful description of the flow of charges in a pulsar magnetosphere. AE might allow to calculate the pulsar emission in full detail, at all energies except the radio. But AE is not a full theory -- the plasma production processes must be added. Here we show that the pulsar magnetosphere and the resulting emission are sensitive to the plasma production rate near the light cylinder. In strong pulsars (high production rate, due to photon-photon collisions), the number of field lines closing beyond the light cylinder decreases. This reduces both the pulsar efficiency (the ratio of the pulsed bolometric luminosity to the spin-down power) and the characteristic photon energy, in overall agreement with the Fermi data. For weak pulsars (low production rate), our numerical implementation of AE works stably and seems to be ready to calculate the lightcurves and spectra. But for strong pulsars, the code (included) needs improvement or better understanding.

astro-ph.HE↗
arXiv2012

Pulsar Efficiency

Pulsar efficiency, defined as the ratio of the pulsed bolometric luminosity to the spin-down power, is calculated to be about 15% (averaged over the spin-dipole inclination angle, ranging between about 50% for the aligned and 10% for the orthogonal). We also estimate the characteristic photon energy and argue that our results agree with the Fermi pulsar catalog -- in a sense.

astro-ph.HE↗