arXiv subjects

Vladimir V. Usov

Publications and source records attributed to Vladimir V. Usov.

At least 19 recordsLinked to original sources

arXiv2011

Gamma-ray emission from strongly magnetized pulsars

In a strong magnetic field, B > 4x10^{12} G, gamma-rays emitted nearly along curved field lines adiabatically convert into bound electron-positron pairs (positronium atoms) rather that decaying into free pairs. This process may modify the polar gaps of strong magnetized pulsars. Unlike free pairs, such bound pairs do not screen the electric field component along the magnetic field in the polar gaps. As a result the total power carried away by both relativistic particles and radiation, from the polar gap into the pulsar magnetosphere, may increase significantly (up to a few tens times) in comparison with the conventional polar gap models where creation of bound pairs is ignored, and it may be a substantial fraction of the spin-down power. We demonstrate that the total power of the modified polar gaps may be enough to explain the observed non-thermal luminosities of all known strongly magnetized, gamma-ray pulsars.

astro-ph.HE↗

arXiv2011

Effective Lagrangian in nonlinear electrodynamics and its properties of causality and unitarity

In nonlinear electrodynamics, by implementing the causality principle as the requirement that the group velocity of elementary excitations over a background field should not exceed the speed of light in the vacuum and the unitarity principle as the requirement that the residue of the propagator should be nonnegative, we establish the positive convexity of the effective Lagrangian on the class of constant fields, also the positivity of all characteristic dielectric and magnetic permittivity constants that are derivatives of the effective Lagrangian with respect to the field invariants. Violation of the general principles by the one-loop approximation in QED at exponentially large magnetic field is analyzed resulting in complex energy ghosts that signal the instability of the magnetized vacuum. Superluminal excitations (tachyons) appear, too, but for the magnetic field exceeding its instability threshold. Also other popular Lagrangians are tested to establish that the ones leading to spontaneous vacuum magnetization possess wrong convexity.

hep-th↗

arXiv2010

Real and virtual photons in an external constant electromagnetic field of most general form

The photon behavior in an arbitrary superposition of constant magnetic and electric fields is considered on most general grounds basing on the first principles like Lorentz- gauge- charge- and parity-invariance. We make model- and approximation-independent, but still rather informative, statements about the behavior that the requirement of causal propagation prescribes to massive and massless branches of dispersion curves, and describe the way the eigenmodes are polarized. We find, as a consequence of Hermiticity in the transparency domain, that adding a smaller electric field to a strong magnetic field in parallel to the latter causes enhancement of birefringence. We find the magnetic field produced by a point electric charge far from it (a manifestation of magneto-electric phenomenon). We establish degeneracies of the polarization tensor that (under special kinematic conditions) occur due to space-time symmetries of the vacuum left after the external field is imposed.

hep-th↗

arXiv2009

Convexity of effective Lagrangian in nonlinear electrodynamics as derived from causality

In nonlinear electrodynamics, by implementing the causality principle as the requirement that the group velocity of elementary excitations over a background field should not exceed unity, and the unitarity principle as the requirement that the residue of the propagator should be nonnegative, we find restrictions on the behavior of massive and massless dispersion curves and establish the convexity of the effective Lagrangian on the class of constant fields, also the positivity of all characteristic dielectric and magnetic permittivity constants. Violation of the general principles by the one-loop approximation in QED at exponentially large magnetic field is analyzed resulting in complex energy tachyons and super-luminal ghosts that signal the instability of the magnetized vacuum. General grounds for kinematical selection rules in the process of photon splitting/merging are discussed.

hep-th↗

arXiv2007

Modified Coulomb Law in a Strongly Magnetized Vacuum

We study electric potential of a charge placed in a strong magnetic field B>>4.4x10^{13}G, as modified by the vacuum polarization. In such field the electron Larmour radius is much less than its Compton length. At the Larmour distances a scaling law occurs, with the potential determined by a magnetic-field-independent function. The scaling regime implies short-range interaction, expressed by Yukawa law. The electromagnetic interaction regains its long-range character at distances larger than the Compton length, the potential decreasing across the magnetic field faster than along. Correction to the nonrelativistic ground-state energy of a hydrogenlike atom is found. In the infinite-magnetic-field limit the modified potential becomes the Dirac delta-function plus a regular background. With this potential the ground-state energy is finite - the best pronounced effect of the vacuum polarization.

astro-ph↗

arXiv2006

Collision of two identical hypersonic stellar winds in binary systems

We investigate the hydrodynamics of two identical hypersonic stellar winds in a binary system. The interaction of these winds manifests itself in the form of two shocks and a contact surface between them. We neglect the binary rotation and assume that the gas flow ahead of the shocks is spherically symmetrical. In this case the contact surface that separates the gas emanated from the different stars coincides with the midplane of the binary components. In the shock the gas is heated and flows away nearly along the contact surface. We find the shock shape and the hot gas parameters in the shock layer between the shock and the contact surface.

astro-ph↗

arXiv2006

Positronium collapse and the maximum magnetic field in pure QED

A maximum value for the magnetic field is determined, which provides the full compensation of the positronium rest mass by the binding energy in the maximum symmetry state and disappearance of the energy gap separating the electron-positron system from the vacuum. The compensation becomes possible owing to the falling to the center phenomenon. The maximum magnetic field may be related to the vacuum and describe its structure.

hep-th↗

arXiv2005

1-100 GeV Emission from Millisecond Pulsars

A number of rotation-powered millisecond pulsars are powerful sources of X-ray emission. We present predictions for the spectral characteristics of these sources at gamma-ray energies, using a model for acceleration and pair cascades on open magnetic field lines above the polar caps. Since these pulsars have low surface magnetic fields, the majority do not produce sufficient pairs to completely screen the accelerating electric field allowing particle acceleration to high altitude. The resulting emission above 1 GeV comes from curvature radiation by primary electrons with radiation-reaction-limited Lorentz factors. The spectra are very hard power-laws with exponential cutoffs between 1 and 50 GeV, and the spectral power peaks near the cutoff energy. Millisecond pulsars are thus ideal targets for air-Cherenkov detectors that are able to reach energy thresholds below 50 GeV.

astro-ph↗

arXiv2004

High-Energy Emission From Millisecond Pulsars

The X-ray and gamma-ray spectrum of rotation-powered millisecond pulsars is investigated in a model for acceleration and pair cascades on open field lines above the polar caps. Although these pulsars have low surface magnetic fields, their short periods allow them to have large magnetospheric potential drops, but the majority do not produce sufficient pairs to completely screen the accelerating electric field. The accelerating particles maintain high Lorentz factors and undergo cyclotron resonant absorption of radio emission, that produces and maintains a large pitch angle, resulting in a strong synchrotron component. The resulting spectra consist of several distinct components: curvature radiation from primary electrons dominating from 1 - 100 GeV, synchrotron radiation from primary and secondary electrons dominating up to about 100 MeV, and much weaker inverse-Compton radiation from primary electrons at 0.1 - 1 TeV. We find that the relative size of these components depends on pulsar period, period derivative, and neutron star mass and radius with the level of the synchrotron component also depending sensitively on the radio emission properties. This model is successful in describing the observed X-ray and gamma-ray spectrum of PSR J0218+4232 as synchrotron radiation, peaking around 100 MeV and extending up to a turnover around several GeV. The predicted curvature radiation components from a number of millisecond pulsars, as well as the collective emission from the millisecond pulsars in globular clusters, should be detectable with AGILE and GLAST. We also discuss a hidden population of X-ray-quiet and radio-quiet millisecond pulsars which have evolved below the pair death line, some of which may be detectable by telescopes sensitive above 1 GeV.

astro-ph↗

arXiv2002

Thermal Evolution and Light Curves of Young Bare Strange Stars

The cooling of a young bare strange star is studied numerically by solving the equations of energy conservation and heat transport for both normal and superconducting strange quark matter inside the star. We show that the thermal luminosity from the strange star surface, due to both photon emission and e+e- pair production, may be orders of magnitude higher than the Eddington limit, for about one day for normal quark matter but possibly for up to a hundred years for superconducting quark matter, while the maximum of the photon spectrum is in hard X-rays with a mean energy of ~ 100 keV or even more. This differs both qualitatively and quantitatively from the photon emission from young neutron stars and provides a definite observational signature for bare strange stars. It is shown that the energy gap of superconducting strange quark matter may be estimated from the light curves if it is in the range from ~ 0.5 MeV to a few MeV.

astro-ph↗

arXiv2002

Photon Splitting in the Superstrong Magnetic Fields of Pulsars

We discuss the polarization selection rules for splitting of the two principal electromagnetic modes that propagate in a vacuum polarized by a superstrong magnetic field (B>0.1B_{cr} ~ 4x10^{12} G). We show that below the threshold of free pair creation the selection rules found by Adler in the limit of weak dispersion remain unaffected by taking the resonant effects into consideration, i.e., splitting of one mode is strictly forbidden, while splitting of the other is allowed.

astro-ph↗

arXiv2001

The observational appearance of strange stars

Strange quark matter with the density of ~ 5\times 10^{14} g cm^{-3} might exist up to the surface of a strange star. This differs qualitatively from the case of a neutron star and opens observational possibilities to distinguish strange stars from neutron stars. The thermal emission of photons and e^+e^- pairs from the bare (or nearly bare) quark surface of hot strange stars is reviewed. Bare strange stars are reasonable candidates for soft gamma-ray repeaters. The bursting activity of a soft gamma-ray repeater may be explained by fast heating of the bare quark surface of a strange star up to the temperature of ~ (1-2)\times 10^9 K and its subsequent thermal emission.

astro-ph↗

arXiv2001

Response of bare strange stars to energy input onto their surfaces

We study numerically the thermal emission of $e^+e^-$ pairs from a bare strange star heated by energy input onto its surface; heating starts at some moment, and is steady afterwards. The thermal luminosity in $e^+e^-$ pairs increases to some constant value. The rise time and the steady thermal luminosity are evaluated. Both normal and colour superconducting states of strange quark matter are considered. The results are used to test the magnetar model of soft gamma-ray repeaters where the bursting activity is explained by fast decay of superstrong magnetic fields and heating of the strange star surface. It is shown that the rise times observed in typical bursts may be explained in this model only if strange quark matter is a superconductor with an energy gap of more that 1 MeV.

astro-ph↗

arXiv2001

Thermal emission from bare quark matter surfaces of hot strange stars

We consider the thermal emission of photons and electron-positron pairs from the bare quark surface of a hot strange star. The radiation of high-energy (> 20 MeV) equilibrium photons prevails at the surface temperature T_S > 5 x 10^{10} K, while below this temperature, 8 x 10^8 < T_S < 5 x 10^{10} K, emission of electron-positron pairs created by the Coulomb barrier at the quark surface dominates. The thermal luminosity of a hot strange star in both photons and pairs is estimated.

astro-ph↗

arXiv2000

Low Frequency Radio Pulses from Gamma-Ray Bursts?

Gamma-ray bursts, if they are generated in the process of interaction between relativistic strongly magnetized winds and an ambient medium, may be accompanied by very short pulses of low-frequency radio emission. The bulk of this emission is expected to be at the frequencies of $\sim 0.1-1$ MHz and cannot be observed. However, the high-frequency tail of the low-frequency radio emission may reach a few ten MHz and be detected, especially if the strength of the magnetic field of the wind is extremely high.

astro-ph↗

arXiv2000

Induction of supernova-like explosions by gamma-ray bursts in close binary systems

We propose that a gamma-ray burst in one member of a binary may induce a supernova-like explosion of a close, white-dwarf companion. Such an explosion might be brought about in rather light companions, which cannot undergo the standard accretion-induced explosion. This would give some GRB-associated supernova an appearance rather unlike that of the typical Type I. GRB 980425, if indeed associated with SN 1998bw, is too weak to have produced it through our proposed mechanism.

astro-ph↗

arXiv1999

Radiating Regions in Pulsar Magnetospheres: From Theory to Observations and Back

We discuss plausible locations of radiating regions in the magnetospheres of pulsars and argue that the mechanisms of pulsar radiation at different frequencies are closely connected with the locations of the radiating regions, especially in the radio range. If the region that is responsible for the bulk of the non-thermal radiation at some frequency is localized in the pulsar magnetosphere then the nature of this radiation will be either determined or, at least, restricted significantly.

astro-ph↗

arXiv1999

Radiation from Relativistic Strongly Magnetized Outflows

Relativistic strongly magnetized winds outflowing from fast-rotating compact objects like millisecond pulsars with surface magnetic fields of $\sim 10^{15}-10^{16}$ G are plausible sources of cosmological $γ$-ray bursts. In such winds, there are at least three regions where extremely powerful X-ray and $γ$-ray emission may be generated. The first radiating region is the wind photosphere that is at a distance of $\sim 10^9$ cm from the compact object. The second radiating region is at a distance of $\sim 10^{13}-10^{14}$ cm. In this region, the striped component of the wind field is transformed into large-amplitude electromagnetic waves. The third radiating region is at a distance of $\sim 10^{16}-10^{17}$ cm, where deceleration of the wind due to its interaction with an ambient medium becomes important. Radiation from all these regions is considered.

astro-ph↗