Powerful laser-produced quasi-half-cycle THz pulses

The Maxwell equations based 3D analytical solution for the terahertz half-cycle electromagnetic wave transition radiation pulse has been found. This solution describes generation and propagation of transition radiation into free space from laser-produced relativistic electron bunch crossing a target-vacuum interface as a result of ultrashort laser pulse interaction with a thin high-conductivity target. The analytical solution found complements the theory of laser initiated transition radiation by describing the generated THz wave shape at the arbitrary distance from the generating target surface domain including near-field zone rather than the standard far-field characterization. The analytical research has also been supplemented with the 3D simulations using the finite-diference time-domain (FDTD) method, which makes it possible for description of much wider spatial domain as compared to that from the particle-in-cell (PIC) approach. The results reported fundamentally shed light on the interfere of an electron bunch field and THz field of broadband transition radiation from laser-plasma interaction studied for a long time in the experiments with solid density plasma and may in future inspire them to targeted measurements and investigations of unique super intense half-cycle THz radiation waves near the laser target

Possible contractions of quantum orthogonal groups

Possible contractions of quantum orthogonal groups which correspond to different choices of primitive elements of Hopf algebra are considered and all allowed contractions in Cayley--Klein scheme are obtained. Quantum deformations of kinematical groups have been investigated and have shown that quantum analog of (complex) Galilei group G(1,3) do not exist in our scheme.Comment: 10 pages, Latex. Report given at XXIII Int. Colloquium on Group Theoretical Methods in Physics, July 31- August 5, 2000, Dubna (Russia

On contractions of classical basic superalgebras

We define a class of orthosymplectic $osp(m;j|2n;\omega)$ and unitary $sl(m;j|n;\epsilon)$ superalgebras which may be obtained from $osp(m|2n)$ and $sl(m|n)$ by contractions and analytic continuations in a similar way as the special linear, orthogonal and the symplectic Cayley-Klein algebras are obtained from the corresponding classical ones. Casimir operators of Cayley-Klein superalgebras are obtained from the corresponding operators of the basic superalgebras. Contractions of $sl(2|1)$ and $osp(3|2)$ are regarded as an examples.Comment: 15 pages, Late

Cayley--Klein Contractions of Quantum Orthogonal Groups in Cartesian Basis

Spaces of constant curvature and their motion groups are described most naturally in Cartesian basis. All these motion groups also known as CK groups are obtained from orthogonal group by contractions and analytical continuations. On the other hand quantum deformation of orthogonal group $SO(N)$ is most easily performed in so-called symplectic basis. We reformulate its standard quantum deformation to Cartesian basis and obtain all possible contractions of quantum orthogonal group $SO_q(N)$ both for untouched and transformed deformation parameter. It turned out, that similar to undeformed case all CK contractions of $SO_q(N)$ are realized. An algorithm for obtaining nonequivalent (as Hopf algebra) contracted quantum groups is suggested. Contractions of $SO_q(N), N=3,4,5$ are regarded as an examples.Comment: The statement of the basic theorem have correct. 30 pages, Latex. Report given at X International Conference on Symmetry Methods in Physics, August 13-19, 2003, Yerevan, Armenia. Submitted in Journal Physics of Atomic Nucle

Properties of the δ\delta Scorpii Circumstellar Disk from Continuum Modeling

We present optical $WBVR$ and infrared $JHKL$ photometric observations of the Be binary system $\delta$ Sco, obtained in 2000--2005, mid-infrared (10 and $18 \mu$m) photometry and optical ($\lambda\lambda$ 3200--10500 \AA) spectropolarimetry obtained in 2001. Our optical photometry confirms the results of much more frequent visual monitoring of $\delta$ Sco. In 2005, we detected a significant decrease in the object's brightness, both in optical and near-infrared brightness, which is associated with a continuous rise in the hydrogen line strenghts. We discuss possible causes for this phenomenon, which is difficult to explain in view of current models of Be star disks. The 2001 spectral energy distribution and polarization are succesfully modeled with a three-dimensional non-LTE Monte Carlo code which produces a self-consistent determination of the hydrogen level populations, electron temperature, and gas density for hot star disks. Our disk model is hydrostatically supported in the vertical direction and radially controlled by viscosity. Such a disk model has, essentially, only two free parameters, viz., the equatorial mass loss rate and the disk outer radius. We find that the primary companion is surrounded by a small (7 $R_\star$), geometrically-thin disk, which is highly non-isothermal and fully ionized. Our model requires an average equatorial mass loss rate of 1.5\times 10^{-9} M_{\sun} yr$^{-1}$.Comment: 27 pages, 9 figures, submitted to Ap

Blazar spectral variability as explained by a twisted inhomogeneous jet

Blazars are active galactic nuclei, which are powerful sources of radiation whose central engine is located in the core of the host galaxy. Blazar emission is dominated by non-thermal radiation from a jet that moves relativistically towards us, and therefore undergoes Doppler beaming1. This beaming causes flux enhancement and contraction of the variability timescales, so that most blazars appear as luminous sources characterized by noticeable and fast changes in brightness at all frequencies. The mechanism that produces this unpredictable variability is under debate, but proposed mechanisms include injection, acceleration and cooling of particles2, with possible intervention of shock waves3,4 or turbulence5. Changes in the viewing angle of the observed emitting knots or jet regions have also been suggested as an explanation of flaring events6,7,8,9,10 and can also explain specific properties of blazar emission, such as intra-day variability11, quasi-periodicity12,13 and the delay of radio flux variations relative to optical changes14. Such a geometric interpretation, however, is not universally accepted because alternative explanations based on changes in physical conditions—such as the size and speed of the emitting zone, the magnetic field, the number of emitting particles and their energy distribution—can explain snapshots of the spectral behaviour of blazars in many cases15,16. Here we report the results of optical-to-radio-wavelength monitoring of the blazar CTA 102 and show that the observed long-term trends of the flux and spectral variability are best explained by an inhomogeneous, curved jet that undergoes changes in orientation over time. We propose that magnetohydrodynamic instabilities17 or rotation of the twisted jet6 cause different jet regions to change their orientation and hence their relative Doppler factors. In particular, the extreme optical outburst of 2016–2017 (brightness increase of six magnitudes) occurred when the corresponding emitting region had a small viewing angle. The agreement between observations and theoretical predictions can be seen as further validation of the relativistic beaming theory

Investigating the multiwavelength behaviour of the flat spectrum radio quasar CTA 102 during 2013-2017

We present a multiwavelength study of the flat-spectrum radio quasar CTA 102 during 2013-2017. We use radio-to-optical data obtained by the Whole Earth Blazar Telescope, 15 GHz data from the Owens Valley Radio Observatory, 91 and 103 GHz data from the Atacama Large Millimeter Array, near-infrared data from the Rapid Eye Monitor telescope, as well as data from the Swift (optical-UV and X-rays) and Fermi (gamma-rays) satellites to study flux and spectral variability and the correlation between flux changes at different wavelengths. Unprecedented gamma-ray flaring activity was observed during 2016 November-2017 February, with four major outbursts. A peak flux of (2158 +/- 63) x 10(-8) ph cm(-2) s(-1), corresponding to a luminosity of (2.2 +/- 0.1) x10(50) erg s(-1), was reached on 2016 December 28. These four gamma-ray outbursts have corresponding events in the near-infrared, optical, and UV bands, with the peaks observed at the same time. A general agreement between X-ray and gamma-ray activity is found. The gamma-ray flux variations show a general, strong correlation with the optical ones with no time lag between the two bands and a comparable variability amplitude. This gamma-ray/optical relationship is in agreement with the geometrical model that has successfully explained the low-energy flux and spectral behaviour, suggesting that the long-term flux variations are mainly due to changes in the Doppler factor produced by variations of the viewing angle of the emitting regions. The difference in behaviour between radio and higher energy emission would be ascribed to different viewing angles of the jet regions producing their emission