The role of loop footpoints rotation in single loop flares

We consider that single loop flares can be caused by the rotation of loop footpoints. Choosing a typical geometry for this case we find from MHD equations self-consistent expressions and a set equations governing behaviour of all physical quantities. Numerical simulations have revealed that under the determined conditions for the initial azimuthal velocity and current the pinch instability takes place. The most important parameters of the problem are the plasma and the ratio of the initial values of longitudinal and poloidal components of the magnetic field-B ₁. Thus, calculations show that the critical pinch time increases with the increase ofB ₁ and decreases with the increase of plasma . So the most effective flares are probable for the most high loops with strong currents. ForB ₁=10 and =0.01 the critical pinch time is 2.5 s. The critical twist angle for magnetic field depends on the initial one. For low intial twist which corresponds to bigB ₁ the critical one is more less. For exampleB ₁=30 gives 1.8 (when ratio of loop length and radius is 10). Geometrical analysis shows that the present model can explain (for high photospheric rotation) single loop flares taking place on different parts of the loop as on the top of it as closer to one of the footpoints. It depends on the relative rotation momentum of loop footpoints. Subject headings: MHD-Sun:flares.     

Characteristics of plasma temperature variation during two-current-loop collisions

Characteristics of plasma temperature variations during two-current-loop collisions are described. It is revealed that plasma temperature has an oscillatory feature with damping amplitude and growing quasi-period in the case of anI-type collision. In the case of aY-type collision, if the initial current becomes strong enough, there also occur pulsations of the temperature. However, the temperature profile of anX-type collision is characterized by a single pulse only.     

Study of nonstationary processes in the close binary system RY Scuti

A new analysis is made of old electrophotometric data on RY Scuti, a close binary of an early spectral class, taken at the Abastumani Astrophysical Observatory, Georgia, during 1972–1985 and at the Maidanak Observatory, Uzbekistan, during 1979–1991. Nonstationary processes occurring from period to period, from month to month, and from year to year are identified in RY Scuti. These variations range from hundredths to tenths of a stellar magnitude. In addition, periodic changes in the light curve of this system with a period of several years have been identified near the first maximum. This result is of considerable interest in connection with similar brightness changes observed in bright blue variable LBV stars, and also is related to the ejection of a nebula surrounding the RY Scuti system. Translated from Astrofizika, Vol. 52, No. 2, pp. 275–289 (May 2009).     

UBV photoelectric observations of the eclipsing binary system XZ Cep

Three color photoelectric observations of the close binary system XZ Cep at the Abastumani Astrophysical Observatory are reported. These data cover the entire light curve and represent, up to now, the best and most extensive observations of this star. The positions of the components of XZ Cep on a mass-log g diagram show that it is in a phase following a rapid transfer of mass. The theoretical light curves are not fully consistent with the observed curves at phases of 0.35-0.40 and 0.6-0.9. This is evidently because of the influence of gaseous flows in the system. The existence of circumstellar matter and gas flows at phases of 0.7-0.9 are also confirmed by spectral data and by changes in the intrinsic polarization of XZ Cep. In addition, a polarimetric study of the star indicates that two or more interstellar clouds with different properties lie between XZ Cep and us. A detailed study of the photoelectric data presented here is also extremely interesting for the confirmation of the latter idea.     

Variable synchrotron emission from BL Lacertae objects. II. Optical and X-ray flares in HBL source 1ES 1959+650

This paper presents the results of the optical R band and 1.5–12 keV band X-ray monitoring of the high-energy peaked BL Lacertae source 1ES 1959+650 performed during 2002–2007 with the 70 cm Meniscus Telescope of Abastumani Astrophysical Observatory (Georgia) and the All-Sky Monitor on board the Rossi X-ray Time Explorer, respectively. The observed long- and short-term outbursts are fitted with the lightcurves obtained by means of the modeling of synchrotron flares that are assumed to be the result of a propagation of the relativistic shock waves through the jet of 1ES 1959+650, pointed to the observer. Different values of the input parameters (shock velocity, particles’ spectral index, sizes of emission region, minimum and maximum Lorentz factors of the particles etc.) are used in order to fit the simulated lightcurves whose constructed by means of observational data. This investigation shows that both shock velocity and physical conditions in the jet of 1ES 1959+650 should be variable from flare to flare. The shocks are found to be mildly relativistic with the apparent speeds β=0.46–0.85, expressed in the units of c. Spectral index of the particle energy distribution varied from 2.10 to 2.17 for the long-term flares while it is higher in the case of short-term outbursts: s=2.32–2.45 that is suggested to be a result of the deceleration of shock front during its passage through the shell situated downstream the Mach disc. The average strength of a turbulent magnetic field ranged from 0.025 gauss to 0.04 gauss for different long-term flares while the values of 0.07–0.14 gauss were adopted for the different short-term outbursts. The lengths of variable jet area found to be of 0.13–0.47 pc with the transverse extents of (0.5–1.0)×10¹⁷ cm in the case of long-term flares. The same characteristics for short-term outbursts were (2.74–5.5)×10¹⁶ cm and (0.2–04)×10¹⁷ cm, respectively. We conclude that both shock velocity and properties of pre-shocked plasma were not the same in 1ES 1959+650 for the different flaring epochs.     

Prompt high-energy particle acceleration during two-current-loop collisions

We present a model of prompt high-energy particle acceleration during two-current-loop collisions. By investigating test proton and test electron motions in the electromagnetic field derived from the MHD equations, we found that high-energy particle acceleration occurs only in the case ofY-type, loop-loop collisions. The results depend strongly on the plasma and initial position of the test particle. When the plasma increases, the particle acceleration rate decreases. The particles near the edge of the collision region can be accelerated to higher energy than the ones inside it. It has been shown that both protons and electrons can be accelerated to 10 GeV within 0.001 s and 5 MeV within 10^–6 s, respectively. In the case ofY-type loop-loop collisions, one may expect that high-energy gamma-ray and neutrons will be generated from interaction between high-energy particles and the low atmospheric plasma.     

Optical R-band variability of seven X-ray-selected BL Lacertae objects

This paper presents the results of optical CCD photometry in the Cousins R band for seven X-ray-selected BL Lacertae objects. Observations have been performed with the 70-cm meniscus telescope of Abastumani Astrophysical Observatory, Georgia. The data obtained provide important information on sources that have been rarely observed in the optical band. The targets show clear long-term variability with time-scales of 1.5–5 yr. The maximum amplitudes are slightly greater than 1 mag. 1ES 0323+022, 0502+675 and 0806+524 show clear non-periodical behaviour. 1ES 1028+511 changes quasi-periodically but further monitoring for periodicity confirmation and period derivation is needed. Overall brightness variation is maximal for 1ES 0647+250 with  Δ R = 1.29  mag. Two-peak maxima, indicating the existence of reverse shocks in blazar jets, are observed for 1ES 0502+675, 0647+250, 0806+524 and 1517+656. The targets do not show intra-night variability. Day-to-day changes with amplitudes of 0.1–0.32 mag coincide mainly with short-term bursts. The latter are seen at faint (quiet) stages as during flaring activities. This fact leads to the suggestion that both hot-spot and shock-in-jet models explaining the origin of short-term bursts should be reasonable.