Broadening of electron cyclotron maser emission lines in a nonuniform magnetic field

The formation of cyclotron maser emission lines in a non-uniform (regular or random) magnetic field is studied. In the presence of sufficiently small inhomogeneity, the line shape can be described by a broadened Gaussian profile. In the case of stronger inhomogeneity, the initial Gaussian profile splits into two Gaussian components, which could be observationally perceived as “harmonics.” A relation between the distribution of local magnetic trap sizes and the distribution of the spectral widths of solar radio spikes is derived. Possible applications of the results to the interpretation of solar radio spikes and related problems are discussed.     

Polarization observations of the OH maser W75N on the Nançay radio telescope

Results of observations of the OH maser in W75N at 18 cm are reported. The observations were obtained on the radio telescope of the Nancay Radio Astronomy Observatory (France) from October 2007 to April 2009. The profiles of the Stokes parameters I, Q, U, and V in the 1665 and 1667 MHz OH lines have been measured. A technique taking into account instrumental polarization has been developed and applied. The emission in the OH lines is strongly polarized both linearly and circularly. The degree of polarization of some emission features reaches almost 100%. There were two flares of the maser emission in 2008–2009. During a flare at a radial velocity of +5.5 km/s, a Zeeman pattern was detected in the 1667 MHz line. The measured intensity of the line-of-sight component of the magnetic field was −1.1 mG, which corresponds to the field being directed away from the observer. The maser flares and the associated enhancement of the magnetic field could be associated with the compression of gas at a shock front.     

Evolution of the H<Subscript>2</Subscript>O and OH Maser Emission in W75 N

The results of a study of H₂O and OH maser emission in the complex region of active star formation W75 N are presented. Observations were obtained using the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) and the Nan3ay radio telescope (France). Flaring H₂O maser features may be identified with maser spots associated with the sources VLA 1 and VLA 2. Themain H₂O flares occurred in VLA 1. The flare emission was associated with either maser clusters having closely spaced radial velocities and sizes up to ~2 AU or individual features. The maser emission is generated in a medium where turbulence on various scales is present. Analysis of the line shapes during flare maxima does not indicate the presence of the simplest structures—homogeneous maser condensations. Strong variability of the OH maser emission was observed. Zeeman splitting of the 1665-MHz line was detected for several features of the same cluster at a radial velocity of +5.5 km/s. The mean line-of-sight magnetic field in this cluster is ~0.5 mG, directed away from the observer. Flares of the OH masers may be due to gas compression at a shock or MHD wave front.     

Magnetic fields in methanol maser condensations based on data for related regions. Seven sources: Astrophysical parameters

The astrophysical parameters of seven OH maser condensations are estimated based on magnetic fields obtained from polarization observations carried out on the Nan cay Radio Telescope (France) in the 1665 and 1667 MHz lines in four Stokes parameters. Regions in the studied sources containing the observed clusters of maser condensations, as well as clusters of Class I and II methanol masers, have been identified. The associations of the masers are real; i.e., the magnetic field in the clusters can also extend to groups of methanol masers. The linear dimensions of these associations have been found. The ratio of the mass to the magnetic flux, ratio of the thermal to the magnetic pressure, and virial relationships between energies (kinetic, magnetic, and gravitational) in the regions containing the OH andmethanol masers have been obtained. In sources whose magnetic fields have been determined fairly reliably, the ratio of the mass to the magnetic flux exceeds a critical value, and the energies of chaotic motions and of the magnetic field are considerably smaller than the gravitational binding energy. On the other hand, in all cases, the ratio of the thermal to the magnetic pressure is <1, suggesting that the clouds may be in amagnetically dominated regime. This inconsistency is related to probable uncertainties in the the magnetic field values and the estimated distances to the sources, which may lead to overestimation of the sizes of the regions studied.     

The possible escape of electron cyclotron maser radiation from hot stellar coronas through a window of transparency

The main argument against the idea that the intense radio emission observed from active regions on the Sun and flare stars is electron-cyclotron maser (ECM) radiation is that such radiation should be strongly absorbed in higher-lying layers where the condition for the cyclotron resonance at harmonics of the electron gyrofrequency is fulfilled. Cyclotron absorption lowers the efficiency of ECM radiation virtually to zero for a broad range of angles between the direction of propagation of the radiation and the magnetic field. Less severe absorption is possible only in narrow angular “windows” along (for ordinary and extraordinary waves) and perpendicular to (for ordinary waves) the magnetic field. However, the ECM radiation that is generated does not fall into these windows of transparency due to the kinematic conditions corresponding to coronal magnetic traps. We investigate the efficiency of induced scattering of ECM radiation on ions in the equilibrium plasma in the source. Under certain conditions, induced scattering leads to the formation of a condensate of ECM radiation with the direction of its wave vectors approximately along the magnetic field, enabling the escape of the radiation through windows of transparency. The most favorable conditions for this phenomenon are realized for ordinary waves. We estimate the optical depths of the sources of the ECM radiation to the scattering and the angular width of the condensate for ordinary and extraordinary waves for the cases of the flare radio emission of the star AD Leo and the sources of type I noise storms in the solar corona. In both cases, the polarization of the emergent radiation should correspond to the ordinary wave.     

Results of Long-Term Monitoring of Maser Emission in the Star-forming Region G 10.623–0.383

The results of a study of the maser source G 10.623-0.383 in the λ = 1.35 cm H₂O line using the 22-mradio telescope of the Pushchino Radio AstronomyObservatory (Russia) and in the main hydroxyl lines (λ = 18 cm) using the Nanзay Radio Telescope (France) are presented. Uncorrelated long-term variations of the integrated intensities and the velocity centroids with characteristic times of 11 yrs (mean value) and 32 yrs, respectively, are studied. The drift of the velocity centroid may be associated with maser condensations whose material is collapsing onto the OB cluster. It is shown that the H₂O maser source contains maser condensation configurations on various scales over a long time, which evolve with time. OH maser emission was only detected in the main lines at 1665 and 1667 MHz. The flux densities of the strongest emission components were variable, but their radial velocities did not change. A Zeeman pair was found at 1667 MHz with a splitting of about 1.44 km/s, corresponding to a line-of-sight magnetic field of 4.1 mG, which was preserved over at least 25 years. The characteristics of the H₂O andOHmaser variability suggests that the masers are located in different parts of G 10.623–0.383.     

Flare-plasma diagnostics from millisecond pulsations of the solar radio emission

Two solar radio bursts exhibiting narrow-band millisecond pulsations in intensity and polarization are analyzed. There were considerable time delays between the left-and right-circularly polarized components of the radio emission. The observed oscillations of the degree of polarization are due to the different group velocities of the ordinary and extraordinary modes in their propagation from the source to the observer; the frequency dependence of the delay is in excellent agreement with the theoretically calculated group delay in a magnetoactive plasma. It unambiguously follows that the pulsed radio emission is generated near the double upper hybrid frequency by the nonlinear plasma mechanism, since the source emission has a low degree of polarization. In addition to dispersion effects, a Fourier analysis also reveals effects associated with the source inhomogeneity. We detected a frequency drift of pulsations (autodelays) with different signs for different polarization components. This drift suggests that, apart from the dispersion effects, there are also the effects related to inhomogeneity of the radio source. It is shown, in particular, that the upper hybrid modes (generating the radio emission) are unstable in regions with enhanced gradients of the plasma density and/or magnetic field.     

The evolution of H<Subscript>2</Subscript>O maser spots in star-forming regions

Strong flares of the H₂O maser emission in sources associated with active star-forming regions are analyzed. The main characteristics of 13 flares in nine sources selected using special criteria are presented. The observed phenomena are explained as flares in double emission features. The approach of two emission features in the spectrum with increasing flux and their recession with decreasing flux is explained using a model with two physically related clumps of material that are partially superposed in the line of sight. Calculations have shown that, in this type of model, exponential amplification (unsaturated maser emission) in the overlapping parts of the clumps can produce the observed line narrowing with increasing flux. In most cases, the maser spots are inhomogeneous. During the evolution of some flares, the maser condensations may split into separate fragments. A less catastrophic evolutionary path may be an initial stage of formation of chainlike structures, which are fairly widespread in envelopes around ultracompact HII regions.     

Evolution of Maser Emission in the Region of Active Star Formation G43.8–0.1. I. OH Maser Emission at 18 cm

The results of observations of OH maser emission in the star-forming region G43.8–0.1 are presented. In spite of strong flux-density variations in the main lines at 1665 and 1667 MHz, the radial velocities of the spectral features varied only slightly. The main spectral features are identified with maser spots in previously published maps for epochs 1993 and 2001. It is suggested that the regions of OH maser emission may be elongated, nonuniform structures with weak radial velocity gradients (larger-scale analogs of water-maser filaments). The line-of-sight magnetic fields are determined for two Zeeman pairs, which remained essentially constant over at least 17 years.

     

Class I methanol masers in low-mass star-forming regions

Results of observations of Class I methanol masers in regions of low-mass star formation (MMIL) are summarized and analyzed. Four masers were detected at 44, 84, and 95 GHz towards “chemically active” bipolar outflows in the low-mass star-forming regions NGC1333 I4A, NGC 1333 I2A, HH 25, and L1157. Another maser was found at 36 GHz towards a similar outflow in NGC 2023. Thus, all the detected MMILs are associated with chemically active outflows. The brightness temperatures of the strongest 44-GHz maser spots in NGC 1333 I4A, HH 25, and L1157 exceed 2000 K, whereas the brightness temperature in NGC 1333 I2A is only 176 K, although a rotational-diagram analysis shows that this last source is also amaser. The flux densities of the newly detectedmasers are no higher than 18 Jy, and are much lower than those of strong masers in regions of high-mass star formation (MMIH). The MMIL luminosities match the maser luminosity-protostar luminosity relation established earlier for MMIHs. No MMIL variability was detected in 2004–2011. The radial velocities of the newly detected masers are close to the systemic velocities of the associated regions, except for NGC 2023, where the maser radial velocity is lower than the systemic velocity by approximately 3.5 km/s. Thus, the main MMILproperties are similar to those of MMIHs. MMILs are likely to be an extension of the MMIH population toward lower luminosities of both the masers and the associated young stellar objects. The results of VLA observations of MMILs can be explained using a turbulent-cloud model, which predicts that compact maser spots can arise in extended sources because the coherence lengths along some directions randomly appear to be longer than the mean coherence length in a turbulent velocity field. However, one must assume that the column density of methanol towardM1, the strongest maser in L1157, is appreciably higher than the mean column density of the clump B0a where the maser arises. The shape of the maser lines in L1157, forming double profiles with a red asymmetry, may indicate that the masers arise in collapsing clumps. However, although this model may be correct for L1157, it is specific to this source, since none of the other masers observed exhibited a double profile.     

Accounting for near-surface inhomogeneities over a horizontally layered section in magnetotelluric sounding

This paper presents a numerical model for the effect of near-surface inhomogeneities over a one-dimensional horizontally layered geoelectric section and the distortions they cause during magnetotelluric sounding (MTS). The electromagnetic field within the layer of near-surface inhomogeneities is calculated using the Trefftz method. Expressions are derived for the boundary conditions on the day surface and on the roof of the underlying inhomogeneity of a horizontally layered medium. These boundary conditions allow for the excitation of TM-mode fields by subsurface inhomogeneities and their penetration into the atmosphere and the underlying medium. The spatial distribution and characteristics of galvanic and inductive distortions over different time periods during MTS have been studied. Experimental data show that accounting for galvanic distortions is possible with synchronous recording of the distribution of components of the electric and magnetic fields in a limited area of the Earth’s surface.     

Variability of the OH and H<Subscript>2</Subscript>O maser emission toward AS 501

The results of observations of OH (λ = 18 cm) and H₂O (λ = 1.35 cm) masers toward AS 501 obtained with the Nançay Observatory Radio Telescope (France) and the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia), respectively, are presented. Nine cycles of H₂O maser activity ranging from 2.8 to 6.0 years were detected, identifying AS 501 as an irregular variable star. Zeeman splitting was found only in the 1612-MHz satellite line at ?59.2 km/s. The splitting is 0.11 km/s, corresponding to a line-of-sight magnetic field strength of 0.48 mG. The field is directed toward the observer. The detected radial-velocity drift of the H₂O emission features can be explained in a model with elongated filaments with radial-velocity gradients.     

An H<Subscript>2</Subscript>O maser in the infrared source IRAS 20126+4104

Results of monitoring of H₂O maser in the infrared source IRAS 20126+4104, which is associated with a cool molecular cloud, are presented. The observations were carried out on the 22-meter radio telescope of the Pushchino Radio Astronomy Observatory (Russia) between June 1991 and January 2006. The spectrum of the H₂O maser emission extends from ? 16.7 to 4.8 km/s and splits into separate groups of emission features. Cyclic variations of the integrated maser flux with a period from 3.4 to 5.5 years were detected, together with strong flares of up to 220 Jy in individual emission features. It is shown that large linewidths in periods of high maser activity are due to small-scale turbulent motions of the material. An expanding envelope around a young star is accepted as a model for the source. The protostar has a small peculiar velocity with respect to the molecular cloud (～2 km/s). Individual emission features form organized structures, including multi-link chains.     

Long-term monitoring of the W44C (G 34.3+0.15) maser in the 1.35 cm water vapor and 18 cm hydroxyl lines

Results of monitoring the H₂O and OH masers in W44C, located near the cometary HII region G34.3+0.15, are reported. Observations in the water-vapor line at λ = 1.35 cm were carried out on the 22-meter radio telescope of the Pushchino Radio Astronomy Observatory (Russia) from November 1979 to March 2011, and in the hydroxyl lines at λ = 18 cm on the large Nançay radio telescope (France). Activity maxima and minima of the water maser alternated. The average period of the activity is ～ 14 years, consistent with results obtained earlier for a number of other sources associated with regions of active star formation. In periods of enhanced maser activity, two series of strong H₂O maser flares were observed, which were related to two different clusters of maser spots located at the front of a shock at the periphery of the ultracompact region UH II. These series of flares may be associated with cyclic activity of the protostellar object in UH II. In the remaining time intervals, there were mainly short-lived flares of single features. The Stokes parameters for the observations in the hydroxyl lines were determined. Zeeman splitting was observed in the profile of the 1667 MHz OH main line at a velocity of 58.5 km/s, and was used to estimate the intensity of the line-of-sight component of the magnetic field (1.2 mG).     

Discontinuous plasma flows near reconnecting current layers in solar flares

A model for magnetic reconnection in high-conductivity plasma in the solar corona is analyzed in a strong-magnetic-field approximation. The model includes a Syrovatskii current layer and magnetohydrodynamic (MHD) discontinuities attached to the ends of the layer. A two-dimensional analytical solution for the magnetic field is used to compute the distributions of the plasma flow velocity and plasma density in the vicinity of the corresponding current configuration. The properties of jumps in the density and velocity along the attached discontinuities are studied. Based on the character of the variations of the magnetic field and plasma flows at the MHD discontinuities, it is shown that, with the parameter values considered, an MHDdiscontinuity can include regions of trans-Alfvénic, fast, and slowshocks. The results obtained could be useful to explain the presence of “super-hot” (with effective electron temperatures exceeding 10 keV) plasma in solar flares. Other possible applications of the theory of discontinuous flows near regions of magnetic reconnection to analogous non-stationary phenomena in astrophysical plasmas are noted.     

Observations of OH and H<Subscript>2</Subscript>O Maser Emission in the Star-Forming Region S128

Results of monitoring hydroxyl and water masers in the star-forming region S128 are presented. A large number of emission features in the 1665 MHz OH line have been detected in both circular polarizations. In spite of the strong variability of the flux density in the main 1665 MHz line, the radial velocities of the features remained constant. Zeeman splitting of the 1720MHz line equal to 0.86 km/s was detected, corresponding to a longitudinal magnetic field of 3.6 mG. The variability of the H²O emission has a cyclic character with a quasi-period of 4–14 yrs. The evolution of individual features confirms that the H²O sources A and B are associated with an ionization front between two colliding CO clouds, and shows that the activity was transferred from maser B to maser A in 1999–2001.     

用TEM反演法进行MT静位移的识别和校正

对瞬变电磁(TEM)资料进行反演时, 采用观测磁场而不用视电阻率, 避免了由于TEM视电阻率定义而引起的误差, 并且由源象磁场和观测磁场之间的拟合进行反演使反演结果受非均匀性影响较小; 将反演结果构置出地电模型, 计算出它的MT曲线作为静位移校正的参考曲线, 使它用于MT静位移校正更为可靠, 同时也解决了两种电磁资料从时间域到频率域的转换问题.由此实现用TEM反演的方法进行MT静位移的识别和校正.实例表明, 这种方法对MT静位移的识别和校正十分有效.      

Returning positron flux in the polar-cap regions of a radio pulsar

An approximate method for calculating the returning positron flux in the polar-cap regions of a radio pulsar is proposed. The pulsar is considered in the Goldreich-Julian model for a regime of free-electron emission from the neutron-star surface in the region of open lines of the dipolar magnetic field. Calculations have been done for the case when the dipolar magnetic moment is aligned with the star's rotational axis. The acceleration of primary electrons is assumed to occur near the neutron-star surface on scales comparable to the transverse radius of the tube of open field lines. The generation of electron-positron pairs by curvature radiation of the primary electrons is taken into account. A considerable contribution to the returning flux is made by the region where the electric field is screened by the electron-positron plasma.     

Formation of accretion disks in close-binary systems with magnetic fields

We have developed a three-dimensional numerical model and applied it to simulate plasma flows in semi-detached binary systems whose accretor possesses a strong intrinsic magnetic field. The model is based on the assumption that the plasma dynamics are determined by the slow mean flow, which forms a backdrop for the rapid propagation of MHD waves. The equations describing the slow motion of matter were obtained by averaging over rapidly propagating pulsations. The numerical model includes the diffusion of magnetic field by current dissipation in turbulent vortices, magnetic buoyancy, and wave MHD turbulence. A modified three-dimensional, parallel, numerical code was used to simulate the flow structure in close binary systems with various accretor magnetic fields, from 10⁵ to 10⁸ G. The conditions for the formation of the accretion disk and the criteria distinguishing the two types of flow corresponding to intermediate polars and polars are discussed.     

Evolution of the H2O maser emission in G10.6-0.4

The water-vapor maser emission in the source G10.6-0.4 associated with an active starforming region (OB star cluster) is analyzed. The maser was monitored from 1981–2004 using the 22-meter radio telescope of the Pushchino Radio Astronomy Observatory. Statistical processing of the results revealed the presence of structural formations on various scales. The individual H₂O maser features may form ordered structures with velocity (V _LSR) gradients, localized in separate clusters of maser features. The statistical variations of the V _LSR values for the maser components may be due to the accretion of material onto the OB star cluster in G10.6-0.4 together with the rotation of the molecular cloud core. A model with a rotating, nonuniform condensation of accreted material in the vicinity of the stellar cluster is proposed to explain the variations of the velocity centroid of the H₂O spectra. The integrated flux variations are explained well by a model in which the central source is an OB star cluster, possibly containing five to six stars. An important role in the evolution of the maser emission, as well as of the source as a whole, may be played by turbulent motions of the gas.