A Statistical Study of a Large Sample of Changing-look Active Galactic Nuclei with Multi-frequency Radio Sky Surveys

Optically “changing-look” active galactic nuclei (AGNs) are a class of objects that exhibit appearance or disappearance of broad lines. Recent studies have shown that these “changing-look” activities very likely result from the accretion activities of the central supermassive black holes. Large variation of the accretion rate might cause jet ejection and fading events, thus lead to a certain variability of observational radio properties. From the published literature, 74 “changing-look” AGNs and 90 “changing-look” AGNs candidates identified in optical observations were collected. Based on this largest, complex, and incomplete sample to date, the radio properties of “changing-look” AGNs were investigated. The radio counterparts of 51 “changing-look” AGNs (including 21 candidates) were discovered from the all-sky surveys conducted by the Australian Square Kilometre Array Pathfinder (ASKAP) and Very Large Array (VLA) covering the frequency range of 0.9–3 GHz. This corresponds to a detection rate of about 41 percent, which is similar to general AGNs. The radio spectral indices between 0.9 and 1.4 GHz and between 1.4 and 3 GHz were calculated, which suggest that these “changing-look” AGNs tend to have flatter radio spectra than normal AGNs. This statistical result indicates that “changing-look” AGNs might host some young, compact, and faint radio jets.     

The three-dimensional geometry of the heliospheric current sheet

The three-dimensional geometry of the heliospheric current sheet seen from fixed points in interplanetary space is constructed for idealized (sinusoidal) magnetic neutral lines (equators) and for an observed magnetic equator on the basis of the “kinematic method” developed by Hakamada and Akasofu (1982). The cross-sections of the wavy current sheet at distances 1, 2 and 5 a.u. are also constructed for the idealized magnetic neutral lines.     

A paradox in measuring the magnetic field of the Sun

Significant discrepancies are often observed among the values of the mean magnetic field (MMF) of the Sun as a star observed by various instruments using various spectral lines. This is conventionally attributed to the measurement errors and “saturation” of a solar magnetograph in fine-structure photospheric elements with a strong magnetic field. Measurements of the longitudinal MMF performed in 1968–2006 at six observatories are compared in this paper. It is shown that the degree of discrepancy (slopes b of linear regression lines) varies significantly over the phase of the 11-year cycle. This gives rise to a paradox: the magnetograph calibration is affected by the state of the Sun itself. The proposed explanation is based on quantum properties of light, namely, nonlocality and “coupling” of photons whose polarization at the telescope-spectrograph output is determined by spacious parts of the solar disk. In this case, the degree of coupling, or “identity,” of photons depends on the field distribution in the photosphere and the instrument design (as Bohr said, “the instrument inevitably affects the result”). The “puzzling” values of slope b are readily explained by the dependence of the coupling on the solar-cycle phase. The very statistical nature of light makes discrepancies unavoidable and requires the simple averaging of data to obtain the best approximation of the actual MMF. A 39-year time series of the MMF absolute value is presented, which is indicative of significant variations in the magnitude of the solar magnetic field with a cycle period of 10.5(7) yr.     

Kinetic Alfvén waves in preflare plasma

Low‐frequency instabilities of plasma waves in the arch structures in solar active regions have been investigated before a flare. In the framework of mechanism of “direct initiation” of instability by slowly increasing (quasi‐static) large‐scale electric field in a loop the dispersion relation has been studied for the perturbations which propagate almost perpendicularly to the magnetic field of the loop. The case has been considered, when amplitude of weak (“subdreicer”) electric field sharply increases before a flare, low‐frequency instability develops on the background of ion‐acoustic turbulence and thickness of this turbulent plasma layer plays the role of mean characteristic scale of inhomogeneity of plasma density. If the values of the main plasma parameters, i.e. temperature, density, magnetic field amplitude allow to neglect the influence of the shear of magnetic strength lines on the instability development, then two types of the waves can be generated in preflare plasma: the kinetic Alfvén waves and some new kind of the waves from the range of slowly magneto‐acoustic ones. Instability of kinetic Alfvén waves has clearly expressed threshold character with respect to the amplitude of “subdreicer” electric field. This fact seems to be useful for the short‐time prediction of a flare in arch structure. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Sun and the transcendental world of binaries

The theory of gravity says that a binary with orbital frequency ν should be a source of gravitational waves at the double frequency and higher harmonics. This implies that long-term exposure of an ensemble of binaries to gravity waves with frequency ν _G can result in (a) a lack of binaries with frequencies near frequency ν _G /2 and its higher harmonics (the effect of unstable orbits) and/or (b) an excess of binaries whose orbital frequencies are “absolutely” incommensurable with ν _G /2 and its higher harmonics (the effect of stable orbits). It is assumed that the stable-orbit frequencies are almost equal to multiples of πν _G /2 and ν _G /2π, where π plays the role of a “perfect” factor ensuring the best antiresonance of binaries. The statistical analysis of frequencies of 5774 Galactic close binary systems (CBSs) with periods P less than 10 days is based on calculating the resonance spectrum that indicates the best common multiple for a given set of frequencies with allowance for the factor π. The CBS distribution turns out to be modulated by the frequency ν _G = 104.4(5) μHz, and this effect is the most pronounced for superfast and compact rotators, such as cataclysmic variables (CVs) and related objects. The frequency ν _G is, within the error, equal to the “enigmatic” frequency ν₀ = 104.160(1) μHz com discovered earlier in the power spectra of the Sun and brightness variations of some extragalactic sources. This confirms the existence of a “coherent cosmic oscillation” of the Universe with frequency ν₀(ν _G ). The new astrophysical phenomenon naturally explains an “CV period gap” at frequencies ≈πν _G /3 (P ≈ 153 min) and maxima at the neighboring frequencies ≈πν _G /2 and ≈πν _G /4 (P ≈ 102 and ≈204 min, respectively). The remarkable and “mysterious” role of the transcendental number π for the world of binaries is emphasized, and the mystery of physical nature of the “universal” oscillation ν₀(ν _G ) is highlighted.     

Observation of a flux transfer event on the earthward side of the magnetopause

The medium energy particle spectrometer (electrons of energy > 20 keV, protons > 25 keV) on board ISEE-2 has measured very similar pitch angle distributions and intensities during “flux transfer” events in the magnetosheath and events previously designated as “inclusion” events in the magnetosphere on a single pass through the magnetopause. This is interpreted as strong evidence that magnetic field lines in the magnetosphere can connect to field lines in the magnetosheath, at least locally and for brief times, allowing the same population ofparticles to be observed on both sides of the boundary. In addition, a simple mathematical model is provided incorporating a time constant for the process re-supplying particles to the open flux tube. The observed data are satisfactorily reproduced using a time constant of 46 s, which is comparable to the half-bounce time of protons at this position.     

Non-LTE effects for Na I lines in X-ray illuminated stellar atmospheres

The formation of Na I lines in X-ray illuminated atmospheres is investigated by abandoning the assumption of local thermodynamic equilibrium (LTE). Calculations are performed on the basis of a 21-level Na I model atom for the LTE model atmospheres of irradiated F-G stars obtained with allowance for a reflection effect in the first approximation. The state of extreme “overrecombination” is shown to exist for the populations of all Na I levels in the case of external illumination. Absorption features in the profiles of “cool” and “normal” Na I lines have been found to be enhanced compared to the LTE approximation. Effects of the angle of incidence and intensity of the external radiation on the formation of level populations and line profiles when abandoning LTE are analyzed. The existence of overrecombination for Na I is explained by the small X-ray heating function and the large optical cooling function. Na I level populations are shown to depend weakly on the presence of “overionization” for Na II in the atmospheres of irradiated stars.     

Magnetospheric VLF line radiation observed at Halley,Antarctica

Spectrograms of broad-band ELF/VLF goniometer data obtained from ground based measurements made at Halley, Antarctica (L = 4.3, conjugate near St. Anthony, Newfoundland) have shown the presence of discrete line radiation of magnetospheric origin, in the frequency range 1–4 kHz. The properties of this radiation are broadly similar to Power Line Harmonic Radiation (PLHR), studied from ground based observations made at Siple, Antarctica (L = 4.1, conjugate—Roberval, Quebec), although there are some interesting differences. Line radiation observed at Halley, is never regularly spaced in frequency by 120 Hz, as one may expect if signals from the Newfoundland power distribution system (60 Hz fundamental) are entering the magnetosphere, and being amplified. Instead, frequency spacings are widely distributed about mean values between 50 and 90 Hz. The lines are observed to trigger emissions and often exhibit 2 hop amplitude modulation, which demonstrates that they are of magnetospheric origin. Events occur mostly in quiet to moderate geomagnetic conditions, and during the late afternoon period of local time. Arrays of lines are often observed to drift upwards together in frequency. Line bandwidths are 20–30 Hz—much larger than the bandwidths of locally generated induction lines. We show that the line spacing of ～80 Hz is too large to correspond to sideband separation for waves of equatorial field strength ～10 pT, and we investigate the conditions required for effective particle trapping by the wave array, of the type described by Nunn, 1974. It is proposed that the line radiation either originates in the signals which enter the magnetosphere from Newfoundland, or is “naturally” generated, possibly by a linear instability which takes place if the electron distribution function has sharp localised gradients in pitch angle.     

Evidence for Flash Mixing in He-rich sdB Stars

We present FUSE spectra of three He-rich sdB stars. Two of these stars, PG1544+488 and JL87, reveal extremely strong C III lines, suggesting that they have mixed triple-α carbon from the deep interior out to their surfaces. Using TLUSTY NLTE line-blanketed model atmospheres, we find that PG1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL87 shows a similar surface enrichment of C and N but still retains a significant amount of hydrogen. In contrast, the third star, LB1766, is devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions arise from a delayed helium-core flash on the white-dwarf cooling curve. During such a flash the interior convection zone will penetrate into the stellar envelope, thereby mixing the envelope with the He- and C-rich core. Such “flash-mixed” stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical EHB stars. Two types of flash mixing are possible:“deep” and “shallow”, depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG1544+488 and JL87 are examples of “deep” and “shallow” flash mixing, respectively.     

Radiative equilibrium model of Titan's atmosphere

A simple global radiative equilibrium model is developed for Titan. It is restricted to the two-stream approximation, is vertically homogeneous in its scattering properties, and is spectrally divided into one thermal and two solar channels. A partially absorbing “violet” channel is responsible for heating in the stratosphere, while a conservatively scattering “red” channel permits heating at the surface. The optical thickness of the atmosphere in the red is 1 < τ₁^r < 3. Between 13 and 33% of the total incident solar radiation is absorbed at the planetary surface. The ratio of violet to thermal infrared absorption cross sections is between 30 and 60 in the stratosphere, leading to the large temperature inversion observed there. The observed and theoretically computed tropopause temperatures are 72 and 69°K, respectively, while their corresponding thermal optical depths are, respectively, ～0.1 and ～0.07. The spectrally integrated mass absorption coefficient at thermal wavelengths is approximately constant throughout the stratosphere and roughly linear with pressure in the troposphere. This in turn implies the presence of a uniformly mixed aerosol in the stratosphere, and suggests pressure-induced absorption by gaseous N₂CH₄H₂ in the troposphere. In addition there appear to be two regions of enhanced opacity near 30 and 500 mbar which may be due to C₂H₂C₂H₆C₃H₈ and CH₄ condensation clouds, respectively.     

The Interaction Between Coronal Mass Ejections and Streamers: A Statistical View over 15 Years (1996 – 2010)

We report on the statistical analysis of the interaction between coronal mass ejections (CMEs) and streamers based on 15 years (from 1996 to 2010 inclusive) of observation of the solar corona with the LASCO-C2 coronagraph. We used synoptic maps and improved the method of analysis of past investigations by implementing an automatic detection of both CMEs and streamers. We identified five categories of interaction based on photometric and geometric variations between the pre- and post-CME streamers: “brightening”, “dimming”, “emergence”, “disappearance”, and “deviation”. A sixth category, “no change”, included all cases where none of the above variations is observed. A “global set” of 21?242 CMEs was considered as well as a subset of the 10 % brightest CMEs (denoted “top-ten”) and three typical periods of solar activity: minimum, intermediate, and maximum. We found that about half of the global population of CMEs are not associated with streamers, whereas 93 % of the 10 % brightest CMEs are associated. When there is a CME-streamer association, approximately 95 % of the streamers experience a change, either geometric or radiometric. The “no change” category therefore amounts to approximately 5 %, but this percentage varies from 1?–?2 % during minimum to 7?–?8 % during intermediate periods of activity; values of 3?–?5 % are recorded during maximum. Emergences and disappearances of streamers are not dominant processes; they constitute 16?–?17 % of the global set and 23 % (emergence) and 28 % (disappearance) of the “top-ten” set. Streamer deviations are observed for 57 % and 70 % of, respectively, the global set and “top-ten” CMEs. The cases of dimming and brightening are roughly equally present and each case constitutes approximately 30?–?35 % of either set, global or “top-ten”.     

Quasar redshift distribution and selection effects

With the Hewitt-Burbidge catalogue of quasars as our sample, separately for 3 sets of respectively 7, 11, and 13 spectral lines as the “key lines” in the identification of quasar redshifts, we derive the redshift distribution arising from the identification of the lines. A comparison between the calculated and observed distributions shows that 1) the two are clearly correlated, with a correlation coefficient of 0.56. 2) Similar peaks are present in both in the z-intervals 0.2 – 0.4, 0.8 – 0.9, 1.4 – 1.5, and particularly around 1.95, as well similar valleys at 0.1 and 1.5 – 1.8. 3) Both rise or fall together at z = 0.2, 0.3, 0.4, 0.8, 1.9, 1.4, 1.5, 1.6 and 1.7. It thus appears that selection effects play an important role in the redshift distribution of the quasars.     

High resolution study of the abundance pattern of the heavy elements in very metal‐poor field stars

The abundances of heavy elements in EMP stars are not well explained by the simple view of an initial basic “rapid” process. In a careful and homogeneous analysis of the “First Stars” sample (eighty per cent of the stars have a metallicity [Fe/H] ≃ –3.1 ± 0.4), it has been shown that at this metallicity [Eu/Ba] is constant, and therefore the europium‐rich stars (generally called “r‐rich”) are also Ba‐rich. The very large variation of [Ba/Fe] (existence of “r‐poor” and “r‐rich” stars) induces that the early matter was not perfectly mixed. On the other hand, the distribution of the values of [Sr/Ba] vs. [Ba/Fe] appears with well defined upper and lower envelopes. No star was found with [Sr/Ba] < –0.5 and the scatter of [Sr/Ba] increases regularly when [Ba/Fe] decreases. To explain this behavior, we suggest that an early “additional” process forming mainly first peak elements would affect the initial composition of the matter. For a same quantity of accreted matter, this additional Sr production would barely affect the r‐rich matter (which already contains an important quantity of Sr) but would change significantly the composition of the r‐poor matter. The abundances found in the CEMP‐r+s stars reflect the transfer of heavy elements from a defunct AGB companion. But the abundances of the heavy elements in CEMP‐no stars present the same characteristics as the the abundances in the EMP stars. Direct stellar ages may be found from radioactive elements, the precision is limited by the precision in the measurements of abundances from faint lines in faint stars, and the uncertainty in the initial abundances of the radioactive elements. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The most resonant pulsation frequency of δ Scuti stars

Pulsation of the Sun with a period of P₀ ≈ 160 min discovered about two decades ago, is still waiting explanation. In view of the hypothesis about its cosmological origin, and attempting to find signature of this P₀ periodicity among other (short-period variable) stars, the pulsation frequencies of δ Sct stars are subjected to specific analysis. With a confidence level ≈ 3.8σ it is found that the frequency v₀ = P₀⁻¹ ≈ 104 m̈Hz, within the error limits, appears indeed to be the most “resonant” one for the total sample of 318 pulsating stars of δ Sct type (the most commensurable, or “synchronizing”, period for all these stars occurs to be 162 ± 4 min). We conjecture that a) the P₀ oscillation might be connected with periodic fluctuations of gravity field (metrics), and b) the primary excitation mechanism of pulsations of δ Sct stars, reffected by this “ubiquitous” P₀ resonance, must be attributed perhaps to superfast rotation of their inner cores (their rates tend to be in near-resonance with the “universal” v₀ frequency). The arguments are given favouring a cosmoogical nature of the P₀ oscillation.     

A family of ionospheric models for different uses

Empirical models of three dimensional electron density distributions in the ionosphere have been constructed for global as well as regional use. The models differ by their degree of complexity and calculation time and therefore have different uses. All are based on “ionogram parameter” (critical frequencies foE, foF1, foF2 and the F2 region transfer parameter M(3000)F2). The models allow the use of global or regional maps for foF2 and M(3000)F2 and use built-in formulations for foE and foF1. Update (instantaneous mapping / nowcasting) versions exist which take foF2 and M(3000)F2 or F2 region peak height and electron density as input. The ground to F2 layer peak part of the profile is identical for all three models and is based on an Epstein formulation. The “quick calculationr” model NeQuick uses a simple formulation for the topside F layer, which is essentially a semi-Epstein layer with a thickness parameter which increases linearly with height. The “ionospheric model” COSTprof is the model which was adopted by COST 251 in its regional “monthly median” form. Its topside F layer is based on O⁺-H⁺ diffusive equilibrium with built-in maps for three parameters, namely the oxygen scale height at the F2 peak, its height gradient and the O⁺-H⁺ transition height. The “ionosphere-plasmasphere” model NeUoG-plas uses a magnetic field aligned “plasmasphere” above COSTprof Typical uses of the models and comparison among them are discussed.     

Investigation of OH and H<Subscript>2</Subscript>O masers in the star-forming region G 188.946+0.886

We present the results of our observations of the maser radio emission source G188.946+0.886 in hydroxyl (OH) molecular lines with the radio telescope of the Nançay Observatory (France) and in the H₂O line at λ = 1.35 cm with the RT-22 radio telescope at the Pushchino Observatory (Russia). An emission feature in the 1720-MHz satellite line of the OH ground state has been detected for the first time. The radial velocity of the feature, V _LSR = 3.6 km s^?1, has a “blue” shift relative to the range of emission velocities in the main 1665- and 1667-MHz OH lines, which is 8–11 km s^?1. This suggests a probable connection of the observed feature in the 1720-MHz line with the “blue” wing of the bipolar outflow observed in this region in the CO line. We have estimated the magnetic field strength for three features (0.90 and 0.8 mG for 1665 MHz and 0.25 mG for 1720 MHz) from the Zeeman splitting in the 1665- and 1720-MHz lines. No emission and (or) absorption has been detected in the other 1612-MHz satellite OH line. Three cycles of H₂O maser activity have been revealed. The variability is quasi-periodic in pattern. There is a general tendency for the maser activity to decrease. Some clusters of H2O maser spots can form organized structures, for example, chains and other forms.     

On the interpretation of the “inverse phase effect” for CO2 equivalent widths on Venus

Computations of the equivalent widths of absorption lines as a function of planetary phase angle are made for a homogeneous cloud with particles having the properties (shape, refractive index, and size distribution) deduced from polarimetry of Venus. The computed equivalent widths show an “inverse phase effect” comparable to that which is observed for CO₂ lines on Venus. This result verifies a recent suggestion of Regas et al. that the existence of an inverse phase effect does not by itself imply the presence of multiple layers of scattering particles in the atmosphere of Venus.     

High-resolution reconstruction of variation in aeolian dust (Kosa) deposition at ODP site 797, the Japan Sea,during the last 200 ka

In order to reconstruct past variations in the east Asian monsoon and to establish a direct link between terrestrial and marine climatic records, we evaluated the temporal variation in aeolian dust (Kosa or yellow sand) flux to the Japan Sea using the late Quaternary hemipelagic sediments in the sea. Contributions of four detrital subcomponents, identified by Irino and Tada [Geochem. J. 34(1) (2000) 59] as Kosa derived from “typical” loess, Kosa from “weathered” loess and fine and coarse arc-derived detritus, have varied in millennial scale, as well as in glacial–interglacial scale, during the last 200 ka. Millennial scale variability of Kosa from “typical” loess suggests the change in dust availability controlled by high-frequency variation in summer monsoon precipitation in the central to east Asia during the last 200 ka, whereas variation in Kosa from “weathered” loess suggests the change in transport pathway of dust which was probably affected by the winter monsoon variation.     

Active galactic nuclei: what’s in a name?

Active galactic nuclei (AGN) are energetic astrophysical sources powered by accretion onto supermassive black holes in galaxies, and present unique observational signatures that cover the full electromagnetic spectrum over more than twenty orders of magnitude in frequency. The rich phenomenology of AGN has resulted in a large number of different “flavours” in the literature that now comprise a complex and confusing AGN “zoo”. It is increasingly clear that these classifications are only partially related to intrinsic differences between AGN and primarily reflect variations in a relatively small number of astrophysical parameters as well the method by which each class of AGN is selected. Taken together, observations in different electromagnetic bands as well as variations over time provide complementary windows on the physics of different sub-structures in the AGN. In this review, we present an overview of AGN multi-wavelength properties with the aim of painting their “big picture” through observations in each electromagnetic band from radio to \(\gamma \)-rays as well as AGN variability. We address what we can learn from each observational method, the impact of selection effects, the physics behind the emission at each wavelength, and the potential for future studies. To conclude, we use these observations to piece together the basic architecture of AGN, discuss our current understanding of unification models, and highlight some open questions that present opportunities for future observational and theoretical progress.     

Near-infrared spectroscopy of 3:1 Kirkwood Gap asteroids: Mineralogical diversity and plausible meteorite parent bodies

The 3:1 Kirkwood gap asteroids are a mineralogically diverse set of asteroids located in a region that delivers meteoroids into Earth-crossing orbits. Mineralogical characterizations of asteroids in/near the 3:1 Kirkwood Gap can be used as a tool to “map” conditions and processes in the early Solar System. The chronological studies of the meteorite types provide a “clock” for the relative timing of those events and processes. By identifying the source asteroids of particular meteorite types, the “map” and “clock” can be combined to provide a much more sophisticated understanding of the history and evolution of the late solar nebula and the early Solar System.A mineralogical assessment of seven 3:1 Kirkwood Gap asteroids has been carried out using near-infrared spectral data obtained over the years 2006–2009 combined with visible spectral data (when available) to cover the spectral interval of 0.4–2.5 μm. We explore the diversity, uniqueness, and possible links between the asteroids (198) Ampella, (329) Svea, (495) Eulalia, (556) Phyllis, (623) Chimaera, (908) Buda, and (1772) Gagarin, which are located adjacent to the 3:1 resonance, and the meteorite types in the terrestrial collections.