A VLBA survey of the core shift effect in AGN jets I. Evidence for dominating synchrotron opacity

The effect of a frequency dependent shift of the VLBI core position (known as the "core shift") was predicted more than three decades ago and has since been observed in a few sources, but often within a narrow frequency range. This effect has important astrophysical and astrometric applications. To achieve a broader understanding of the core shift effect and the physics behind it, we conducted a dedicated survey with NRAO's Very Long Baseline Array (VLBA). We used the VLBA to image 20 pre-selected sources simultaneously at nine frequencies in the 1.4-15.4 GHz range. The core position at each frequency was measured by referencing it to a bright, optically thin feature in the jet. A significant core shift has been successfully measured in each of the twenty sources observed. The median value of the core shift is found to be 1.21 mas if measured between 1.4 and 15.4 GHz, and 0.24 mas between 5.0 and 15.4 GHz. The core position, r, as a function of frequency, n, is found to be consistent with an r n^-1 law. This behavior is predicted by the Blandford & Koenigl model of a purely synchrotron self-absorbed conical jet in equipartition. No systematic deviation from unity of the power law index in the r(n) relation has been convincingly detected. We conclude that neither free-free absorption nor gradients in pressure and/or density in the jet itself and in the ambient medium surrounding the jet play a significant role in the sources observed within the 1.4-15.4 GHz frequency range. These results support the interpretation of the parsec-scale core as a continuous Blandford-Koenigl type jet with smooth gradients of physical properties along it.Comment: 31 pages, 6 figures, 5 tables; accepted to Astronomy & Astrophysic

Nonlinear acoustic waves in channels with variable cross sections

The point symmetry group is studied for the generalized Webster-type equation describing non-linear acoustic waves in lossy channels with variable cross sections. It is shown that, for certain types of cross section profiles, the admitted symmetry group is extended and the invariant solutions corresponding to these profiles are obtained. Approximate analytic solutions to the generalized Webster equation are derived for channels with smoothly varying cross sections and arbitrary initial conditions.Comment: Revtex4, 10 pages, 2 figure. This is an enlarged contribution to Acoustical Physics, 2012, v.58, No.3, p.269-276 with modest stylistic corrections introduced mainly in the Introduction and References. Several typos were also correcte

Magnetoelastic nature of solid oxygen epsilon-phase structure

For a long time a crystal structure of high-pressure epsilon-phase of solid oxygen was a mistery. Basing on the results of recent experiments that have solved this riddle we show that the magnetic and crystal structure of epsilon-phase can be explained by strong exchange interactions of antiferromagnetic nature. The singlet state implemented on quaters of O2 molecules has the minimal exchange energy if compared to other possible singlet states (dimers, trimers). Magnetoelastic forces that arise from the spatial dependence of the exchange integral give rise to transformation of 4(O2) rhombuses into the almost regular quadrates. Antiferromagnetic character of the exchange interactions stabilizes distortion of crystal lattice in epsilon-phase and impedes such a distortion in long-range alpha- and delta-phases.Comment: 11 pages, 4 figures, Changes: corrected typos, reference to the recent paper is adde

Survey of Instantaneous 1-22 GHz Spectra of 550 Compact Extragalactic Objects with Declinations from -30deg to +43deg

We present observational results for extragalactic radio sources with milliarcsecond components, obtained with the 600 meter ring radio telescope RATAN-600 from 1st to 22nd December, 1997. For each source, a six frequency broad band radio spectrum was obtained by observing simultaneously with an accuracy up to a minute at 1.4, 2.7, 3.9, 7.7, 13 and 31 cm. The observed list is selected from Preston et al. (1985) VLBI survey and contains all the sources in the declinations between -30deg and +43deg with a correlated flux density exceeding 0.1 Jy at 13 cm. The sample includes the majority of sources to be studied in the current VSOP survey and the future RadioAstron Space VLBI mission.Comment: 26 pages, 4 figures, 5 tables, published in the A&AS; figure 4 with the broad-band spectra plots is included in the preprint; tables 1 and 5, in electronic form, as well as the ReadMe file can be extracted from the preprint sourc

High-energy neutrino-induced cascade from the direction of the flaring radio blazar TXS 0506+056 observed by the Baikal Gigaton Volume Detector in 2021

The existence of high-energy astrophysical neutrinos has been unambiguously demonstrated, but their sources remain elusive. IceCube reported an association of a 290-TeV neutrino with a gamma-ray flare of TXS 0506+056, an active galactic nucleus with a compact radio jet pointing to us. Later, radio blazars were shown to be associated with IceCube neutrino events with high statistical significance. These associations remained unconfirmed with the data of independent experiments. Here we report on the detection of a rare neutrino event with the estimated energy of 224 +- 75 TeV from the direction of TXS 0506+056 by the new Baikal-GVD neutrino telescope in April 2021 followed by a radio flare observed by RATAN-600. This event is the highest-energy cascade detected so far by Baikal-GVD from a direction below horizon. The result supports previous suggestions that radio blazars in general, and TXS 0506+056 in particular, are the sources of high-energy neutrinos, and opens up the cascade channel for the neutrino astronomy.Comment: 12 pages, 6 figures, 2 table