Mercury in Hydrobionts and Their Habitat in Grønfjorden,West Spitsbergen,in Early Springtime

The first data were obtained on the total mercury content in hydrobionts and their habitat in Grønfjorden, Spitsbergen, at the waste discharge sites of the settlement of Barentsburg in early spring 2017. The Hg concentration was below the detection limit in the water and varied from 7.1 to 42.3 ng/g of dry weight in the bottom sediments. Mercury concentration in the hydrobionts increased toward the inner fjord and was higher near the mouth of the Grøndalen River, which flows into the fjord. Elevated Hg concentrations at the mouth of the Grøndalen River indicate that much of the toxic metal is brought to the inner part of the fjord with riverine runoff, and this Hg source is likely more important than the surface supply of Hg transferred from local surface pollution centers at Barentsburg. The Hg concentration depended on the position of the marine organisms in the trophic chain and was the highest in the detritophage mollusks Thyasira gouldi, Cardium sp., and Macoma calcarea, the specialized predatory sea snail Cryptonatica affinis, and the cod Gadus morhua, which is a benthosophage–secondary predator. The total Hg concentrations in the hydrobionts and their habitat in Grønfjorden were generally relatively low and close to the background one.     

Optical Spectroscopy of SRG/eROSITA Objects with 2.5-m Telescope at the Caucasus Mountain Observatory of the SAI MSU

Astronomy Letters - Based on observations with the new transient double-beam spectrograph (TDS) at the 2.5-m telescope of the Caucasus Mountain Observatory of SAI MSU, we have determined the types...     

Spectroscopic Redshift Determination for a Sample of Distant Quasars Detected by the SRG Observatory Based on RTT-150 Observations. II

Astronomy Letters - Results of the spectroscopic observations at the 1.5-m Russian–Turkish telescope for another group of 12 X-ray sources discovered by the eROSITA telescope onboard the SRG...     

Sample of cataclysmic variables detected in the 400d X-ray survey

A sample of cataclysmic variables (CVs) detected among the X-ray sources of the 400 square degree (400d) survey performed based on ROSAT pointings is presented. A technique for selecting CVs among the X-ray sources using additional optical and infrared data, based on Sloan Digital Sky Survey andWISE data, is described. We present the optical observations of the selected objects carried out mainly with the Russian–Turkish 1.5-m telescope (RTT-150) and the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (BTA). Some observations have also been performed with the 1.6-m AZT-33IK telescope of the Institute of Solar–Terrestrial Physics, the Siberian branch of the Russian Academy of Sciences. Eight CVs, four of which were detected in our work, have been selected by now. Based on this sample, we have obtained preliminary constraints on the X-ray luminosity function of CVs in the solar neighborhood in the range of low luminosities, L _X ~ 10²⁹–10³⁰ erg s^?1 (0.5–2 keV). The logarithmic slope of the CV luminosity function in this luminosity range is shown to become gentler than that at L _X > 10³¹ erg s^?1. It follows from our estimates of the CV luminosity function that several thousand CVs will be detected in the SRG all-sky survey at high Galactic latitudes, which will allow much more accurate measurements of their X-ray luminosity function to be obtained.     

First geomagnetic measurements in the Antarctic region

Based on data from literature and archival sources, we have further processed and analyzed the results of geomagnetic measurements made during the 1772–1775 Second World Expedition by James Cook and the 1819–1821 overseas Antarctic Expedition by Russian mariners Bellingshausen and Lazarev. Comparison with the GUFM historical model showed that there are systematic differences in the spatial structure of both the declination and its secular variation. The results obtained can serve as a basis for the construction of regional models of the geomagnetic field for the Antarctic region.     

Optical Spectroscopy of Quasars Discovered by SRG/eROSITA with a 2.5-m Telescope at the Caucasus Mountain Observatory of SAI MSU

Astronomy Letters - Based on observations with the transient double-beam spectrograph (TDS) at the 2.5-m telescope of the Caucasus Mountain Observatory of SAI MSU, we have determined the redshifts...     

Catalog of candidates for quasars at 3 &lt; <Emphasis Type="Italic">z</Emphasis> &lt; 5.5 selected among X-Ray sources from the 3XMM-DR4 survey of the XMM-Newton observatory

We have compiled a catalog of 903 candidates for type 1 quasars at redshifts 3 < z < 5.5 selected among the X-ray sources of the “serendipitous” XMM-Newton survey presented in the 3XMMDR4 catalog (the median X-ray flux is ≈5 × 10^?15 erg s^?1 cm^?2 in the 0.5–2 keV energy band) and located at high Galactic latitudes |b| > 20° in Sloan Digital Sky Survey (SDSS) fields with a total area of about 300 deg². Photometric SDSS data as well infrared 2MASS and WISE data were used to select the objects. We selected the point sources from the photometric SDSS catalog with a magnitude error δ _mz′ < 0.2 and a color i′ ? z′ < 0.6 (to first eliminate the M-type stars). For the selected sources, we have calculated the dependences χ²(z) for various spectral templates from the library that we compiled for these purposes using the EAZY software. Based on these data, we have rejected the objects whose spectral energy distributions are better described by the templates of stars at z = 0 and obtained a sample of quasars with photometric redshift estimates 2.75 < z _phot < 5.5. The selection completeness of known quasars at z _spec > 3 in the investigated fields is shown to be about 80%. The normalized median absolute deviation (Δz = |z _spec ? z _phot|) is σ _Δz /(1+z _spec) = 0.07, while the outlier fraction is η = 9% when Δz/(1 + z _спек.) > 0.2. The number of objects per unit area in our sample exceeds the number of quasars in the spectroscopic SDSS sample at the same redshifts approximately by a factor of 1.5. The subsequent spectroscopic testing of the redshifts of our selected candidates for quasars at 3 < z < 5.5 will allow the purity of this sample to be estimated more accurately.     

Measuring the Probabilistic Photometric Redshifts of X-ray Quasars Based on the Quantile Regression of Ensembles of Decision Trees

We present empirical machine learning algorithms for measuring the probabilistic photometric redshifts (photo-z) of X-ray quasars based on the quantile regression of ensembles of decision trees. Relying on the data of present-day photometric sky surveys (e.g., SDSS, GALEX, WISE, UKIDSS, 2MASS, FIRST), the proposed methods allow one to make high-quality photo-z point predictions for extragalactic objects, to estimate the confidence intervals, and to reconstruct the full probability distribution functions for all predictions. The quality of photo-z predictions has been tested on samples of X-ray quasars from the 1RASS and 3XMM DR7 surveys, which have spectroscopic redshift measurements in the SDSS DR14Q catalog. The proposed approaches have shown the following accuracy (the metrics are the normalized median absolute deviation σNMAD and the percentage of outliers n_>0.15): σ_NMAD, n_>0.15 = 0.043, 12% (SDSS + WISE), 0.037, 8% (SDSS + WISE + GALEX) and 0.032, 8.6% (SDSS + WISE + GALEX + UKIDSS) on the RASS sample; σ_NMAD, n_>0.15 = 0.054, 13% (SDSS + WISE), 0.045, 7.6% (SDSS + WISE + GALEX), and 0.037, 6.6% (SDSS + WISE + GALEX + UKIDSS) on the 3XMM sample. The presented photo-z algorithms will become an important tool for analyzing the multi-wavelength data on X-ray quasars in the forthcoming Spectrum–Roentgen–Gamma sky survey.     

Spectroscopic Redshift Determination for a Sample of Distant Quasars Detected by the SRG Observatory Based on RTT-150 Observations. I

Astronomy Letters - Results of the first spectroscopic observations at the 1.5-m Russian–Turkish telescope for the X-ray sources discovered by the eROSITA telescope onboard the SRG space...     

Magnetic declination measurements over European Russia and Siberia in the 18th century

The paper presents the history of measurements of the geomagnetic field parameters over the territory of Russia in the 18th century derived from archival and literature sources. Topographical mapping of the European territory and neighboring seas of the Russian Empire from the late 17th to the mid 18th century during which magnetic measurements were made was of great importance for determining magnetic declination. The magnetic declination in Siberia and its neighboring seas was measured for the first time during the first expedition of V. Bering in 1728 and then during his second expedition (the Great Northern Expedition) in 1733–1742. Magnetic measurements were carried out along the northern coast of Siberia and in the Bering Sea. The academic group of the expedition performed magnetic declination measurements over southern regions of Siberia (Krasnoyarsk, Irkutsk, Nerchinsk, Yakutsk, etc.) in 1735 and 1736. During the second half of the 18th century, Russian expeditions determined geographical coordinates of the cities of European Russia and carried out magnetic declination measurements for them. During these expeditions Inokhodtsev paid attention to the anomalous magnetic declination in the Kursk region. In his opinion, the anomaly could have been caused by the presence of iron ore.