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Optical Observations (optical + observation)
Selected AbstractsObservations of the quiescent X-ray transients GRS 1124,684 (=GU Mus) and Cen X-4 (=V822 Cen) taken with ULTRACAM on the VLT,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010T. Shahbaz ABSTRACT We present high time-resolution multicolour optical observations of the quiescent X-ray transients GRS 1124,684 (=GU Mus) and Cen X-4 (=V822 Cen) obtained with ULTRACAM. Superimposed on the secondary stars' ellipsoidal modulation in both objects are large flares on time-scales of 30,60 min as well as several distinct rapid flares on time-scales of a few minutes, most of which show further variability and unresolved structure. Not significant quasi-periodic oscillations are observed and the power density spectra of GRS 1124,684 and Cen X-4 can be described by a power law. From the colour,colour diagrams of the flare events, for GRS 1124,684 we find that the flares can be described by hydrogen gas with a density of NH, 1024 nucleons cm,2, a temperature of ,8000 K and arising from a radius of ,0.3 R,. Finally we compile the values for the transition radius (the radius of the hot advection-dominated accretion flow) estimated from quasi-periodic oscillations and/or breaks in the power density spectrum for a variety of X-ray transients in different X-ray states. As expected, we find a strong correlation between the bolometric luminosity and the transition radius. [source] X-ray diffraction study of the phase transition of K2Mn2(BeF4)3: a new type of low-temperature structure for langbeinitesACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2001A. Guelylah The potassium manganese tetrafluoroberyllate langbeinite compound has been studied in the temperature range 100,300,K. Using DSC measurements, a phase transition has been detected at 213,K. The space group of the low-temperature phase was determined to be P1121 using X-ray diffraction experiments and optical observations of the domain structure. The b axis is doubled with respect to the prototypic P213 cubic phase. Lattice parameters were determined by powder diffraction data [a = 10.0690,(8), b = 20.136,(2), c = 10.0329,(4),Ĺ, , = 90.01,(1)°]. A precise analysis of the BeF4 tetrahedra in the low-temperature phase shows that two independent tetrahedra rotate in opposite directions along the doubled crystallographic axis. A symmetry mode analysis of the monoclinic distortion is also reported. This is the first report of the existence of such a phase transition in K2Mn2(BeF4)3 and also of a new type of low-temperature structure for langbeinite compounds. [source] Error analysis of proper motions in decination obtained for 807 Hipparcos stars from PZT observations over many decadesASTRONOMISCHE NACHRICHTEN, Issue 8 2010G. Damljanovi Abstract After publication of the Hipparcos catalogue (in 1997), a few new astrometric catalogues have appeared (TYCHO-2, ARIHIP, etc.), as a good combination of the Hipparcos satellite and ground-based data, to get more accurate coordinates and proper motions of stars than the Hipparcos catalogue ones. There are also investigations on improving the Hipparcos coordinates and proper motions by using the astrometric observations of latitude and universal time variations (via observed stars referred to Hipparcos catalogue), together with Hipparcos data, carried out during the last few years. These kind of ground-based data were collected at the end of the last century by J. Vondrák. There are about 4.4 million optical observations made worldwide at 33 observatories and with 47 instruments during 1899.7,1992.0; our Belgrade visual zenith telescope data (for the period 1949.0-1986.0) were included. First of all, these data were used to determine the Earth Orientation Parameters , EOP, but they are also useful for the opposite task , to check the accuracy of coordinates and proper motions of Hipparcos stars which were observed from the ground over many decades. Here, we use the latitude part of ten Photographic Zenith Tubes , PZT data (more than 0.9 million observations made at 6 observatories during the time interval 1915.8,1992.0), and combine them with the Hipparcos catalogue ones, with suitable weights, in order to check the proper motions in declination for 807 common PZT/Hipparcos stars (and to construct the PZT catalogue of ,, for 807 stars). Our standard errors in proper motions in declination of these stars are less than or equal to the Hipparcos ones for 423 stars. The mean value of standard errors of 313 stars observed over more than 20 years by PZT is 0.40 mas/yr. This is 53% of 0.75 mas/yr (the suitable value from the Hipparcos catalogue). We used the Least Squares Method , LSM with the linear model. Our results are in good agreement with the Earth Orientation Catalogue , EOC-2 and the new Hipparcos ones. The main steps of the method and the investigations of systematic errors in determined proper motions (the proper motion differences with respect to the Hipparcos values, the EOC-2 ones and the new Hipparcos ones, as a function of ,, ,, and magnitude) are presented here. A comparison of the four catalogues by pairs shows that there is no significant relationship between the differences of their ,, values and magnitudes and color indices of the common 807 stars. All catalogues have relatively small random and systematic errors which are close to each other. However, the comparison shows that our formal errors are too small. They are underestimated by a factor of nearly 1.7 (for EOC-2, it is 2.0) if we take the new Hipparcos (or Hipparcos) data as reference (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] On the relation between supersoft X-ray sources and VY Scl stars: The cases of V504 Cen and VY Scl,ASTRONOMISCHE NACHRICHTEN, Issue 2 2010J. Greiner Abstract We summarise our optical monitoring program of VY Scl stars with the SMARTS telescopes, and triggered X-ray as well as optical observations after/during state transitions of V504 Cen and VY Scl (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||