G. T. Petrov

Experimental investigation of weld pool formation in electron beam welding

Abstract The energy transfer mechanism from an electron beam to a metal target, weld pool and keyhole of formation during electron beam welding was studied using a collector for secondary emitted particles, light photo diodes and CCD techniques. The physical processes in the welding pool and plasma cavity are discussed. It is shown that the nature of the heat source in a weld pool is non-stationary and the dynamic processes occurring in both the welding pool and the plasma cavity play a dominant role in the formation of the welding seam during electron beam welding of metals. Periodical molten metal flows were determined at low oscillation frequencies of the welding seams root for frequency between 0.5 and 50 Hz and on the surface of the welding pool in the interval 5–850 Hz. The frequency spectrum of the secondary electron emission signal is in the order of a few kHz.

CCD standards for U and I in the open cluster NGC 7790

Photometric U and I standard sequences in the eld of the open cluster NGC 7790 are presented. The intention is to achieve wide ranges in magnitude and colour, making these sequences suitable for calibrating deep CCD photometry. The 84 standard stars extend the BVR sequences of Odewahn et al. (1992) to the near UV and IR, respectively.

Photometric monitoring of the blazar 3C 345 for the period 1996–2006

We present the results of the blazar 3C 345 monitoring in Johnson-Cousins BVRI bands for the period 1996–2006. We have collected 29 V and 43 R data points for this period; the BI light curves contain a few measurements only. The accuracy of our photometry is not better than 0.03 mag in the VR bands. The total amplitude of the variability obtained from our data is 2.06 mag in the V and 2.25 mag in the R band. 3C 345 showed periods of flaring activity during 1998/99 and 2001: a maximum of the blazar brightness was detected in 2001 February – 15.345 mag in the V and 14.944 mag in the R band. We confirm that during brighter stages 3C 345 becomes redder; for higher fluxes the colour index seems to be less dependent on the magnitude. The intra-night monitoring of 3C 345 in three consecutive nights in 2001 August revealed no significant intra-night variability; 3C 345 did not show evident flux changes over timescales of weeks around the period of the intra-night monitoring. This result supports the existing facts that intra-night variability is correlated with rapid flux changes rather than with specific flux levels. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

BV photometry of a possible open star cluster pair NGC 7031/NGC 7086

We present a CCD BV photometry of the possible binary open star cluster NGC 7031/NGC 7086. The aim is to confirm or disprove their common nature on the grounds of their age and distance. An age of 224 ± 25 Myr and a distance of 831 ± 72 pc was determined for NGC 7031, and 178 ± 25 Myr, 955 ± 84 pc for NGC 7086, respectively. Based on these differences in age and distance we conclude that the two clusters are most likely not formed together from one and the same giant molecular cloud and thus do not form a true binary cluster. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

New galaxies with high surface brightness in the Nilson system

The data for 47 new Galaxies with High Surface Brightness (HSBG) are presented. The surface brightness is evaluated by Arkelians relations in the system of theUppsala General Catalogue of Galaxies. Out of total 193 Arakelian galaxies listed in UGCG, 46 were found to be HSBG in the Nilson system as well.

Photometric Monitoring of Selected Quasars: The Highly Luminous Quasar HS 1946+7658

B.M. Mihov, R.S. Bachev, A.A. Strigachev, L.S. Slavcheva, G.T. PetrovInstitute of Astronomy, 72 Tsar. chausse Blvd., 1784 So a, Bulgariae-mail: bmihov@astro.bas.bgAbstract. We present and discuss Johnson { Cousins V and R pho-tometry of the highly luminous quasar HS 1946+7658 (z = 3:051). Wefound no evidence for strong variability during one year of monitoring.1. IntroductionThe quasar HS 1946+7658 is the most luminous quasar discovered up to now {it has an absolute magnitude of 31:1 (H

Star burst activity in high surface brightness galaxies

High surface brightness galaxies are also galaxies with high star-forming activity. About a half of them omit, on the average, twice as much energy in the IR than in the blue. The rates of star formation are 10–30 times higher than those in normal galaxies. On average 100–300 solar masses gas are converted into stars every year and 10–30 are massive stars.

A proposed program of observations of extragalactic X-ray sources

It is proposed to carry out an observational test of the hypothesis that active galactic nuclei contain a massive black hole, surrounded by an accretion disk as an energy source. The observational program included 64 objects, selected according the Aldrovandis model.

Some spectrophotometric data for 31 galaxies from Karchentsev's list

The equivalent widths and relative intensities of the emission lines of 31 Karachentsev galaxies are determined on spectra obtained with a 5-m telescope incorporating a 512-channel scanner. The physical conditions in the nuclei of the galaxies are analysed and the relative oxygen content is defined. The parameters derived are compared to similar parameters of Seyfert, Arakelian, and Markarian galaxies.

Physical conditions in the nuclei of type 1 seyfert galaxies

Osterbrocks spectrophotometric results were used to determine in a unified manner the electron densities and temperatures in the nuclei of 36 Seyfert galaxies of type 1 and five radio galaxies. It was found that in the O(2+) emission zones of all the investigated objects the temperatures are close to 10,000 K and the densities to 5 million per cu cm. The dispersion of both these parameters is very small. The spread of the values of the same parameters in the O(+) emission zones is much greater.