Sh. A. Ehgamberdiev

Uzbekistan Academy of Sciences, Toshkent, Toshkent Shahri, Uzbekistan

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Publications (15)23.12 Total impact

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    ABSTRACT: The results of photometric observations of the main-belt asteroid 4727 Ravel (1979 UD1) from 2013 August 13- 16 at the Maidanak Astronomical Observatory of the Ulugh Beg Astronomical Institute (UBAI), Uzbekistan Academy of Sciences are presented. During the period of observations, the lightcurve amplitude was 0.32 ± 0.05 mag and the synodic rotation period was 0.185 ± 0.009 days (4.44 ± 0.22 h).
    03/2014;
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    ABSTRACT: Since the launch of the Fermi satellite, BL Lacertae has been moderately active at γ-rays and optical frequencies until 2011 May, when the source started a series of strong flares. The exceptional optical sampling achieved by the GLAST-AGILE Support Program of the Whole Earth Blazar Telescope in collaboration with the Steward Observatory allows us to perform a detailed comparison with the daily γ-ray observations by Fermi. Discrete correlation analysis between the optical and γ-ray emission reveals correlation with a time lag of 0 ± 1 d, which suggests cospatiality of the corresponding jet emitting regions. A better definition of the time lag is hindered by the daily gaps in the sampling of the extremely fast flux variations. In general, optical flares present more structure and develop on longer time-scales than corresponding γ-ray flares. Observations at X-rays and at millimetre wavelengths reveal a common trend, which suggests that the region producing the mm and X-ray radiation is located downstream from the optical and γ-ray-emitting zone in the jet. The mean optical degree of polarization slightly decreases over the considered period and in general it is higher when the flux is lower. The optical electric vector polarization angle (EVPA) shows a preferred orientation of about 15°, nearly aligned with the radio core EVPA and mean jet direction. Oscillations around it increase during the 2011-2012 outburst. We investigate the effects of a geometrical interpretation of the long-term flux variability on the polarization. A helical magnetic field model predicts an evolution of the mean polarization that is in reasonable agreement with the observations. These can be fully explained by introducing slight variations in the compression factor in a transverse shock waves model.
    Monthly Notices of the Royal Astronomical Society 12/2013; 436(2):1530-1545. · 5.52 Impact Factor
  • O. A. Burkhonov, Sh. A. Ehgamberdiev, K. E. Ergashev
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    ABSTRACT: The main-belt asteroid 4527 Schoenberg (1982 OK) has been observed between June 28 and July 1, 2012 at Maidanak astronomical observatory of the Ulugh Beg Astronomical Institute (UBAI), Uzbekistan Academy of Sciences. On the basis of data analysis it is found a synodic rotation period of 2.6928" b0.0384 hour (0.1122±0.0016 day) and lightcurve amplitude of 0.31±0.05 mag.
    Minor Planet Bulletin. 10/2013;
  • D. O. Mirzaqulov, Sh. A. Ehgamberdiev, M. Villata, C. M. Raiteri
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    ABSTRACT: With reference to ATel #5411, the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT) reports on the recent observation of a strong optical brightening of the gamma-loud quasar 3C 454.3. This is one of the 28 blazars for which the GASP performs a long-term, multiwavelength monitoring.
    The Astronomer's Telegram. 09/2013;
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    ABSTRACT: The quasar-type blazar 4C 38.41 (B3 1633+382) experienced a large outburst in 2011, which was detected throughout the entire electromagnetic spectrum. We present the results of low-energy multifrequency monitoring by the GASP project of the WEBT consortium and collaborators, as well as those of spectropolarimetric/spectrophotometric monitoring at the Steward Observatory. We also analyse high-energy observations of the Swift and Fermi satellites. In the optical-UV band, several results indicate that there is a contribution from a QSO-like emission component, in addition to both variable and polarised jet emission. The unpolarised emission component is likely thermal radiation from the accretion disc that dilutes the jet polarisation. We estimate its brightness to be R(QSO) ~ 17.85 - 18 and derive the intrinsic jet polarisation degree. We find no clear correlation between the optical and radio light curves, while the correlation between the optical and \gamma-ray flux apparently fades in time, likely because of an increasing optical to \gamma-ray flux ratio. As suggested for other blazars, the long-term variability of 4C 38.41 can be interpreted in terms of an inhomogeneous bent jet, where different emitting regions can change their alignment with respect to the line of sight, leading to variations in the Doppler factor \delta. Under the hypothesis that in the period 2008-2011 all the \gamma-ray and optical variability on a one-week timescale were due to changes in \delta, this would range between ~ 7 and ~ 21. If the variability were caused by changes in the viewing angle \theta\ only, then \theta\ would go from ~ 2.6 degr to ~ 5 degr. Variations in the viewing angle would also account for the dependence of the polarisation degree on the source brightness in the framework of a shock-in-jet model.
    Astronomy and Astrophysics 07/2012; 545. · 5.08 Impact Factor
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    ABSTRACT: Optical observations at the Mount Maidanak Observatory in the framework of the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT) reveal a rapid optical brightening of BL Lacertae. This is one of the 28 blazars for which the GASP performs a long-term, multiwavelength monitoring. The source brightness has recently increased from R = 13.66 +/- 0.01 on July 19.82 to R = 12.60 +/- 0.01 on July 22.97.
    The Astronomer's Telegram. 07/2012;
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    ABSTRACT: Table 2 contains relative magnitudes of images A and B of UM673 in the R band. The fluxes of image B presented in the table are not corrected for the lensing galaxy flux contribution (see paper for details). (1 data file).
    VizieR Online Data Catalog. 07/2012;
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    ABSTRACT: We study brightness variations in the double lensed quasar UM673 (Q0142-100) with the aim of measuring the time delay between its two images. In the paper we combine our previously published observational data of UM673 obtained during the 2003 - 2005 seasons at the Maidanak Observatory with archival and recently observed Maidanak and CTIO UM673 data. We analyze the V, R and I-band light curves of the A and B images of UM673, which cover ten observational seasons from August 2001 to November 2010. We also analyze the time evolution of the difference in magnitudes between images A and B of UM673 over more than ten years. We find that the quasar exhibits both short-term (with amplitude of \sim 0.1 mag in the R band) and high-amplitude (\sim 0.3 mag) long-term variability on timescales of about several months and several years, respectively. These brightness variations are used to constrain the time delay between the images of UM673. From cross-correlation analysis of the A and B quasar light curves and error analysis we measure the mean time delay and its error of 89 \pm11 days. Given the input time delay of 88 days, the most probable value of the delay that can be recovered from light curves with the same statistical properties as the observed R-band light curves of UM673 is 95{+5/-16}{+14/-29} days (68 and 95 % confidence intervals). Analysis of the V - I color variations and V, R and I-band magnitude differences of the quasar images does not show clear evidence of the microlensing variations between 1998 and 2010.
    Astronomy and Astrophysics 04/2012; · 5.08 Impact Factor
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    ABSTRACT: CCD-photometry of three Jupiter Trojan asteroids were carried out to study their opposition effect. We obtained well-sampled magnitude-phase curves for (588) Achilles, (884) Priamus, and (1143) Odysseus in the maximal attainable phase angle range down to 0.1-0.2°. The magnitude-phase relations have a linear behavior in all observed range of phase angles and do not show any non-linear opposition brightening. We have not found any confident differences between phase slopes measured in B, V and R bands. The values of the measured phase slopes of Trojans are different from available data for Centaurs. They are within the range of phase slopes measured for some low-albedo main belt asteroids, also exhibit a linear behavior down to small phase angles. An absence of non-linear opposition brightening puts constraints on the surface properties of the studied objects, assuming very dark surfaces where single scattering plays dominating role.We also determined the rotation periods, amplitudes, the values of color indexes B-V and V-R, and the absolute magnitudes of these asteroids
    Icarus 01/2012; 217(1):202-208. · 3.16 Impact Factor
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    ABSTRACT: The GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT) reports on the recent observation of a strong optical brightening of the gamma-loud quasar 4C 38.41. This is one of the 28 blazars for which the GASP performs a long-term, multiwavelength monitoring. In the current optical observing season, the source has shown multiwavelength activity (see also ATels #3238, #3333, #3335, #3360), so that the GASP has intensified the observations with a dedicated campaign (contact person: C.
    The Astronomer's Telegram. 07/2011;
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    ABSTRACT: A 2-month series of quasi-simultaneous imaging photometric observations of the Moon and the Sun has been performed at Maidanak Observatory (Uzbekistan). New absolute values of lunar albedo have been obtained. Maps of lunar apparent albedo and equigonal albedo at phase angles 1.7–73° at wavelength 603 nm are presented. The standard deviation of our data from a best-fitted phase curve is 2%. The average ratio of the Clementine albedo to ours is 1.41. While the ratio of ROLO albedo to ours is 0.87, our data are in agreement with independent measurements of absolute albedo by Saiki et al. (Saiki, K., Saito, K., Okuno, H., Suzuki, A., Yamanoi, Y., Hirata N., Nakamura, R. [2008]. Earth Planets Space 60, 417–424) at a phase angle near 7°. A phase ratio imaging near opposition (1.6°/2.7°) shows almost the same ratio for maria and highlands, though bright craters (e.g., Tycho, Copernicus, Aristarchus) clearly reveal smaller slopes of phase function. This is an unexpected result, as the craters are bright and one could anticipate a manifestation of the coherent backscattering effect resulting in the opposition spike increasing at so small phase angles.Highlights► New determination of absolute apparent albedo of the Moon is carried out. ► Difference near 13% is found between ROLO and our albedo. ► Smaller slope of phase curve was found for bright craters at small phase angles.
    Icarus 01/2011; 214(1):30-45. · 3.16 Impact Factor
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    ABSTRACT: We have determined the realistic seeing of the 1.5-m AZT-22 telescope of the Mt. Maidanak Observatory (Astronomical Institute, Uzbek Academy of Sciences) using more than 20 000 CCD frames with stellar images in the UBV RI bands acquired in 1996–2005: ε = 1.065″ in the V band. The characteristic seeing reduced to unit air mass, ε med V (M(z) = 1), is 0.945″. We derived color equations for the CCD detectors used with the telescope. Atmospheric-extinction coefficients in different photometric bands were also determined. The mean V -band atmospheric extinction is 0.20 m ± 0.04 m . The time needed for the conditions to settle, in the free atmosphere as well as inside the telescope dome, is 2–2.5 hours after the end of astronomical twilight. For nights with ε med V > 0.9″, we find a persistent difference between the seeing found at this telescope and measured simultaneously with a differential image motion monitor, amounting to ∼0.1 m .
    Astronomy Reports 01/2010; 54(11):1019-1031. · 0.76 Impact Factor
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    ABSTRACT: Since 1997, a program of observations of gravitational lens systems (GLS) with the 1.5-m telescope of the high-altitude Maidanak Observatory has been carried out by joint efforts of seven institutions from five countries. The Q2237+0305, Q0957+561, SBS 1520+530, and other GLS were observed in VRI spectral bands using the TI 800×800, Pictor 416 and ST-7 CCD cameras with the aim to obtain the high-precise estimates of magnitudes and colours of the lensed quasar components at different epochs. The results of photometric image processing are presented. This work was made possible in part by Award No.UP2-302 of the U.S. Civilian Research and Development Foundation for the Independent States of the former Soviet Union (CRDF), and with the kind support of the Maidanak Foundation established in 1998 in Norway. Funding from the Uzbek-Ukrainian program of developing the Maidanak Observatory was also very important, as well as the 98--02--17490 and 1.2.5.5 grants from the Russian Basic Research Foundation. The Maidanak Observatory also participated in the successful Qouc-around-the-Clock program monitoring the Q0957+561 quasar continuously with 8 telescopes around the globe for 10 nights in January 2000.
    Kinematika i Fizika Nebesnykh Tel Supplement. 09/2000;
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    ABSTRACT: The brightness of four components of the gravitational lens system QSO 2237+0305 was measured in the R band, made from 2 Jul - 14 Nov 1997. The image processing algorithm is described, and possible sources of errors are discussed. Brightness variations are observed. Based on a very simple microlensing model, the authors estimate a typical mass of the microlenses which produce the brightness variations. It was found to be 0.24 Msun, in good agreement with other estimates made with the use of more complicated models.
    Kinematics and Physics of Celestial Bodies 01/1999; 15(4):257. · 0.35 Impact Factor
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    ABSTRACT: The brightness of four components of the gravitational lens system QSO 2237+0305 was measured in the R band, made from 2 Jul - 14 Nov 1997. The image processing algorithm is described, and possible sources of errors are discussed. Brightness variations are observed. Based on a very simple microlensing model, the authors estimate a typical mass of the microlenses which produce the brightness variations. It was found to be 0.24 Msun, in good agreement with other estimates made with the use of more complicated models.
    Kinematics i Fizika Nebesnyck Tel. 01/1999; 15:338.