Abastumani, Georgia
Abastumani, Georgia

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Gupta A.C.,Aryabhatta Research Institute of Observational science ARIES | Krichbaum T.P.,Max Planck Institute for Radio Astronomy | Wiita P.J.,The College of New Jersey | Rani B.,Aryabhatta Research Institute of Observational science ARIES | And 35 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We report results from a one-week multiwavelength campaign to monitor the BLLacertae object (BL Lac) S5 0716+714 (on 2009 December 9-16). Nine ground-based telescopes at widely separated longitudes and one space-based telescope aboard the Swift satellite collected optical data. Radio data were obtained from the Effelsberg and Urumqi observatories and X-ray data from Swift. In the radio bands, the source shows rapid [∼(0.5-1.5) d] intraday variability with peak amplitudes of up to ∼10 per cent. The variability at 2.8cm leads by about 1d the variability at 6 and 11cm. This time lag and more rapid variations suggest an intrinsic contribution to the source's intraday variability at 2.8cm, while at 6 and 11cm, interstellar scintillation (ISS) seems to predominate. Large and quasi-sinusoidal variations of ∼0.8mag were detected in the V, R and I bands. The X-ray data (0.2-10keV) do not reveal significant variability on a 4d time-scale, favouring reprocessed inverse Compton over synchrotron radiation in this band. The characteristic variability time-scales in radio and optical bands are similar. A quasi-periodic variation of 0.9-1.1d in the optical data may be present, but if so it is marginal and limited to 2.2 cycles. Cross-correlations between radio and optical bands are discussed. The lack of a strong radio-optical correlation indicates different physical causes of variability (ISS at long radio wavelengths, source intrinsic origin in the optical) and is consistent with a high jet opacity and a compact synchrotron component peaking at ≃100GHz in an ongoing very prominent flux-density outburst. For the campaign period, we construct a quasi-simultaneous spectral energy distribution, including γ-ray data from the Fermi satellite. We obtain lower limits for the relativistic Doppler boosting of δ ≥ 12-26, which for a BLLac-type object is remarkably high. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Bhatta G.,Florida International University | Webb J.R.,Florida International University | Hollingsworth H.,Florida International University | Dhalla S.,Florida International University | And 44 more authors.
Astronomy and Astrophysics | Year: 2013

Context. The international Whole Earth Blazar Telescope (WEBT) consortium planned and carried out three days of intensive micro-variability observations of S5 0716 + 714 from February 22, 2009 to February 25, 2009. This object was chosen due to its bright apparent magnitude range, its high declination, and its very large duty cycle for micro-variations. Aims. We report here on the long continuous optical micro-variability light curve of 0716+714 obtained during the multi-site observing campaign during which the Blazar showed almost constant variability over a 0.5 mag range. The resulting light curve is presented here for the first time. Observations from participating observatories were corrected for instrumental differences and combined to construct the overall smoothed light curve. Methods. Thirty-six observatories in sixteen countries participated in this continuous monitoring program and twenty of them submitted data for compilation into a continuous light curve. The light curve was analyzed using several techniques including Fourier transform, Wavelet and noise analysis techniques. Those results led us to model the light curve by attributing the variations to a series of synchrotron pulses. Results. We have interpreted the observed microvariations in this extended light curve in terms of a new model consisting of individual stochastic pulses due to cells in a turbulent jet which are energized by a passing shock and cool by means of synchrotron emission. We obtained an excellent fit to the 72-hour light curve with the synchrotron pulse model. © ESO, 2013.

Akyuz A.,NASA | Thompson D.J.,NASA | Donato D.,NASA | Donato D.,University of Maryland University College | And 12 more authors.
Astronomy and Astrophysics | Year: 2013

Aims. The observation of γ-ray flares from blazar 0836+710 in 2011, following a period of quiescence, offered an opportunity to study correlated activity at different wavelengths for a high-redshift (z = 2.218) active galactic nucleus. Methods. Optical and radio monitoring, plus Fermi-LAT γ-ray monitoring provided 2008-2012 coverage, while Swift offered auxiliary optical, ultraviolet, and X-ray information. Other contemporaneous observations were used to construct a broad-band spectral energy distribution. Results. There is evidence of correlation but not a measurable lag between the optical and γ-ray flaring emission. In contrast, there is no clear correlation between radio and γ-ray activity, indicating radio emission regions that are unrelated to the parts of the jet that produce the γ rays. The γ-ray energy spectrum is unusual in showing a change of shape from a power law to a curved spectrum when going from the quiescent state to the active state. © ESO, 2013.

Raiteri C.M.,National institute for astrophysics | Villata M.,National institute for astrophysics | Bruschini L.,University of Turin | Capetti A.,National institute for astrophysics | And 53 more authors.
Astronomy and Astrophysics | Year: 2010

Aims. In a previous study we suggested that the broad-band emission and variability properties of BL Lacertae can be accounted for by a double synchrotron emission component with relatedinverse-Compton emission from the jet, plus thermal radiation from the accretion disc. Here we investigate the matter with further data extending over a wider energy range. Methods. The GLAST-AGILE Support Program (GASP) of the whole earth blazar telescope (WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical frequencies to follow its flux behaviour. During this period, high-energy observations were performed by XMM-Newton, Swift, and Fermi. We analyse these data with particular attention to the calibration of Swift UV data, and apply a helical jet model to interpret the source broad-band variability. Results. The GASP-WEBT observations show an optical flare in 2008 February-March, and oscillations of several tenths of mag on a few-day time scale afterwards. The radio flux is only mildly variable. The UV data from both XMM-Newton and Swift seem to confirm a UV excess that is likely caused by thermal emission from the accretion disc. The X-ray data from XMM-Newton indicate a strongly concave spectrum, as well as moderate (∼4-7%) flux variability on an hour time scale. The Swift X-ray data reveal fast (interday) flux changes, not correlated with those observed at lower energies. We compare the spectral energy distribution (SED) corresponding to the 2008 low-brightness state, which was characterised by a synchrotron dominance, to the 1997 outburst state, where the inverse-Compton emission was prevailing. A fit with an inhomogeneous helical jet model suggests that two synchrotron components are at work with their self inverse-Compton emission. Most likely, they represent the radiation from two distinct emitting regions in the jet. We show that the difference between the source SEDs in 2008 and 1997 can be explained in terms of pure geometrical variations. The outburst state occurred when the jet-emitting regions were better aligned with the line of sight, producing an increase of the Doppler beaming factor. Conclusions. Our analysis demonstrates that the jet geometry can play an extremely important role in the BL Lacertae flux and spectral variability. Indeed, the emitting jet is probably a bent and dynamic structure, and hence changes in the emitting regions viewing angles are likely to happen, with strong consequences on the source multiwavelength behaviour. © 2010 ESO.

Agudo I.,Institute Astrofisica Of Andalucia | Marscher A.P.,Boston University | Jorstad S.G.,Boston University | Jorstad S.G.,Saint Petersburg State University | And 40 more authors.
Astrophysical Journal Letters | Year: 2011

We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and γ-ray wavelengths of the BL Lacertae object AO 0235+164. We analyze the timing of multi-waveband variations in the flux and linear polarization, as well as changes in Very Long Baseline Array images at λ = 7mm with 0.15 milliarcsec resolution. The association of the events at different wavebands is confirmed at high statistical significance by probability arguments and Monte Carlo simulations. A series of sharp peaks in optical linear polarization, as well as a pronounced maximum in the 7 mm polarization of a superluminal jet knot, indicate rapid fluctuations in the degree of ordering of the magnetic field. These results lead us to conclude that the outburst occurred in the jet both in the quasi-stationary "core" and in the superluminal knot, both parsecs downstream of the supermassive black hole. We interpret the outburst as a consequence of the propagation of a disturbance, elongated along the line of sight by light-travel time delays, that passes through a standing recollimation shock in the core and propagates down the jet to create the superluminal knot. The multi-wavelength light curves vary together on long timescales (months/years), but the correspondence is poorer on shorter timescales. This, as well as the variability of the polarization and the dual location of the outburst, agrees with the expectations of a multi-zone emission model in which turbulence plays a major role in modulating the synchrotron and inverse Compton fluxes. © 2011. The American Astronomical Society. All rights reserved.

Fan J.H.,Guangzhou University | Fan J.H.,Astronomy Science and Technology Research Laboratory | Kurtanidze O.,Abastumani Observatory | Kurtanidze O.,Kazan Federal University | And 6 more authors.
Astrophysical Journal, Supplement Series | Year: 2014

Variability is one of the most observable characteristics of active galactic nuclei, and it is important when considering the emission mechanism. In this paper, we report optical photometry monitoring of two nearby brightest quasars, PHL 1811 and 3C 273, using the ST-6 camera attached to the Newtonian focus and the Ap6E CCD camera attached to the primary focus of the 70 cm meniscus telescope at the Abastumani Observatory, Georgia. PHL 1811 was monitored during the period from 2002 September to 2012 December, while 3C 273 was monitored during the period from 1998 February to 2008 May. During our monitoring period, the two sources did not show any significant intra-day variability. The largest detected variations are ΔR = 0.112 ± 0.010 mag. for PHL 1811, ΔB = 0.595 ± 0.099 mag, ΔV = 0.369 ± 0.028 mag, ΔR = 0.495 ± 0.076 mag, and ΔI = 0.355 ± 0.009 mag for 3C 273. When the periodicity analysis methods are adopted for the observations of the sources, a period of p = 5.80 ± 1.12 yr is obtained for PHL 1811 in the R light curve in the present work, and periods of p = 21.10 ± 0.14, 10.00 ± 0.14, 7.30 ± 0.09, 13.20 ± 0.09, 2.10 ± 0.06, and 0.68 ± 0.05 yr are obtained for 3C 273 based on the data in the present work combined with historical works. © 2014. The American Astronomical Society. All rights reserved..

Britzen S.,Max Planck Institute for Radio Astronomy | Zamaninasab M.,Max Planck Institute for Radio Astronomy | Aller M.,University of Michigan | Aller H.,University of Michigan | And 6 more authors.
Journal of Physics: Conference Series | Year: 2012

We report the detection of the first VLBI ring-structure around the core of an AGN - around the core of the quasar 3C454.3. This ring-structure starts being visible in VLBI maps around 1996. It expands with an apparent velocity between 0.11±0.01mas/yr and 0.18±0.01mas/yr and dominates the pc-scale structure for at least 14 years. This is the result of a re-analysis of 41 VLBA data sets at six different radio frequencies observed between 1995.57 and 2011.48. We observe a correlation between radio flaring, flux-density variability, a ring-structure and kinematic properties of the jet. Taken together, it is tempting to see a causal connection and to explain all of this geometrically. The kinematic changes as well as the changes in the flaring characteristics might be caused by a change of the angle to the line of sight towards the observer. This behaviour resembles our findings for 0735+178 - with 3C454.3 being the second AGN to reveal kinematic mode changes. These mode changes could be explained by the presence of a supermassive binary black hole. 3C454.3 had been modelled as a binary black hole before.

Britzen S.,Max Planck Institute for Radio Astronomy | Qian S.-J.,CAS National Astronomical Observatories | Witzel A.,Max Planck Institute for Radio Astronomy | Krichbaum T.P.,Max Planck Institute for Radio Astronomy | And 6 more authors.
Astronomy and Astrophysics | Year: 2013

Context. 2251+158 (3C 454.3) is a well-studied quasar with rather unusual properties. It is among the most variable and brightest gamma-ray emitting active galactic nucleus (AGN) in the sky observed by Fermi. The multi-wavelength flux density emission of this source is peculiar, and so is the very long baseline interferometry (VLBI) structure. While it is usually assumed that the jet and jet-components in a given AGN show more or less the same properties (e.g., apparent velocities and paths) with time, we demonstrate here that unusual morphologies (arc-like structure) can occur as temporal phenomena. We also show that the kinematic properties of jet components change from slow to fast apparent motion. Aims. We present the detection of an arc around the core region. To understand the physical nature of this and other peculiar kinematic properties of the parsec-scale jet of 2251+158 we performed detailed radio-interferometric studies of this prototypical AGN. Methods. We (re-)analyzed 32 Very Long Baseline Array (VLBA) observations covering 16 years in time (between 1995.57 and 2011.48), observed at 15 GHz from the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE/2 cm) survey and derived the parameters of the observed VLBI structure. Three 43 GHz VLBA observations from 2001.22, 2005.38, and 2005.76 have been reprocessed. We fitted the components of the VLBI structure with Gaussian components. We studied the properties of light-curves obtained in the radio (4.8, 8.0, and 14.5 GHz) and optical regime (R-band), and studied the correlations between the flaring properties and the VLBI structure of the source. Results. We report the detection of an arc-like structure around the core of an AGN. This ring-structure becomes visible in VLBI maps around 1996. It expands with an apparent proper motion between 0.12 ± 0.01 mas yr-1 and 0.19 ± 0.01 mas yr-1 (corresponding to 5.4 ± 0.3c to 9.0 ± 0.4c) and dominates the parsec-scale structure for about 14 years. In addition, we report a significant change in the kinematic properties of jet components close to the core. A phase of slow apparent radial motion (directed away from the core) of two inner jet components (A and B) and no component ejection after a major radio-flare in 1995 is atypical for AGN. This phase is followed by a more typical behaviour in which the same components separate with higher apparent speeds from the core and two new components appear to be ejected from the central region. Conclusions. We observe significant changes in the morphological and kinematic properties of the parsec-scale jet. Taken together, it is tempting to see a causal connection between the different phenomena. We briefly suggest and discuss several possible physical scenarios to explain the observed phenomena. The kinematic changes as well as the changes in the flaring characteristics could be explained geometrically because of a change in the angle to the line of sight towards the observer assuming a helical structure of the jet. Another possible explanation for the observed phenomena and the correlations among them could be a precessing jet (precession period on timescale of 14.5 years) in combination with its interaction with an external inhomogeneous medium surrounding the core. These and other possible explanations will be investigated further and manuscripts presenting the results are in preparation. © 2013 ESO.

Fan J.H.,Guangzhou University | Fan J.H.,Astronomy Science and Technology Research Laboratory | Liu Y.,Guangzhou University | Liu Y.,Astronomy Science and Technology Research Laboratory | And 4 more authors.
Journal of Astrophysics and Astronomy | Year: 2011

Variability is one of the characteristics of blazars. The rapid variability is superposed on the long term variation. In this work, the variability on different time scales, such as intra-day (IDV), short-term (STV) and long-term (LTV) variations are presented for some sources. We also presented our own observations of some selected objects, for which the historical data were compiled for periodicity analysis using several methods. The parameters of the binary black hole system OJ 287 are determined. © 2011 Indian Academy of Sciences.

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