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Nechaeva M.,Ventspils International Radio Astronomy Center | Antipenko A.,Radiophysical Research Institute RRI | Bezrukovs V.,Ventspils International Radio Astronomy Center | Bezrukov D.,Ventspils International Radio Astronomy Center | And 17 more authors.
Baltic Astronomy | Year: 2013

An experiment on radar location of space debris objects using of the method of VLBI was carried out in April, 2012. The radar VLBI experiment consisted in irradiation of some space debris objects (4 rocket stages and 5 inactive satellites) with a signal of the transmitter with RT-70 in Evpatoria, Ukraine. Reflected signals were received by a complex of radio telescopes in the VLBI mode. The following VLBI stations took part in the observations: Ventspils (RT-32), Urumqi (RT-25), Medicina (RT-32) and Simeiz (R.T-22). The experiment included measurements of the Doppler frequency shift and the delay for orbit refining, and measurements of the rotation period and sizes of objects by the amplitudes of output interferometer signals. The cross-correlation of VLBI-data is performed at a correlator NIRFI-4 of Radiophysical Research Institute (Nizhny Novgorod). Preliminary data processing resulted in the series of Doppler frequency shifts, which comprised the information on radial velocities of the objects. Some results of the experiment are presented.

Raiteri C.M.,National institute for astrophysics | Villata M.,National institute for astrophysics | Aller M.F.,University of Michigan | Gurwell M.A.,Harvard - Smithsonian Center for Astrophysics | And 71 more authors.
Astronomy and Astrophysics | Year: 2011

Context. The blazar 3C 454.3 is one of the most active sources from the radio to the γ-ray frequencies observed in the past few years. Aims. We present multiwavelength observations of this source from April 2008 to March 2010. The radio to optical data are mostly from the GASP-WEBT, UV and X-ray data from Swift, and γ-ray data from the AGILE and Fermi satellites. The aim is to understand the connection among emissions at different frequencies and to derive information on the emitting jet. Methods. Light curves in 18 bands were carefully assembled to study flux variability correlations. We improved the calibration of optical-UV data from the UVOT and OM instruments and estimated the Lyα flux to disentangle the contributions from different components in this spectral region. Results. The observations reveal prominent variability above 8 GHz. In the optical-UV band, the variability amplitude decreases with increasing frequency due to a steadier radiation from both a broad line region and an accretion disc. The optical flux reaches nearly the same levels in the 2008-2009 and 2009-2010 observing seasons; the mm one shows similar behaviour, whereas the γ and X-ray flux levels rise in the second period. Two prominent γ-ray flares in mid 2008 and late 2009 show a double-peaked structure, with a variable γ/optical flux ratio. The X-ray flux variations seem to follow the γ-ray and optical ones by about 0.5 and 1 d, respectively. Conclusions. We interpret the multifrequency behaviour in terms of an inhomogeneous curved jet, where synchrotron radiation of increasing wavelength is produced in progressively outer and wider jet regions, which can change their orientation in time. In particular, we assume that the long-term variability is due to this geometrical effect. By combining the optical and mm light curves to fit the γ and X-ray ones, we find that the γ (X-ray) emission may be explained by inverse-Comptonisation of synchrotron optical (IR) photons by their parent relativistic electrons (SSC process). A slight, variable misalignment between the synchrotron and Comptonisation zones would explain the increased γ and X-ray flux levels in 2009-2010, as well as the change in the γ/optical flux ratio during the outbursts peaks. The time delays of the X-ray flux changes after the γ, and optical ones are consistent with the proposed scenario. © 2011 ESO.

Mohan P.,Aryabhatta Research Institute of Observational science ARIES | Agarwal A.,Aryabhatta Research Institute of Observational science ARIES | Agarwal A.,Gorakhpur University | Mangalam A.,Indian Institute of Astrophysics | And 13 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

We study the core shift effect in the parsec-scale jet of the blazar 3C 454.3 using the 4.8-36.8 GHz radio light curves obtained from three decades of continuous monitoring. From a piecewise Gaussian fit to each flare, time lags Δt between the observation frequencies ν and spectral indices α based on peak amplitudes A are determined. From the fit Δt α ν1/kr , kr = 1.10 ± 0.18 indicating equipartition between the magnetic field energy density and the particle energy density. From the fit Aανα, α is in the range -0.24 to 1.52. A mean magnetic field strength at 1 pc, B1 = 0.5 ± 0.2 G, and at the core, Bcore = 46 ± 16 mG, are inferred, consistent with previous estimates. The measure of core position offset is Ω = 6.4 ± 2.8 pc GHz1/kr when averaged over all frequency pairs. Based on the statistical trend shown by the measured core radius rcore as a function of ν, we infer that the synchrotron opacity model may not be valid for all cases. A Fourier periodogram analysis yields power-law slopes in the range -1.6 to -3.5 describing the power spectral density shape and gives bend timescales in the range 0.52-0.66 yr. This result, and both positive and negative α, indicate that the flares originate from multiple shocks in a small region. Important objectives met in our study include: the demonstration of the computational efficiency and statistical basis of the piecewise Gaussian fit; consistency with previously reported results; evidence for the core shift dependence on observation frequency and its utility in jet diagnostics in the region close to the resolving limit of very long baseline interferometry observations. © 2015 The Authors.

Gaur H.,Chinese Academy of Sciences | Gupta A.C.,Chinese Academy of Sciences | Gupta A.C.,Aryabhatta Research Institute of Observational science ARIES | Bachev R.,Bulgarian Academy of Science | And 20 more authors.
Astronomy and Astrophysics | Year: 2015

Context. We extensively observed the prototype blazar, BL Lacertae, in optical and radio bands during an active phase in the period 2010-2013 when the source showed several prominent outbursts. We searched for possible correlations and time lags between the optical and radio band flux variations using multi-frequency data to learn about the mechanisms producing variability. Aims. During an active phase of BL Lacertae, we searched for possible correlations and time lags between multi-frequency light curves of several optical and radio bands. We tried to estimate any possible variability timescales and inter-band lags in these bands. Methods. We performed optical observations in B, V, R, and I-bands from seven telescopes in Bulgaria, Georgia, Greece, and India and obtained radio data at 36.8, 22.2, 14.5, 8, and 4.8 GHz frequencies from three telescopes in Crimea, Finland, and USA. Results. Significant cross-correlations between optical and radio bands are found in our observations with a delay of cm-fluxes with respect to optical bands of ∼250 days. The optical and radio light curves do not show any significant timescales of variability. BL Lacertae showed many optical "mini-flares" on short timescales. Variations on longer term timescales are mildly chromatic with the superposition of many strong optical outbursts. In radio bands, the amplitude of variability is frequency dependent. Flux variations at higher radio frequencies lead the lower frequencies by days or weeks. Conclusions. The optical variations are consistent with being dominated by a geometric scenario where a region of emitting plasma moves along a helical path in a relativistic jet. The frequency dependence of the variability amplitude supports an origin of the observed variations intrinsic to the source. © ESO, 2015.

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