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Astron. Astrophys. 329, 504-510 (1998)

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2. Observations

An intense flare of radio emission of Cygnus X-3 was detected with the Green Bank Interferometer with a maximum of 15 Jy and 13 Jy on 1991 January 20 at 3.6 cm and 13.3 cm respectively, following a small outburst with amplitude of order 3 Jy at 3.6 cm on 1991 January 10. Whereas the maximum was reached within one day, the total duration of the flaring event was about 40 days, with at least four superimposed outbursts exceeding a flux density of 5 Jy.

The Green Bank Interferometer (GBI) monitoring data for this event are displayed in Fig. 1. The observational aspects of these data are described in Johnston et al. (1986), Fiedler et al. (1987), and Waltman et al. (1994), the latter describe the relationship between very low flux densities ([FORMULA] mJy) at 2.25 and 8.3 GHz, which are observed prior to major flares, and the January 1991 flare. In summary Cygnus X-3 was observed simultaneously at 13.3 cm (2.25 GHz: S-band) and 3.6 cm (8.3 GHz: X-band) in two circular polarizations. The flux density scale is based on 3C286, which is assumed to have a flux density of 11.85 Jy and 5.27 Jy at 13.3 and 3.6 cm respectively. A detailed error analysis of GBI data is presented in Fiedler et al. (1987).

Following the report of the radio outburst on January 18 an adhoc VLBI campaign on four consecutive days using telescopes of the European VLBI network at 6 cm wavelength was organized. Data were taken with the 32 m telescopes at Medicina and Noto (Italy) on January 24. On January 25 and 26 the 100 m telescope at Effelsberg (Germany), one 25 m antenna of the Westerbork array (The Netherlands) and the 32 m telescopes at Medicina and Noto (the latter only on January 25) were used, and on January 27 the antennas at Medicina and Noto observed the source. The wideband MkIII recording system (Rogers et al., 1983) was used at a bandwidth of 56 MHz (in Mode A: 4958.99 - 5010.99 MHz), the angular resolution of the baselines was in the range of 5 to 26 mas. The observational details are summarized in Table 1. The VLBI scans on Cygnus X-3 were of 780 s duration. Every hour one 780 s block was split to record 480 s data on the quasar 2005+403, which serves as structural calibrator, and the remaining 300 s were observed on Cygnus X-3 again.


Table 1. Observational parameters of the 1991 January adhoc VLBI- experiment on Cygnus X-3. Notes: I.H.A.: Interferometer Hour Angle; Telescopes: Effelsberg (Germany) 100 m (B), single 25 m antenna (W) of the Westerbork array (The Netherlands), and the 32 m antennas at Medicina (M) and Noto (N) (Italy). The MkIII system with a total bandwidth of 56 MHz was used (14 channels with 4 MHz bandwidth each, and sky-frequencies 4958.99 - 5010.99 MHz), with the lowest [FORMULA] -detection limit (B-baselines) of 10 mJy. The angular resolution range of the baselines was 5 to 26 mas at 6 cm wavelength.

The data were correlated at the MkIII correlator of the Max-Planck- Institut für Radioastronomie, Bonn (Germany). In order to improve the signal-to- noise ratio on the weaker baselines and reduce the noise on the closure phases, the data were refringed with narrow windows making use of closure relations to determine and fix the delays and delay rates (Alef & Porcas 1986). The results obtained for Cygnus X-3 especially on the "weak" baselines were cross-checked with those obtained for the structural calibrator 2005+403, which was well detected on all baselines. The calibration of the amplitudes was performed by applying the system temperatures recorded between VLBI scans and the reported gain curves for each telescope. The flux density scale was obtained by performing cross scans with the 100 m telescope on 3C295, adopting a flux density of [FORMULA] Jy at 6 cm wavelength from Ott et al. (1994). As a further check of the consistency, the flux densities of Cygnus X-3 and 2005+403 have been interpolated from the Green Bank Interferometer flux density measurements at 13.3 and 3.6 cm wavelength and compare well with those obtained using 3C295.

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© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997