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Astron. Astrophys. 357, L45-L48 (2000)

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2. Observations and data reduction

In 1997 we detected the onset of a new major radio outburst in III Zw 2 and we initiated a target of opportunity program to monitor the spectral evolution of the burst with the VLA and its structural evolution with the VLBA.

The VLA observations were made at six frequencies ranging from 1.4 GHz to 43 GHz in A, B, C and D configuration from 1998 September until now. The three epochs discussed in this paper where obtained on 1998 November 04, 1999 March 23 and 1999 July 07 in CnB, D and A configuration respectively. The source 3C48 was used as the primary flux density calibrator, and III Zw 2 was self-calibrated and mapped with the Astronomical Image Processing System (AIPS). Since the 1.4 GHz observation in March 1999 was heavily confused by the nearby sun, we estimated the flux density by interpolation between earlier and later epochs.

We observed III Zw 2 with the VLBA on 1998 February 16, 1998 June 13, 1998 September 22, 1998 December 12, and 1999 July 15 at 43 and 15 GHz. We used a total bandwidth of 64 MHz for a full 8 hour scan, except for the last observation which was a 4 hour scan. We spent three-quarters of the available observing time at 43 GHz and one quarter of it at 15 GHz. For the second epoch, we used the Effelsberg 100 m telescope in combination with the VLBA. We reduced the data using the software packages AIPS and DIFMAP (Shepherd, Pearson, & Taylor 1994). Fringes were detected in the III Zw 2 data on all baselines. We calibrated the gains using system temperature information and applied atmospheric opacity corrections. To obtain a reliable total flux estimate, amplitude gains for stations with bad weather conditions were scaled up to match the other antennas. The data were then self-calibrated, first using phase-only and later phase-amplitude self-calibration with solution intervals slowly decreasing down to one minute. A more detailed discussion of the VLA spectra and the 15 GHz VLBA-data will be presented in a forthcoming paper.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000