D. McConnell ,
R. Deacon ,
J.G. Ables
, PASA, 18 (2), in press.
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Observations and Data Reduction
Observations of 47 Tuc were made with the ATCA, as described in MA2000, over two 128 MHz bands centred at 1.408 GHz and 1.708 GHz in a number of Array configurations. Several reference positions close to the cluster centre were used to minimise the effects of artefacts at the phase reference centre and to flatten the sensitivity profile over the cluster. Table 1 lists the details of each observation.Date | Time | Integration | Array | ||
(UT) | (UT) | (hours) | (J2000) | (J2000) | |
1998 Oct 16 | 13:36 | 1.5 | 00:24:06 | -72:05:00 | 1.5D |
1998 Nov 13 | 10:30 | 7.5 | 00:24:06 | -72:05:00 | 6D |
1998 Nov 24 | 02:21 | 3.9 | 00:24:06 | -72:05:00 | 6D |
1999 Jan 25 | 23:23 | 9.9 | 00:24:06 | -72:03:00 | 0.75B |
1999 Feb 12 | 01:40 | 1.9 | 00:24:06 | -72:03:00 | 6C |
1999 Mar 23 | 00:29 | 6.2 | 00:24:06 | -72:03:00 | 1.5B |
1999 May 04 | 16:40 | 10.3 | 00:24:06 | -72:03:00 | 6A |
1999 May 08 | 21:59 | 4.5 | 00:24:06 | -72:03:00 | 6A |
1999 Jun 03 | 16:31 | 9.5 | 00:24:06 | -72:03:00 | 6A |
1999 Aug 17 | 11:30 | 11.1 | 00:24:06 | -72:03:00 | 6D |
1999 Aug 18 | 08:25 | 11.0 | 00:24:06 | -72:03:00 | 6D |
1999 Aug 30 | 07:37 | 12.9 | 00:23:44 | -72:03:15 | 6D |
1999 Sep 02 | 22:45 | 8.1 | 00:23:49 | -72:03:15 | 6D |
1999 Sep 11 | 20:28 | 1.4 | 00:23:49 | -72:03:15 | 6A |
1999 Sep 22 | 17:41 | 2.1 | 00:23:49 | -72:03:15 | 6A |
1999 Oct 04 | 16:30 | 5.2 | 00:23:49 | -72:03:15 | 6A |
1999 Dec 26 | 04:31 | 7.1 | 00:23:49 | -72:03:15 | 1.5A |
1999 Dec 31 | 02:21 | 24.8 | 00:24:06 | -72:03:00 | 1.5A |
The MIRIAD data reduction package was used to calibrate the data and form images. The antenna gains were calibrated using brief observations of the source B2353-686 (
S1.4 = 1.05 Jy,
S1.7 = 1.00 Jy) and B0252-712 (S1.4 = 5.5 Jy, S1.7 = 4.6 Jy). The flux scale was referred to the ATCA primary flux calibrator B1934-638 (
S1.4 = 14.9 Jy;
S1.7 = 14.0 Jy). The image we present here is derived from all datasets listed in Table 1 combined with Compact Array data from 1992. Before combination, separate images corresponding to each dataset were made to assist in the identification of data affected by radiofrequency interference or instrumental faults. The MIRIAD software supports the reduction of data collected from a mosaic of pointing centres, and tasks INVERT and MOSMEM were used to produce and deconvolve images at each observing frequency. Both images have reference (tangent) point at (2000) = 00:24:053, (2000) = -72:04:52
7, the nominal cluster centre (De Marchi et al., 1996). An imaging cell size of 2
was used to provide adequate sampling of the synthesized beam which is 8
3 x 7
2 at 1.4 GHz and 8
6 x 5
7 at 1.7 GHz. The size of each image is 2755 x 2820 pixels (91.8 x 94 arcmin2). The final image analysis was performed over a region of approximate diameter 53 within which the sensitivity varied by less than a factor of 3.5. Within this region the variation of sensitivity was modelled taking account of both primary beam attenuation towards the field edges and the increased fluctuations close to strong sources. The sensitivity model allowed uniform discrimination of sources across the entire analysed field. Sources with peak flux density greater than 5 times the local rms brightness fluctuation were selected. We note that from the analysis of the earlier ATCA image of 47 Tuc, two sources were reported (numbered 9 and 11 in Table 2 of MA2000) with fluxes of 200 or 4.8 times the image rms brightness fluctuation. Neither source is detected in the more sensitive image reported here, whereas all sources with flux in the original report are detected with the new data.
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Title/Abstract Page: Radio Images of the
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© Copyright Astronomical Society of Australia 1997