Australia Telescope National Facility

Radio Images of the Globular Cluster 47 Tucanae
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.

**Table 1:** Summary of new observations. The pointing and phase reference position and Compact Array configuration for each observation is tabulated. The data listed here were combined with data obtained from the Compact Array in 1992 (McConnell and Ables, 2000). The total effective integration time on the cluster core, including the 1992 observations, is 170 hours.
Date	Time	Integration	$\alpha$	$\delta$	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 (

S_1.4 = 1.05 Jy,

S_1.7 = 1.00 Jy) and B0252-712 (S_1.4 = 5.5 Jy, S_1.7 = 4.6 Jy). The flux scale was referred to the ATCA primary flux calibrator B1934-638 (

S_1.4 = 14.9 Jy;

S_1.7 = 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 $\alpha$ (2000) = 00:24:05 $.\!\!^{s}$ 3, $\delta$ (2000) = -72:04:52

$.\!\!^{\prime\prime}$ 7, the nominal cluster centre (De Marchi et al., 1996). An imaging cell size of 2

$^{\prime \prime }$ was used to provide adequate sampling of the synthesized beam which is 8

$.\!\!^{\prime\prime}$ 3 x 7

$.\!\!^{\prime\prime}$ 2 at 1.4 GHz and 8

$.\!\!^{\prime\prime}$ 6 x 5

$.\!\!^{\prime\prime}$ 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 $^\prime$ within which the sensitivity varied by less than a factor of $\sim$ 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 $\sim$ 200 $\mu\mbox{Jy}$ 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 $S > 5\sigma$ in the original report are detected with the new data.

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