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2. Observations and data analysis

The observations were carried out with a 30-m telescope of the Instituto Argentino de Radioastronomıa during March 1996. The telescope was equipped with a 1.42 GHz-continuum receiver of single beam, corrugated, dual-channel feed. The bandwidth was 20 MHz and the system temperature tex2html_wrap_inline1107 K. The HPBW of the antena is tex2html_wrap_inline1109 arcminutes at the observing frequency. The observations were made at night, in order to reduce the effects of changes in telescope structure due to ambient temperature fluctuations and terrestrial interfering signals, and intercalated in blanks during a variability monitoring campaign of the radio source PKS 1830-211 (Romero et al. 1997).

A region of tex2html_wrap_inline1113 around the "best" position of 2EG tex2html_wrap_inline1041 (see Thompson et al. 1995 and Mattox et al. 1996) was mapped by means of repeated fast (tex2html_wrap_inline1117/min) scans in declination, regularly spaced in right ascension. Each group of scans were averaged and processed by standard techniques (e.g. Combi & Romero 1995 and references therein). The non-variable, powerful radio sources PKS 1814-63, PKS 1932-46, and PKS 2152-69 were observed for flux density and pointing calibration. The resulting flux density scale was fixed according to Wills (1975).

The result of these observations was the map shown (in galactic coordinates) in Fig. 1 (click here). The rms noise of this map is tex2html_wrap_inline1125 mK. The strong contamination produced by the radiation originated in the galactic plane avoids a faithful discrimination of the fine radio features in this image. With the aim of removing this difficulty we have applied a filtering method originally developed by Sofue & Reich (1979) and used by several authors in studies of regions close to the plane (e.g. Combi et al. 1995; Duncan et al. 1995). The map shown in Fig. 1 (click here) was convolved with a filtering Gaussian beam of HPBW tex2html_wrap_inline1127 yielding brightness temperatures T01 and residuals tex2html_wrap_inline1131, where T stands for the original temperatures. A new set of temperatures T'01 was computed according to tex2html_wrap_inline1137 for tex2html_wrap_inline1139, and T'01=T for tex2html_wrap_inline1143. The procedure was repeated in order to generate T02, tex2html_wrap_inline1147, and T'02, and so on. After n=6 iterations, when |T0n-T0n-1| became smaller than the rms noise, a residual distribution tex2html_wrap_inline1155 was obtained. The resulting final map, where smooth emission with sizes scales larger than tex2html_wrap_inline1127 has been eliminated, is shown in Fig. 2 (click here). The original map can be recovered just by simple addition of the background component to this new map.

Figure 1: Total continuum emission map at 1.42 GHz of the region around the position of 2EG tex2html_wrap_inline1041. Contour lines are shown at 8, 8.3, ..., 11.9 K in brightness temperature

Figure 2: The same region shown in Fig. 1 (click here) after the subtraction of the diffuse disk contribution. The confidence contours of the likelihood test statistics of 2EG tex2html_wrap_inline1041 are also shown as a gray-scale. The contours of the radio emission are labelled in steps of 0.06, 0.16, ..., 0.96; 1.3, 1.6, and 1.9 K in brightness temperature. The gray-scaled levels represent the 99%, 95%, 68%, and 50% statisitcal probability that the source lies within each contour

Several radio sources can be clearly seen in the filtered image. Just two of them have been previously detected: the gravitational lensed QSO PKS 1830-211 (Subrahmanyan et al. 1990) and the SNR tex2html_wrap_inline1165 (Green 1996). In Table 1 (click here) we list the main characteristics of all sources in the frame.


Source (l, b) tex2html_wrap_inline1173 (K) tex2html_wrap_inline1175 (Jy) tex2html_wrap_inline1177 Id
G8.7-5 (8.7, -4.9) 0.51 1.6 0.1 SNR
G8.5-6.7 (8.5, -6.7) 0.36 1.9 0.21 SNR?
G10.2-3.5 (10.2, -3.5) 0.56 5.7 -0.16 SNR?
G10.7-5.4 (10.7, -5.4) 0.81 17.7 -0.71 SNR
G11.9-3.6 (11.9, -3.6) 0.34 1.1 -0.75 SNR
PKS 1830-211 (12.1, -5.7) 1.88 11.8 0.12 Lensed QSO
G12.7-3.9 (12.7, -3.9) 0.36 1.9 -0.56 SNR
Table 1: Main radio sources at 1.42 GHz in the field around 2EG tex2html_wrap_inline1041


In order to obtain some information about the nature of the new sources discovered in the field, we have used data from the 408 MHz all-sky survey by Haslam et al. (1982) for computing spectral indices of the emission. These lower frequency data were processed in similar way than the 1.42 GHz data and, after the removing of the background radiation, a spectral index distribution was calculated using the procedure described by Combi & Romero (1995, 1997). The resulting spectral index map is shown in Fig. 3 (click here), where contour values lower than 5 rms have been excluded. The angular resolution of this map is tex2html_wrap_inline1221 arcminutes due to the convolution of the 1.42-GHz beam to the larger 408-MHz beam. Mean errors tex2html_wrap_inline1223 have been estimated as in Combi & Romero (1997). The accuracy of the given spectral index values can be checked with PKS 1830-211, which is a well-established flat-spectrum source (Pramesh Rao & Subrahmanyan 1988).

Figure 3: Spectral index distribution computed between 408 MHz and 1.42 GHz. The values of the spectral indices (defined as tex2html_wrap_inline1227) are indicated in the map

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