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Astron. Astrophys. 363, L17-L20 (2000)

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2. Radio observations

A field centred at the pressumed core position taken from the FIRST map (cf. Sect. 1) was observed with the VLA B-array at 4.86 GHz. This observing frequency and the array configuration did not allow us to map the brightness distribution in the extended lobes with their brightest regions lying 5-6 arc min apart from the core.

The observations were conducted on December 13, 1999. [FORMULA] of the synthesized beam was [FORMULA]. With the integration time of [FORMULA] min, the rms fluctuations were about 26 µJy beam-1. The [FORMULA] core component of 0.7 mJy beam-1 and the integral flux density of 1.1 mJy was detected at [FORMULA], [FORMULA] (J2000). Besides the core, very much attenuated emission was detected from the hotspot in the NE lobe.

Fig. 1 shows the radio contours of the source taken from the NVSS map C1348P36.IQU.1 overlayed on the brightness from our 4.86-GHz map (gray scale). Because of very large scale of the entire source, poorly visible dot indicating the radio core is marked by the cross. The most compact structure in the NE lobe, reproduced from the 1.4-GHz FIRST map (cf. Sect. 1), and our 4.86-GHz map resolving the hotspot are shown in Fig. 2. The deconvolved size of the hotspot at 1.4 GHz is [FORMULA] at PA=+83o. This hotspot is resolved at 4.86 GHz into two components with the deconvolved sizes of [FORMULA] and [FORMULA]. These sizes are used in Sect. 4 to estimate the area of the bowshock and ambient density of the environment in front of a head of the NE lobe.

[FIGURE] Fig. 1. 1.4-GHz NVSS contour map. The radio core (marked by the cross) and fine structure of the NE hotspot detected at 4.86 GHz are shown in gray scale. The small rectangle area marked in the NE lobe is enlarged in Fig. 2. The inset shows position of this core in the DSS image of the identified galaxy

[FIGURE] Fig. 2. a Compact structure in the NE lobe reproduced from the 1.4-GHz FIRST map suggesting presence of a hotspot. b Our 4.86-GHz map of this hotspot

The degree of asymmetry of J1343+3758 is low. The ratio of separation between the core and brightest regions in the lobes and the misalingnment angle are 1.32 and 3 deg, respectively. The widths of the lobes (322 kpc and 274 kpc) are determined using the prescription of Leahy & Williams (1984), i.e. the deconvolved half-power widths are multiplied by the factor [FORMULA]. The average of the above values give the overall axial ratio of 10.5.

The flux densities available for the source at frequencies from 151 MHz to 5 GHz are given in Table 1.


Table 1. Radio flux densities of the source and its lobes.
a) original B3 flux density is multiplied by 1.087,
i.e. adjusted to the common scale of Baars et al. (1977)

The integrated spectrum of the entire source and its lobes is shown in Fig. 3. In order to calculate the total radio luminosity, we fit the observed flux density data with an assumed functional form. The best fit to the data in column 3 of Table 1 is achieved with a parabola [FORMULA][mJy][FORMULA], where [FORMULA][GHz]. This fit gives the best-fitted 1.4 GHz total flux density of 136 mJy, and the fitted spectral indices of [FORMULA] and [FORMULA] at 151 MHz and 5 GHz, respectively. Though the quoted errors of spectral indices are large, the integrated spectrum clearly steepens at high frequencies. A change of the spectrum slope of about 0.5 suggests that the break frequency is likely between 151 MHz and 5 GHz, however its too short frequency range did not allow a reliable fit of any theoretical spectrum accounting for radiative losses to the observations.

[FIGURE] Fig. 3. Radio spectrum of the entire source and its lobes

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000