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Astron. Astrophys. 360, 99-106 (2000)

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5. The X-ray halo of 3C 273

For the comparison of the quasar's profile with that of the white dwarf, the normalization constant had been optimised over the range 3" to 13", where the profiles show excellent agreement. However, it is evident from Fig. 5 that the quasar profile lies systematically above the profile of the stellar source beyond radii of about 15". Although the error of individual data points is large, we regard the systematic deviation as real and attribute it to an extended X-ray halo of the quasar (3C 273 had been observed on-axis, the pointing for HZ 43 was 1´ off-axis). To isolate the X-ray flux from this halo we have used the scaling from Fig. 5 and subtracted the azimuthally averaged HZ 43-profile from the averaged quasar profile (Fig. 10). This differential profile was fitted with a King profile (Jones & Forman 1984)

[FIGURE] Fig. 10. Azimuthally averaged radial profile of the quasar halo's X-ray emission. The fit with a King profile is shown between [FORMULA] and 80" from the core.

[EQUATION]

with P (0) = 3.6, a = [FORMULA] [FORMULA] 28.8 kpc, [FORMULA] = 0.6758 determined from a least square fit to the data points between [FORMULA] and 80" from the core. Although the formal errors for these parameters are large, we nevertheless used them for our numerical estimates, as they describe the data reasonably well. The King-profile was integrated up to give the total X-ray flux from the halo of 232 nJy corresponding to a luminosity in the HRI band of [FORMULA] erg/sec (spectral index [FORMULA] assumed). To convert this into an estimate of the physical parameters we assumed a plasma temperature of [FORMULA] K as in the M 87 halo (Böhringer et al. 1994) and that the plasma is confined with constant density to within the core radius. From these assumptions a density of [FORMULA] H-atoms/m3 is derived for the central part of the halo. This is a factor of 10 above the upper limit derived from EINSTEIN observations by Willingale as quoted by Conway et al. (1981). The typical density of the thermal plasma in the cores of rich clusters of galaxies is a factor of 10 below this value (Jones & Forman 1984) but the density derived above for the halo of 3C 273 matches the central density of [FORMULA] m-3 derived for the central density derived for the intra-cluster medium around Cygnus A (Carilli et al. 1994).

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000
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