High resolution radio study of the pulsar wind nebula within the supernova remnant G0.9+0.1

Aims. We have conducted a study in radio wavelengths and in X-rays of the pulsar wind nebula (PWN) in the supernova remnant (SNR) G0.9+0.1 with the goal of investigating in detail its morphology and to accurately determine its characteristic parameters. Methods. To carry out this research we have observed the PWN at lambda 3.6 and 6 cm using the Australia Telescope Compact Array (ATCA) and combined these data with existing multiconfiguration VLA data and single dish observations in order to recover information at all spatial scales. We have also reprocessed VLA archival data at lambda 20 cm. From all these observational data we have produced high-fidelity images at the three radio frequencies with angular resolution better than 3 ''. The radio data were compared to X-ray data obtained with Chandra and in two different observing runs with XMM-Newton. Results. The new observations revealed that the morphology and symmetry suggested by Chandra observations (torus and jet-like features) are basically preserved in the radio range in spite of the rich structure observed in the radio emission of this PWN, including several arcs, bright knots, extensions and filaments. The reprocessed X-ray images show for the first time that the X-ray plasma fills almost the same volume as the radio PWN. Notably the X-ray maximum does not coincide with the radio maximum and the neutron star candidate CXOU J174722.8-280915 lies within a small depression in the radio emission. From the new radio data we have refined the flux density estimates, obtaining S (PWN) similar to 1.57 Jy, almost constant between lambda 3.6 and lambda 20 cm. For the whole SNR (compact core and shell), a flux density S (20 cm) = 11.5 Jy was estimated. Based on the new and the existing lambda 90 cm flux density estimates, we derived a spectral index alpha(PWN) = -0.18 +/- 0.04 and alpha(shell) = -0.68 +/- 0.07. From the combination of the radio data with X-ray data, a spectral break is found near nu similar to 2.4 x 10(12) Hz. The total radio PWN luminosity is L(radio) = 1.2 x 10(35) erg s(-1) when a distance of 8.5 kpc is adopted. By assuming equipartition between particle and magnetic energies, we estimate a nebular magnetic field B = 56 mu G. The associated particle energy turns out to be U(part) = 5 x 1047 erg and the magnetic energy U(mag) = 2 x 10(47) erg. The high ratio between magnetic and particles flux energy density suggests that the pulsar wind just started to become particle dominated. Based on an empirical relation between X-ray luminosity and pulsar energy loss rate, and the comparison with the calculated total energy, a lower limit of 1100 yr is derived for the age of this PWN.