The X-ray properties of young radio-loud AGN

We present XMM-Newton observations of a complete sample of five archetypal young radio-loud active galactic nucleus (AGN), also known as Compact Symmetric Objects (CSO) or Gigahertz Peaked Spectrum (GPS) sources. They are among the brightest and best studied GPS/CSO sources in the sky, with radio powers in the range L-5 GHz= 10(43-44) erg s(-1) and with four sources having measured kinematic ages of 570-3000 yr. All five sources are detected, and have 2-10 keV luminosities ranging from 0.5 to 4.8 x 10(44) erg s(-1). A detailed analysis was performed, comparing the X-ray luminosities and N-H absorption column densities of the GPS/CSO galaxies with their optical and radio properties, and with those of the general population of radio galaxies. We find the following. (i) GPS/CSO galaxies show a wide range in absorption column densities with a distribution not different from that of the general population of radio galaxies. We therefore find no evidence that GPS/CSO galaxies could reside in a significantly more dense circumnuclear environment such that they could be 'frustrated' radio sources - hampered in their development. (ii) The ratio of radio to X-ray luminosity is significantly higher for GPS/CSO sources than for classical radio sources. This is consistent with an evolution scenario in which young, compact radio sources are more efficient radio emitters than large extended objects, at a constant accretion power. (iii) Taking the X-ray luminosity of radio sources as a measure of their ionization power, we find that GPS/CSO sources are significantly underluminous in their [O (III)](5007 a) line luminosity, including a weak trend with age. This is consistent with the fact that the Stromgren sphere should still be expanding in these young objects. If true, this would mean that here we are witnessing the birth of the narrow-line region of radio-loud AGN.