High frequency X-ray and radio emission from AGN not correlated
23 August 2013
Our Sydney based researcher Dr Davide Burlon, together with other CAASTRO members and colleagues from Australia, Italy, and the US, looked for counterparts of X-ray sources at high radio frequencies to better understand and characterise the core properties of Active Galactic Nuclei (AGN). It had previously been suggested that radio and X-ray emissions were correlated, indicating that all X-ray emitting AGN were also radio emitters, at least at low radio frequencies, with some of these sources (~10%) launching relativistic radio jets (‘blazars’). The research team used six years of X-ray data from the Burst Alert Telescope (BAT), mounted on the Swift satellite and currently providing the deepest unbiased scan of the sky at 15-55keV, and data from the Australia Telescope Compact Array AT20G survey, one of largest blind surveys from the ground at 20GHz. Both of these surveys cover vast areas of the sky (AT20G ~20,000 deg2 of the southern sky).
The team’s Bayesan cross-matching algorithm found 37 matching AGN in the two data sets. Of these sources, 19 were identified as local Seyfert galaxies that are traditionally considered to be radio-quiet but are radio emitting at low levels; the other 18 sources were distant blazars. The apparent relation between X-ray and radio luminosities could be exclusively attributed to the distance of these sources. The team also tested for an effect of angular resolution but filtering their results down to the 21 most ‘compact’ AGN still did not reveal any correlation.
In their recent publication in the Monthly Notices of the Royal Astronomical Society, the researchers conclude that the absence of such a correlation must be a genuine core property. Only an estimated ~20% of X-ray sources actually had radio counterparts, of which, not surprisingly, blazars had a higher detection rate of ~65% compared to Seyfert galaxies at ~14%.