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Radio and X-ray absorption in compact radio galaxies

Submitter: Raffaella Morganti & Luisa Ostorero
Description: Compact radio galaxies with a GHz-peaked spectrum (GPS), such as reproduced in the background of this image, have sizes smaller than 1 kpc and likely represent the youngest population radio-galaxies. They are complex systems, still not fully understood. Thanks to the sensitivity of XMM and Chandra, they have been increasingly detected in the X-ray domain. A big problem is, however, that the low spatial resolution of the X-ray data does not allow to locate the production site of the X-ray photons. The accretion disc surrounding the active supermassive black hole, the interstellar medium of the host galaxy shocked by the expanding radio mini lobes, and the lobes themselves are all proposed as X-ray factories.

Discriminating between different X-ray emission scenarios can help to clarify the origin and evolution of these enigmatic sources. One strategy is to compare the properties of absorbing material detected in X-ray with those of absorbers detected in radio observations.

A plot of the available data is shown in the figure where the column density of absorpbers as determined from radio (x-axis) is plotted against that determined from X-ray data (y-axis). The complication is that to estimate the gas density from radio data, one has to assume a temperature for the gas. For the black symbols 100 K was assumed (i.e. cold gas), while for the red symbols 5500 K (warm gas). There appears to be a correlation between the densities derived from X ray and from radio data, suggesting that both techniques are measuring the same absorbers. The data also seem to indicate that the absorbers are fairly warm (because only then the estimated densities are similar)

To obtain more data on a larger sample, a program of observations of additional GPS galaxies with the WSRT was started at ASTRON, as part of the Helena Kluyver female visitor programme. The investigation is now under way.

Confirming the correlation would imply co-spatiality of the X-ray and radio emission, thus supporting models where most of the X-rays are generated far away from the accretion disc. It would also provide an indirect estimate of the spin temperature of the circumnuclear gas.
Copyright: Radio map B0108+388: after Pearson & Readhead 1988; NH-NHI plot by Luisa Ostorero
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