Correlation between X-ray and radio absorption in compact radio galaxies

TL;DR

This study finds a significant positive correlation between X-ray and radio hydrogen column densities in compact radio galaxies, suggesting co-spatial or related absorbing structures, with implications for understanding their absorption properties.

Contribution

The paper provides the first statistical analysis of the relationship between X-ray and radio absorbers in GPS/CSOs, including new radio data and a compiled sample, revealing a correlation and its possible physical origin.

Findings

$N_{H}$ and $N_{HI}$ are positively correlated.

The correlation suggests co-spatial or related absorption components.

Large intrinsic spread may be due to fluctuations in $T_{s}/C_{f}$.

Abstract

Compact radio galaxies with a GHz-peaked spectrum (GPS) and/or compact-symmetric-object (CSO) morphology (GPS/CSOs) are increasingly detected in the X-ray domain. Their radio and X-ray emissions are affected by significant absorption. However, the locations of the X-ray and radio absorbers are still debated. We investigated the relationship between the column densities of the total ($N_{\mathrm{H}}$) and neutral ($N_{\mathrm{HI}}$) hydrogen to statistically constrain the picture. We compiled a sample of GPS/CSOs including both literature data and new radio data that we acquired with the Westerbork Synthesis Radio Telescope for sources whose X-ray emission was either established or under investigation. In this sample, we compared the X-ray and radio hydrogen column densities, and found that $N_{\mathrm{H}}$ and $N_{\mathrm{HI}}$ display a significant positive correlation, with $N_{\mathrm{HI}} \propto N_{\mathrm{H}}^b$, where $b=0.47$ and $b=0.35$, depending on the subsample. The $N_{\mathrm{H}}$ - $N_{\mathrm{HI}}$ correlation suggests that the X-ray and radio absorbers are either co-spatial or different components of a continuous structure. The correlation displays a large intrinsic spread that we suggest to originate from fluctuations, around a mean value, of the ratio between the spin temperature and the covering factor of the radio absorber, $T_{\rm s}/C_{\rm f}$.