X-ray Emission of the $\gamma$-ray Loud Young Radio Galaxy NGC 3894

Abstract

The radio source 1146+596 is hosted by an elliptical/S0 galaxy NGC\,3894, with a low-luminosity active nucleus. The radio structure is compact, suggesting a very young age of the jets in the system. Recently, the source has been confirmed as a high-energy (HE, $>0.1$\,GeV) $\gamma$-ray emitter, in the most recent accumulation of the {\it Fermi} Large Area Telescope (LAT) data. Here we report on the analysis of the archival {\it Chandra} X-ray Observatory data for the central part of the galaxy, consisting of a single 40\,ksec-long exposure. We have found that the core spectrum is best fitted by a combination of an ionized thermal plasma with the temperature of $\simeq 0.8$\,keV, and a moderately absorbed power-law component (photon index $\Gamma =1.4\pm 0.4$, hydrogen column density $N_{\mathrm{H}}/10^{22}$\,cm${}^{-2}$\,$=2.4\pm 0.7$). We have also detected the iron K$\alpha$ line at $6.5\pm 0.1$\,keV, with a large equivalent width of EW\,$=1.0_{-0.5}^{+0.9}$\,keV. Based on the simulations of the {\it Chandra}'s Point Spread Function (PSF), we have concluded that, while the soft thermal component is extended on the scale of the galaxy host, the hard X-ray emission within the narrow photon energy range 6.0--7.0\,keV originates within the unresolved core (effectively the central kpc radius). The line is therefore indicative of the X-ray reflection from a cold neutral gas in the central regions of NGC\,3894. We discuss the implications of our findings in the context of the X-ray Baldwin effect. NGC\,3894 is the first young radio galaxy detected in HE $\gamma$-rays with the iron K$\alpha$ line.

Revised version, accepted for publication in the Astrophysical Journal

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