The `harder when brighter' X-ray behaviour of the low luminosity active galactic nucleus NGC 7213

TL;DR

This study provides the first robust evidence of an anti-correlation between X-ray photon index and luminosity in the LLAGN NGC 7213, showing it behaves similarly to black hole X-ray binaries in the hard state.

Contribution

It demonstrates a clear 'harder when brighter' spectral trend in NGC 7213, a low luminosity AGN, using long-term X-ray monitoring data, and compares its spectral energy distribution with other low-ionization nuclei.

Findings

NGC 7213 exhibits a 'harder when brighter' X-ray spectral behavior.

The anti-correlation between nd X-ray luminosity is robust and consistent.

The spectral energy distribution resembles that of low-ionization nuclear emission-line regions.

Abstract

We present the first robust evidence of an anti-correlation between the X-ray photon index, \Gamma, and the X-ray luminosity in a single low luminosity active galactic nuclei (LLAGN), NGC 7213. Up to now, such anti-correlation trends have been seen only in large samples of LLAGN that span a wide range of X-ray fluxes, although the opposite behaviour (i.e. a positive correlation between \Gamma and X-ray luminosity) has been extensively studied for individual X-ray bright active galactic nuclei. For NGC 7213, we use the long-term X-ray monitoring data of Rossi X-ray Timing Explorer (RXTE), regularly obtained on average every two days from March 2006 to December 2009. Based on our X-ray data, we derive the \Gamma versus flux and the hardness ratio versus flux relations, indicating clearly that NGC 7213 follows a `harder when brighter' spectral behaviour. Additionally, by analysing radio and optical data, and combining data from the literature, we form the most complete spectral energy distribution (SED) of the source across the electromagnetic spectrum yielding a bolometric luminosity of 1.7*10^43 erg s^-1. Phenomenologically, the SED of NGC 7213 is similar to that of low-ionization nuclear emission-line region. The robust anti-correlation trend that we find between \Gamma and X-ray luminosity together with the low accretion rate of the source, 0.14 per cent that of Eddington limit, make NGC 7213 the first LLAGN exhibiting a similar spectral behaviour with that of black hole X-ray binaries in `hard state'.