Joint NuSTAR and Chandra analysis of the obscured quasar in IC 2497 - Hanny's Voorwerp system

TL;DR

This study combines NuSTAR and Chandra X-ray data with other observations to reveal that the obscured AGN in IC 2497 has significantly decreased in luminosity, indicating a change in accretion state rather than a typical changing-look AGN.

Contribution

It provides the first combined NuSTAR and Chandra analysis of the obscured AGN in IC 2497, proposing a change in accretion state as the cause of luminosity decline.

Findings

The AGN is Compton-thick with $N_{\rm H} \sim 2 \times 10^{24}$ cm$^{-2}$.

Luminosity dropped by a factor of ~50 over 100 kyr.

The AGN likely transitioned from a radiatively efficient to inefficient state.

Abstract

We present new Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the core of IC 2497, the galaxy associated with Hanny's Voorwerp. The combined fits of the Chandra (0.5-8 keV) and NuSTAR (3-24 keV) X-ray spectra, together with WISE mid-IR photometry, optical longslit spectroscopy and optical narrow-band imaging, suggest that the galaxy hosts a Compton-thick AGN ($N_{\rm H} \sim 2 \times 10^{24}$ cm$^{-2}$, current intrinsic luminosity $L_{\rm bol} \sim 2-5 \times 10^{44}$ erg s$^{-1}$) whose luminosity dropped by a factor of $\sim$50 within the last $\sim 100$ kyr. This corresponds to a change in Eddington ratio from $\rm \lambda_{Edd} \sim$ 0.35 to $\rm \lambda_{Edd} \sim$ 0.007. We argue that the AGN in IC 2497 should not be classified as a changing-look AGN, but rather we favour the interpretation where the AGN is undergoing a change in accretion state (from radiatively efficient to radiatively inefficient). In this scenario the observed drop in luminosity and Eddington ratio corresponds to the final stage of an AGN accretion phase. Our results are consistent with previous studies in the optical, X-ray and radio although the magnitude of the drop is lower than previously suggested. In addition, we discuss a possible analogy between X-ray binaries and an AGN.